Normal and Refractory Concretes for LM FBR Applications

LMFBR Applications Volume 2: Evaluation of Concretes for

NP-2437, Volume 2 Research Projects 1704-14, 1704-19

Final Report, June 1982

Prepared by

NORTHWESTERN UNIVERSITY Tec hnolog ica I I ns t i t u t e

Center for Concrete and Geomaterials Evanston. Illinois 60201

Principal Investigators Z. P. Baiant, Coordinator

J. C. Chern

PORTLAND CEMENT ASSOCIATION Construction Technology Laboratories

Old Orchard Road Skokie, Illinois 60077

Principal Investigators M. S. Abrams M. P. Gillen

Prepared for

Electric Power Research Institute 3412 Hillview Avenue

Palo Alto, California 94304

EPRl Project Manager J. Matte, I l l

Developing Applications and Technology Program Nuclear Power Division

ORDER I N G I N FORM AT ION

Requests for copies of this report should be directed to Research Reports Center (RRC), Box 50490, Palo Alto, CA 94303, (415) 965-4081. There is no charge for reports requested by EPRI member utilities and affiliates, contributing nonmembers, U.S. utility associations, U.S. government agencies (federal, state, and local), media, and foreign organizations with which EPRI has an information exchange agreement. On request, RRC will send a catalog of EPRl reports.

NOTICE This report was prepared by the organization(s) named below as an account of work sponsored by the Electric Power Research Institute, Inc. (EPRI) Neither EPRI. members of EPRI, the organization(s) named below, nor any person acting on behalf of any of them: (a) makes any warranty, express or implied, with respect to the use of any information, apparatus, method, or process disclosed in this report or that such use may not infringe private ly owned rights; or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process disclosed in this report.

Prepared by Northwestern University Evanston, Illinois and Portland Cement Association Skokie, Illinois

EPRI PERSPECTIVE

PROJECT DESCRIPTION

Virtually without exception, conventional reinforced concrete is used for constructing nuclear power plant foundations and supports, shielding walls and floors, and containment building shells because of the eminent suitability of the material under normal design conditions. In such applications, and particularly in liquid metal fast breeder reactor (LMFBR) plants, this material is challenged by high-temperature operating environments and by the possibility of contact with sodium leaks or spills; meeting these challenges has led to complex auxiliary systems for cooling and protecting the concrete.

The recent trend in LMFBR technology toward achievement of reliabil- ity and ease of operation by utilizing inherent characteristics rather than depending on complex systems suggested another approach. That approach was to seek alternative castable construction materials which can be used at elevated temperatures and are compatible with sodium. This two-volume report for RP1704-14 and RP1704-19 is a disciplined first step in that search.

PROJECT OBJECTIVES

The basic objective of this project was to identify and characterize certain types of high-temperature or refractory concretes that may have beneficial applications in the civil engineering portions of an LMFBR plant. A few examples of such applications were offered to guide the researchers: structural service at elevated temperatures, sodium spill containment, and anhydrous shielding material (for overreactor applications). A secondary objective of the work was to report any serendipitous findings that could bear on providing a substructure which would be a heat sink rather than an energy source in the improbable event of a core melt challenging containment.

iii

PROJECT FGSULTS

The results of this project are presented in two volumes. Volume 1 consists of a selective, critical survey of material properties information relevant to the project objectives, which concern both normal portland cement concrete and various refractory concretes. This survey was based on an extensive literature search; the bibliography of consulted sources is included in this volume.

Volume 2 reports the comparison and evaluation of the high- temperature performance of various concretes and the selection of the more promising candidates for possible LMFBR applications. Based on presently available data, the report recommends that four varieties of refractory concrete be tested further since they show promise of having beneficial applications for both LMFBR and certain possible LWR applications. Specific follow-on activities are recommended in this volume.

This report should be of interest primarily to LMFBR design organi- zations and their sponsoring utilities or agencies. Certain aspects of this work may be of benefit in the design and operational analysis of LWR systems.

Joseph Matte, 111, Project Manager Nuclear Power Division

iv

ABSTRACT

The extensive literature on the properties and behavior at elevated temperature of portland cement concrete and various refractory concretes was reviewed to collect in concise form the physical and chemical properties of castable refractory concretes and of conven- tional portland cement concretes at elevated temperature. This survey, together with an extensive bibliography of source documents, is presented in Volume 1. A comparison was made of these properties, the relative advantages of the various concretes was evaluated for possible liquid metal fast breeder reactor applications, and a selec- tion was made of several materials of interest for such applications. Volume 2 concludes with a summary of additional knowledge needed to support such uses of these materials together with recommendations on research to provide that knowledge.

V

ACKNOWLEDGMENT

This work has been carried out jointly by Northwestern University and Construction Technology Laboratories of PCA under contracts No. RP 1704-14 and RP 1704-19 with the Electric Power Research Institute, Palo Alto, California.

v i i

TABLE OF CONTENTS

Section Page

1. INTRODUCTION 1-1

2. COMPARISON OF VARIOUS MATERIALS

2.1 Strength Comparison 2.2 Comparison of Deformation Data 2.3 Comparison of Insulation and Sodium Resistance

3. ADVANTAGES, DISADVANTAGES AND SELECTION OF MATERIALS OF INTEREST

3.1 Listing of Advantages and Disadvantages of

3.2 Improvement of Performance 3.3 Ranking of Materials for Various Properties 3.4 Selection of Materials of Main Interest 3.5 Materials of Interest for Typical Applications

Various Materials of Interest

in LMFBR

4. PRINCIPAL GAPS IN KNOWLEDGE

4.1 Portland Cement Concretes 4 . 2 High Alumina Cement Concretes 4.3 Non-Hydraulic Concretes 4.4 Refractory Brick Linings (Masonry) 4.5 Materials for Radiation Shielding 4.6 Admixtures and Surface Treatments

5. RECOMMENDATION FOR FURTHER RESEARCH

5.1 Experimental Work 5.2 Theoretical Work

2-1

2-2 2-8

2-10

3-1

3-1 3-12 3-14 3-19 3-24

4-1

4-1 4-2

4-3 4-4 4-5 4-5

5-1

5-1 5-5

6. SUMMARY 6-1

ix

Section 1

INTRODUCTION

Concrete has important applications in nuclear reactor containments and vessels and, in particular, appears to be very effective for structures and supports in liquid metal cooled fast breeder reactors (LMFBR's). This material permits casting of massive walls that offer excellent performance in case of various destructive dynamic loads, such as hypothetical core-destructive accidents, external and inter- nal missiles, earthquake, blasts, etc. This excellent performance is due mainly to the very large energy absorption capability of reinforced concrete, ductility of reinforcement, and absence of brittle fracture modes that are of great concern for metallic structures.

The designer, however, is unable to realize the full potential of reinforced concrete in the confinement of nuclear reactors. This limitation is due to gaps in knowledge, as well as lack of easy availability of existing knowledge with regard to thermal response of concrete. Material and structural tests indicate a significant loss of strength, stiffness, and serviceability when concrete is subjected to temperatures much greater than 100°C.

While concrete is not ordinarily considered a high-temperature con- struction material, practical experience has shown that concrete structures have performed adequately in service when subjected to fire or heat from industrial processes. However, some losses in strength and other mechanical properties have resulted. Under very intense or prolonged high temperature exposures, failures of concrete structures have occurred. Various methods have been developed for the design of concrete structures to withstand fire. These methods have been based on laboratory results from short duration fire tests of full-scale methods exist temperatures.

concrete members. Unfortunately, no comparable design for concrete exposed for prolonged periods to elevated

1-1

Because of the lack of a design procedure and the of material behavior, current practice in nuclear as embodied in existing codes (Vol. 1, Ref. 21)*, exposure of concrete in excess of 150 F (200°F in under normal operating conditions.

0

limited knowledge containment design, avoids temperature special areas)

At the same time, it is widely acknowledged that various types of high-temperature exposure of concrete would be permissible, provided one could carry out a sufficiently realistic analysis of the thermal effects, based on measurements of material properties. This is explicitly recognized by existing nuclear code requirements governing concrete temperatures, such as ASME BPV-111-2, subsection CC-3440, (Vol. 1, Ref. 21):

"(c) Higher temperatures than given in (a) and (b) above may be allowed in the concrete if tests are provided to evaluate the reduction in strength and this reduction is applied t o the design allowables. Also, evidence s h a l l be provided which verifies that the increased tempera- tures do not cause deterioration of the concrete either with or without load."

Any advances in this direction are likely to have a high payoff. Considerable economies would be possible in the construction of reactor structures and supports, if properly designed to operate at temperatures over 100 C. Furthermore, the safety assessment of certain nuclear reactor systems could change considerably--for example, if a concrete structure could be relied upon to contain a spill of hot sodium from a sodium-cooled fast breeder reactor. In this sense, further advances in concrete technology could make a major contribution for economical and safe nuclear power.

0

Consideration cannot be limited to normal portland cement concretes. Although refractory and various other special concretes have not been used for large load-bearing structures, they offer very attrac- tive thermal properties that might be successfully exploited. Literature on refractory concretes as well as the high-temperature behavior of normal concrete is quite extensive, but little effort

*Designates reference number given in Volume 1 of the present report.

1-2

h a s b e e n made t o col lect a n d o r g a n i z e t h e s e d a t a i n a c o n c i s e form. Two e x c e p t i o n s a r e t h e b o o k s by Nekrasov (Vol. 1, R e f . 2 3 2 ) , and by P e t z o l d a n d R o h r s , (Vol. 1, R e f . 2 5 3 ) . However, t h e s e t w o v o l u m e s c o v e r h i g h t e m p e r a t u r e c o n c r e t e r e s e a r c h d a t a p u b l i s h e d o n l y b e f o r e

1 9 6 7 .

A c c o r d i n g l y , t h e f i r s t t a s k o f t h i s s t u d y was a r e v i e w o f e x i s t i n g

l i t e r a t u r e o n h i g h - t e m p e r a t u r e b e h a v i o r o f n o r m a l a n d r e f r a c t o r y c o n c r e t e s . T h i s was a c c o m p l i s h e d i n Volume 1 o f t h i s repor t . T h a t volume s e r v e s a s b a c k g r o u n d f o r t h e p r e s e n t s t u d y . O b j e c t i v e s o f t h e p r e s e n t report a re t o compare h i g h - t e m p e r a t u r e p e r f o r m a n c e o f v a r i o u s c o n c r e t e s , e s p e c i a l l y r e f r a c t o r y c o n c r e t e s ; i d e n t i f y t h e i r a d v a n t a g e s and d i s a d v a n t a g e s i n v a r i o u s s t r u c t u r a l f u n c t i o n s and t y p e s o f e x p o s u r e ; a n d d e t e r m i n e p r i n c i p a l g a p s i n e x i s t i n g knowledge w i t h t h e c o n s e q u e n t n e e d s for f u r t h e r r e s e a r c h .

I t m u s t be e m p h a s i z e d t h a t t h e p u r p o s e o f t h i s report is t o select c o n c r e t e ma te r i a l s o f i n t e r e s t a n d i d e n t i f y t h o s e p roper t ies t h a t m a k e t h e s e mater ia ls p o t e n t i a l l y a t t r a c t i v e for v a r i o u s LMFBR app l i - c a t i o n s . I t i s n o t t h e p u r p o s e or i n t e n t o f t h i s report t o m a k e con- c l u s i v e r e c o m m e n d a t i o n s o f ma te r i a l s f o r a n y LMFBR or o t h e r n u c l e a r app 1 i c a t i o n s .

The f i n a l s e l e c t i o n o f mater ia l s f o r a n u c l e a r reactor s t r u c t u r e e x p o s e d t o h i g h t e m p e r a t u r e d e p e n d s o n many f a c t o r s i n c l u d i n g d e s i g n parameters, and t y p e s o f a c c i d e n t s t o be c o n s i d e r e d . Such a selec- t i o n c a n n o t be a c c o m p l i s h e d w i t h o u t e x t e n s i v e d e s i g n c a l c u l a t i o n s based o n appropriate e x p e r i m e n t a l e v i d e n c e . T h i s t y p e o f e x p e r i m e n - t a l a n d d e s i g n a n a l y s i s is beyond t h e scope of t h e p r e s e n t s t u d y .

T h i s report i d e n t i f i e s mater ia l s t h a t show p o t e n t i a l f o r LMFBR

a p p l i c a t i o n s , b a s e d o n a v a i l a b l e p u b l i s h e d i n f o r m a t i o n o n h i g h - t e m p e r a t u r e propert ies b e h a v i o r . The repor t a lso i n d i c a t e s w h e r e a d d i t i o n a l d a t a a re n e e d e d b e f o r e t h e p o t e n t i a l o f t h e s e mater ia l s f o r n u c l e a r a p p l i c a t i o n s c a n b e e v a l u a t e d .

Among t h e numerous known r e f r a c t o r y mater ia l s , t h i s report d i s c u s s e s

o n l y t h o s e wh ich show t h e g r e a t e s t promise f o r LMFBR a p p l i c a t i o n s . A s i d e f rom n o r m a l p o r t l a n d c e m e n t c o n c r e t e s , t h e s e i n c l u d e : (1)

1-3

h i g h - a l u m i n a cemen t c o n c r e t e s w i t h d i f f e r e n t t y p e s o f r e f r a c t o r y

a g g r e g a t e s , s u c h a s chrome ore, t a b u l a r a l u m i n a , corundum, c h a m o t t e ( c r u s h e d f i r e b r i c k ) , and r e f r a c t o r y l i g h t w e i g h t a g g r e g a t e s : ( 2 ) w a t e r g l a s s - b o n d e d c o n c r e t e s w i t h a g g r e g a t e s s u c h as c h a m o t t e or chrome ore: ( 3 ) phospha te -bonded c o n c r e t e s , w i t h a g g r e g a t e s s u c h as corundum, c h a m o t t e , or r e f r a c t o r y l i g h t w e i g h t a g g r e g a t e ; and ( 4 )

m a g n e s i a c o n c r e t e s w i t h chrome ore a g g r e g a t e . Fo r c o m p l e t e n e s s some i n f o r m a t i o n w i l l a l s o be g i v e n o n r e f r a c t o r y b r i c k masonry , wh ich is n o t a c o n c r e t e b u t c a n b e s u b s t i t u t e d f o r it i n some a p p l i c a t i o n s .

A l t h o u g h t h i s w o r k is m o t i v a t e d by t h e n e e d s o f a LMFBR's, i ts c o n c l u s i o n s may b e e q u a l l y v a l i d f o r mater ia l s i n o t h e r reactor s y s t e m s , s u c h as l i g h t

n u c l e a r reactors.

s a f e d e s i g n o f i m p o r t a n t u s e s water c o o l e d

J o f

1 - 4

Section 2

COMPARISON OF VARIOUS MATERIALS

To facilitate comparison of the performance of various available concretes at high-temperature exposure, a list of principal charac- teristic properties that the desigEer needs to know is presented. This information, based on Volume 1 of this report, is compiled in Tables 2.1, 2.2, and 2.3.

The listing of various properties involved in comparing materials is split into three separate tables; one on strength at various tempera- tures (Table 2 .1 ) , one on deformation as related to temperature (Table 2 .2 ) , and one on basic thermal properties and resistance to sodium attack (Table 2 . 3 ) .

In studying these tables, it should be realized that the need for brevity prevents presentation of all qualifying conditions and range limitations. The tables are intended as a first guide. Further literature must be consulted before making a final choice.

2-1

TABLE 2 . 1 - STRENGTH COMPARISON TABLE

Mater i a 1

~~ ~

St reng th ( i n p s i ) Type of T e s t

25 to 3OO0C 300 t o 6OO0C over 6OO0C

R e m a r k s

(1) Compressive 2,800 to 8,000 a t 1,480 t o 5,600 a t N o r m a l Po r t l and Cement Concre t e s wi th N a t u r a l 6,800 a t 300 C 2,800 f o r sili- Aggregates ( s h o r t term) cate aggrega te

s t r e n g t h , hot* 25 C to 1 ,708 to 600 C (700 to

conc re t e )

s h o r t term h o t 1. f ' = 2,000 to 8,000 p s i

abou t 870 C 2. S t r e n g t h a t e l e v a t e d

s t r e n g t h >25% f; C

temper a t ur e i nc r ease s i f s ta t ic l o a d p r e s e n t du r i ng hea ti ng

Compressive 2,000 to 8,000 0.65 f ' af ter L i t t l e or no r e s i d u a l s t r e n g t h 3. S t r e n g t h losses g r e a t e r

for long term hea t ing s t r e n g t h , for no preheat to prehea: to 300°c cooled * * 1,300 to 5,200 to 0.40 f ' for af ter preheat

f o r m e h e a t to C 3 0 O o c prehea t t o 6OO0C

(0.3 f ' for si l i- c a t e aggrega te conc re t e )

C

above 870°C 4. U s e of ceramic s t a b i l -

Modulus of T e n s i l e and Rupture, ho t* f l e x u r a l s t r e n g t h

losses comparable to compressive s t r e n g t h losses (losses s l i g h t l y mre seve re )

(2) Po r t l and Cement + Compressive 4,700 a t 23O$ to 3,250 a t 6OO0C 1,400 a t l lOO°C Chamotte s t r e n g th , 3,800 a t 300 C

cooled**

i z e r s reduce s t r e n g t h loss a t e l e v a t e d tem- p e r a t u r e

c

*Hot. P r o p e r t i e s de te rmined wh i l e c o n c r e t e was main ta ined a t i n d i c a t e d tempera ture . **Cooled. Concre te i n i t i a l l y pre-heated t o i n d i c a t e tempera ture , t hen cooled. P r o p e r t i e s determined a t

ambient tempera ture .

TABLE 2 . 1 - STRENGTH COMPARISON TABLE (Continued)

N

S t r e n g t h ( i n p s i ) Material Type of T e s t R e m a r k s

25 to 3OO0C 300 to 6OO0C over 6OO0C

(3) Cmpr ess ive 6,400 to 4,250 4,250 to 3,800 7 ,000 a t 120OoC High Alumina Cement + s t r e n g t h , coo led Chrome O r e based on

Nekrasov et a1

M i t u s c h (20% 4,800 to 3,550 3,550 to 2,850 2,400 a t 1200°C alumina cement), cooled

Modulus of N o d a t a R u p t u r e , h o t

700 a t 48OoC 1 , 3 0 0 a t 900°C

I ( 4 ) Cmpr essive 7,000 80 12,000 7,000 &o 12,000 7,080 to 12 ,000 High s t r e n g t h

High Alumina Cement + s t r e n g t h , a t 110 C a t 538 C 816 C T a b u l a r Alumina cooled

Modulus of 3,000 R u p t u r e , h o t

I f pre-g ir e d 3,800 to 1700 C

2 ,100 a t 48OoC 1 ,800 a t 900°C

3,800 a t 48OoC 3,000 a t 900°C

(5) Cmpressive 2,650 a t 3OO0C 2 ,130 a t 6OO0C 2 ,800 a t 120OOC High Alumina Cement + s t r e n g t h , Corundum cooled

TABLE 2 . 1 - STRENGTH COMPARISON TABLE (Continued)

S t r e n g t h ( i n ps i ) Material Type of T e s t Remarks

25 to 3OO0C 300 t o 6OO0C o v e r 6OO0C

(6) Compress ive High Alumina Cement + s t r e n g t h , h o t Chamot te (based on 1,000 to 1 ,800 1,000 t o 2,100 800 to 1,100

Nekrasov , Babcock and Wilcox) 4,200

Compr es si ve s t r e n g t h , c o o l e d (based on Nekrasov , 3,550 t o 3,600 Mi tusch , 1 ,400 a t 3OO0C Rudolph , N o d a t a Babcock and Wilcox)

Modulus of R u p t u r e , h o t (based on N o d a t a

N e k r a s w , Babcock a n d Wilcox)

3,800

a t ~ O O O ~ C

5,000

2,250 t o 3,8!0 2,500 to 3,70! 1 ,400 a t 600 C 4 ,200 a t 1400 C N o d a t a 2,100 tg 4,000

a t 1200 C

1 ,050 a t 48OoC 800 a t 900°C

( 7 ) Compr es s i v e For i n s u l a t i o n High Alumina Cement s t r e n g t h , + Refractory L i g h t w e i g h t c o o l e d A g g r e g a t e s d e n s i t y ~ 2,000 a t 25OC 1 , 9 0 0 a t 3OO0C 1 , 4 0 0 a t 6OO0C

1400 g/cm’

3 d e n s i t y 1200 g/cm

1,400 a t 25OC 840 a t 3OO0C 560 a t 6OO0C

TABLE 2 . 1 - STRENGTH COMPARISON TABLE ( C o n t i n u e d )

S t r e n g t h ( i n psi) Material Type of T e s t

25 to 3OO0C 300 to 6OO0C Over 6OO0C

Remarks

(8 1 Compressive 2,400 to 3,700 3,400 to 3,550 2,700 a t 120OoC P r e h e a t i n g p r o c e s s is Waterg l as s-Chamo t t e s t r e n g t h , h o t good for hot and c o l d Concre t e I f p rehga ted compressive s t r e n g t h ,

to 1000 c

C m p r e s s i v e 700 to 2,560 2,700 a t 6OO0C 2,840 ( a t 7 0 0 3 ) w i t h p r e h e a t t empera tu re s t r e n g t h , 5,500 ( a t 1000oC cooled 6,700 ( a t 1200 C

i.e., s t r e n g t h improves

(9 ) Compressive 2,300 to 2,600 2,800 to 3,000 2,300 too3,000

Waterglass-Chrome Ore s t r e n g t h , hot a t 1,200 C

Modulus of N o d a t a N o d a t a 800 a t 6 5 0 3 Rupture , hot 1,000 a t 80OOC

250 a t 1100 C

(10) Compressive 2,700 to 4,800 6,700 a t 6OO0C 8 , 0 0 0 a t 1,200°C 1. Low i n i t i a l s t r e n g t h Phosphate-bound Corundum s t r e n g t h , ho t 140 to 700 ps i

based on Babcock and 2. Very h i g h s t r e n g t h Wilcox (90% af te r h e a t i n g up A1203)

(11) Compressive 100 a t 2 3 3 1,700 a t 6OO0C 1,600 a t 135OoC Phosphate-bound Chamotte s t r e n g t h , 1 ,200 a t 300 C

cooled

(12) N o d a t a Phosphate-bound Light - weight Corundum

For i n s u l a t i o n - not s t r u c t u r a l material

hl I cn

TABLE 2 . 1 - STRENGTH COMPARISON TABLE ( C o n t i n u e d )

S t r e n g t h ( i n p s i ) Mater i a1 Type o f T e s t R e m a r k s

25 to 3OO0C 300 to 6OO0C over 600°C

(13) C m p r e s s i v e 1,700 to 3,600 1,200 t o 1,700 400 too 1 , 2 0 0 Low s t r e n g t h a& tempera-o Magnesia-bound Chrome s t r e n g t h , a t 1200 C t u r e range 300 C to 1200 C Or e cooled 3,550 t g 4,000

a t 1400 C

(14) compressive R e f r a c t o r y B r i c k s t r e n g t h ,

Dense HA B r i c k cooled (Tabular alumina -+ high alumina cement) Modulus o f

Rupture , cooled

Magnesia B r i c k

Z i r con ia B r i c k

Modulus o f Rupture , h o t

(Har b i son- Walker)

Foesse l and Tref f n e r (Phosphate- bonded 1

Compressive s t r e n g t h , cooled

Modulus o f Rupture

N o d a t a N o d a t a 9 ,000 to 16,000 hard f i r e d by manufacturer

2,500 to 3,500 N o d a t a N o d a t a hard f i r e d

1 ,000 to 2,000 a t N o d a t a 23OoC

500 t o 1 , 0 0 0 up to 150OOC

2,300 a t 3OO0C 1,000 a t 6OO0C 2,600 a t l,OOO°C

7,000 to 11 ,000 N o d a t a a t 23OC

N o d a t a

2,300 t o 3,300 a t N o d a t a N o d a t a 23OC

Graph i t e B r i c k N o d a t a

TABLE 2 . 1 - STRENGTH COMPARISON TABLE (Cont inued)

Strength ( i n p s i ) Material Type of Test Remar k s

25 to 3OO0C 300 to 6OO0C over 6OO0C

Dense Superduty Compressive 2 ,000 a t 4 ,000 a t N o data 7,000 to 11 ,000 F i r e c l a y Brick s t r eng th , 23OC a t ~ O O O ~ C

cooled

Modulus of 700 to 1,500 N o data Rupture ( for normal)

2,700 to 3,700 (for pref ired)

No data

TABLE 2 . 2 - COMPARISON OF DEFORMATION DATA

Material Remarks Deformat ion C a p a c i t y Thermal Expans ion

(1) Ta* = 38OoC 5OO0C 55 to 1 4 0 x I O - ~ P C N o r m a l P o r t l a n d Cement (for l i m e s t o n e c o n c r e t e ) for normal concrete C o n c r e t e s w i t h N a t u r a l Aggrega te s

-7 0 80 to 180 X 10 / C for l i m e s t o n e c o n c r e t e

(2) Ta = 148OOC 75 to 90 x I O - ~ P C High Alumina Cement + Chrome Ore

Chromi te a g g r e g a t e -1 0 60 X 1 0 / C

f o r Chrome Ore w i t h p o r t l a n d cement

(3) Ta = 140OoC High Alumina Cement + T a b u l a r Alumina

N o d a t a

-7 0 (4 ) Ta = 155OoC 25 to 50 X / C High d e f o r m a t i o n c a p a c i t y , High Alumina Cement + l o w t h e r m a l c o e f f i c i e n t of Corundum e x p a n s i o n

(5 ) Ta = 135OoC 50 to 80 X 1 0 / C High Alumina Cement + Chamot te

(6) Ta = 104OoC Depend on a g g r e g a t e s P o o r e r t h a n t h e normal High Alumina Cement + (foamed a lumina cement) we igh t r e f r a c t o r y c o n c r e t e

-7 0

Ta = llOO°C ( s m e l t e d a lumina cement )

Refract or y L i g h t w e i g h t A g g r e g a t e s

*Ta = d e f i n e s t h e t e m p e r a t u r e a t which t h e l o a d e d t e s t piece is compressed by 0.6% from t h e p o i n t of maximum expans ion .

TABLE 2.2 - COMPARISON OF DEFORMATION DATA (Con t inued)

Material R e m a r k s Deformation C a p a c i t y Thermal Expans ion

(7) Ta = 104OOC 60 to 80 X 10-7/C Waterg lass -Chamo t te Shr inkage 0.5% Concrete up to 120oOc

(8) Ta = llOO°C 90 x ~ o - ~ / o c Wat e r g l a s s - C h r ome O r e Concrete

(9 1 Ta = 170OOC 60 80 X 1 0 / C Phosphate-bound Shr inkage 0.4% Corundum up to 1500°C

-7 0

(10) Ta = l3OO0C 60 165 X With v e r y small s h r i n k a g e Pho sph a t e-bound a t h i g h t e m p e r a t u r e up to A l u m i n a p h o s p h a t e

depends on bond ing) Chamotte c a s t a b l e ( v a r i e t y 120oOc

(11) Can be s u b j e c t e d to N o d a t a Phospha te-bound L i g h t w e i g h t Corundum Ta = 1300 C

(12) Ta = 148OoC 130 X 10 / C Magnes ia-bound Chrome Ore

temperatuke up t o 150OOC

-7 0

(13) N o d a t a R e f r a c t o r y B r i c k

from 5 x I O - ~ ~ C for z i r c o n i a b r i c k to

15 X 10-7/C for magnesia b r i c k

TABLE 2.3 - COMPARISCN OF INSULATION AND SODIUM RESISTANCE

Thermal P r o p e r t ies

Material ~~~ ~

Sodium Resistance Thermal C o n d u c t i v i t y S p e c i f i c Heat 0 0 cal/cm-sec- C cal/g- C

Remarks

N I P 0

~~

(1 1 Normal w e i g h t 0.18 to 0.28 Not good U s e of s u r f a c e t r e a t m e n t s Normal P o r t l a n d Cement 0.003 0.0060at 25OC 25OC to 1000°C can increase t h e sodium C o n c r e t e s w i t h N a t u r a l 0.003 a t 1000 C Basal t c o n c r e t e resistance A g g r e g a t e s L i g h t w e i g h t c o n c r e t e Cp = 0.22 0.40

0.001 Basalt c o n c r e t e 0.0046 0.0017

(2 1 P o r t l a n d + Chrome O r e 0.22 High Cr203, S i 0 2 High Alumina Cement + (unf i r e d ) and FeO con ten t Chrome O r e 0.0015 0.0021 may react w i t h

Alumina + Chrome O r e sodium 0.0028

(3) 0.12 to 0.17 0.08 0.10 Good High Alumina Cement + T a b u l a r Alumina

(unf i r e d ) High c o n d u c t i v i t y due to h i g h d e n s i t y , 170 pcf

( 4 ) 0.0055 a t 25OC 0.31

0.005 a t 1000°C High Alumina Cement + Corundum

Good High Cp

(5) 0.002 High Alumina Cement + Chamot te

0.20 to 0.25 High S i 0 2 c o n t e n t

may react w i t h sod ium

TABLE 2.3 - CUMPARISON OF INSULATION AND SODIUM RESISTANCE (Continued)

Mate r i a l

Thermal P r o p e r t i e s

Sodium Resis tance Thermal Conductivity S p e c i f i c Heat 0 cal/g- 0 C cal/cm-sec- C

R e m a r k s

(6) I n s u l a t i n g chamotte N o d a t a 1. Depends on High Alumina Cement + 0.001 0.0005 aggregate chemistry Refrac tory Lightweight I n s u l a t i n g alumina Aggregates cement concrete 2. High p o r o s i t y

0.0003 0.0004 e a s i l y l ead to f o r d e n s i t y 0.2 g/cm chemical a t t a c k 0.0016 f o r dens i ty

3 1.6 g/cm

( 7 ) N o d a t a Waterglass-Chamotte Concrete

N o d a t a High Si02 con ten t

may react with sodium

(8) 0.0034 Waterglass-Chrome Ore Concrete

N o da t a C r 2 0 3 and FeO

con ten t may react with sodium

(9 ) N o d a t a N o d a t a N o d a t a , but Po ros i ty is high Phosphate-bound widely used i n g e n e r a l l y w i l l n o t resist Corundum s p e c i a l p l a n t to Na pene t r a t ion , but no

da ta on p o s s i b l e chemical resist chemical a t tack i n t e r a c t i o n

(10) N o d a t a N o d a t a N o da t a , but Po ros i ty is high Phosphate-bound widely used i n g e n e r a l l y w i l l n o t resist Chamot te special p l a n t to N a pene t r a t ion , bu t no

d a t a on p o s s i b l e chemical resist chemical a t t a c k i n t e r a c t ion

TABLE 2.3 - COMPARISON OF INSULATION AND SODIUM RESISTANCE (Continued)

Thermal P r o p e r t i e s

Mater ia l Sodium Resis tance Thermal Conduct ivi ty S p e c i f i c Heat

R e m a r k s

cal/cm-sec- 0 C c a l/g-OC

N o d a t a , b u t Po ros i ty is high widely used i n gene ra l ly w i l l no t r e s i s t

r e s i s t chemical d a t a on poss ib l e chemical a t t ack i n t e r a c t ion

(10) N o d a t a N o d a t a Phos ph a te- bound Lightweight Corundum s p e c i a l p l a n t to N a pene t r a t ion , bu t no

Cr203 and FeO may (12) 0.004 N o d a t a

Magnesia-bound Chrome r e a c t with sodium O r e

(13) F i r ec l ay br ick N o d a t a Exce l len t : Ref rac tory B r i c k 0.0033 to 0.0052 High dens i ty h igh

alumina b r i ck 50 C o t o 1500°C

High alumina b r i ck Zi rconia b r i ck Good :

0.0027 0.004

5OoC to 150OoC Quest ionable: Carbon br ick

(graph i te) Superduty f i r e - c l a y b r i ck

Section 3

ADVANTAGES, DISADVANTAGES, AND SELECTION OF MATERIALS OF INTEREST

Now that the essential properties relevant to a comparison of various materials available have been listed, the principal advantages and disadvantages of which the designer must be aware in making a selec- tion of materials for high-temperature exposure may be identified. Again, it should be realized that statements of advantages and dis- advantages are simplified and do not permit distinguishing small differences in performance, ranges of temperature, and other parameters.

3.1 LISTING OF ADVANTAGES AND DISADVANTAGES OF VARIOUS MATERIALS OF INTEREST

(1) Normal Portland Cement Concretes with Natural Aggregates Advantages

No preheating or prefiring is needed to develop adequate strength; needs only moist curing conditions to produce max imum strength . Conventional concrete has a relatively high compressive strength at ambient temperatures. The material retains a significant portion of its compres- sive strength on heating up to 450°F. for prolonged heating periods. For basalt aggregate con- cretes, the residual strength after heating more than 2 years at 450°F is about 65% of that for unheated con- crete. The same behavior is true of the tensile strength. The elastic modulus loss is somewhat greater. The material has a proven stability of microstructure and of dimensions at ambient and moderate elevated tempera- tures. This information is obtained from performance of gas-cooled nuclear reactor vessels and waste storage tanks. Thermal properties up to about 500 to 7OO0C are acceptable if the aggregate is properly selected. Preferable is

This is true even

3-1

basalt or limestone, but even siliceous aggregate concretes perform relatively well below 54OoC. The material has relatively low conductivity, making it a good (but not superior) insulating material. Relatively extensive information on thermal properties and performance is already available in the literature. How- ever, certain conspicuous gaps exist, particularly €or the case of prolonged heat exposure. Design and construction technology are well developed, although €or the case of high temperature applications they may be much too conservative.

Disadvantages The material liberates much water on heating, particularly at temperatures over 50OoC. Creep increases significantly as a result of heating and is much larger than for certain other refractory materials. Related to this is a significant drop of the apparent short-time elastic modulus. At temperatures under 5OO0C the loss of strength is relatively minor. However, it becomes significant at higher temperatures. At 76OoC the strength is near zero. The loss of water as well as significant thermal deforma- tion and thermal shrinkage may lead to significant cracking at temperatures above 100°C, depending on the particular design and reinforcement. Material is susceptible to explosive spalling from rapid heating in moist condition, particularly under conditions of restraint. However, counter-measures in design seem possible. If unprotected, the material is quite susceptible to sodium attack. The performance depends on many conditions including surface exposure conditions, reaction product transport, surface sodium convection, reinforcement, restraint, and dimensions.

(2 ) High-Alumina Cement (HAC) Concretes Advantages (a) Like portland cement concretes, these concretes harden and

gain strength at ambient temperature. No heat-treatment is required to develop strength.

3-2

Compressive as well as tensile strength of this material is relatively good and comparable to that of portland cement concrete. If pre-f ired above 1000°C, this material retains signifi- cant strength at much higher temperatures than portland cement concretes. This is due to the formation of ceramic bonds at firing temperatures above 1000°C. After firing, thermal deformations are reversible and easily predictable. That is, after firing, no further permanent length changes occur with subsequent heating and cooling . After firing, aluminous cement concretes using high-quality refractory aggregates, such as chamotte and chrome ore exhibit smaller creep deformations at elevated temperatures than portland cement concretes.

Disadvantages

(a) To obtain maximum strength at temperatures above 6OO0C where portland cement concrete cannot serve, and to achieve reversibility of deformations, the material must be fired at temperatures over 1000 C. While this is no problem for small blocks or thin linings used in chemical processing technology vessels, the need for firing presents formidable difficulties when massive nuclear reactor structures are considered.

0

(b) High alumina cement concretes are susceptible to the well- known conversion reaction (Vol. 1, Ref. 2 4 7 ) . This can lead to loss of strength at temperatures slightly above ambient. The probability of conversion occurring can be reduced by controlling temperatures during curing. Conver- sion reactions occur when hydrated aluminous cement con- crete is heated. These reactions lead to some loss of concrete strength. However, with care in mix design and curing, residual strengths may be adequate for structural applications at elevated temperatures. Firing concrete at 1000°C can eliminate the conversion problem, since fired concrete has high strength due to permanent ceramic bonds, not to formation of hydration products.

3-3

Without firing, these refractory materials are not as chemically stable as portland cement concretes at lower temperatures. High-alumina cements develop considerably higher hydration heat than do portland cements. This causes large thermal expansions, particularly in more massive concrete walls in which the hydration heat produces a large temperature rise. This presents difficulties in the design of these structures against cracking. However, even in a massive structure the deleterious effects of hydration heat can be eliminated by cooling through pipes embedded in the concrete. Drying shrinkage of high alumina cement concretes on heating is generally larger than for portland cement concretes. This makes the structure more susceptible to cracking, due to the higher water requirements for curing. Compared to portland cement concretes, mechanical behavior of these materials is generally less well understood, owing to their lack of use in structural load-bearing applications. The technology of design and construction is less developed than that for portland cement concretes. Overall, for moderately high temperatures (up to 400OC) high alumina cement concretes appear to afford no signifi- cant advantages over portland cement concretes for struc- tural applications. Their main advantages are realized at temperatures above 600 C, where normal portland cement concrete has little or no strength.

0

A more detailed discussion of high alumina cement concretes with different types of aggregate follows.

(2a) High Alumina Cement Concrete with Chrome Ore Aggregate Advantages (a) This aggregate gives the material very good creep

resistance. Creep deformations are quite low com- pared to normal portland cement concrete. The same is true of shrinkage. Even 15 to 20% of finely ground chrome ore added to the mix significantly reduces shrinkage.

(b) The use of chrome ore also increases strength.

3-4

(c) Thermal conductivity is low compared to concrete made with other normal weight aggregates. The material has good insulating properties.

Disadvantages (a) Chromite, the main constituent of chrome ore, is sen-

sitive to oxidation. This leads to the formation of Fe203 from FeO and results in a destruction of refractory spinel structure. Control of the atmos- phere of the firing furnace is therefore very important .

(b) Tests on refractories incorporating chrome ore aggregates have shown strong reactivity to molten sodium, even when chrome ore is present only as a minor constituent (10 to 15% of total aggregate volume).

(2b) Hiqh Alumina Cement Concrete with Tabular Alumina Aggreqate (99%+ A1203) Advan taqes (a) If prefired at 17OO0C, the material exhibits high

strength (modulus of rupture) at temperatures up to 15OOOC.

(b) In the case of high purity tabular A1203, the aggregates show excellent resistance to molten sodium attack.

temperatures of 18OOOC. ( c ) Highly refractory material withstands service

Disadvantages (a) Firing to 1700 C is difficult and expensive. (b)

0

Knowledge of the mechanical properties of this mate- rial is quite limited. No data seem to exist for hot compressive strengths. Concrete exhibits high tbermal conductivity and creep at elevated temperatures, compared to other refractory concretes.

(c)

(d) Cost of high purity tabular alumina aggregate is very high.

(2c) Hiqh Alumina Cement Concrete with Corundum Aggregate (50 to 90% A1203)

3-5

A d v a n t a g e s ( a ) C o e f f i c i e n t o f t h e r m a l e x p a n s i o n is l o w ( 6 0 t o 80 x

( b ) M e l t i n g p o i n t i s h i g h ( d 8 0 O O C ) . (c ) S t r e n g t h is h i g h (2800 p s i i n c o m p r e s s i o n a t

-7 0 10 / C ) .

12OOOC).

( d ) C h e m i c a l r e s i s t a n c e is g e n e r a l l y good . ( e ) U s e o f g r o u n d corundum t e n d s t o r e d u c e d r y i n g s h r i n k -

a g e o f h e a t e d c o n c r e t e .

I f A 1 2 0 3 is o f h i g h p u r i t y , r e s i s t a n c e t o sod ium

a t t a c k is g o o d .

( f ) Heat c a p a c i t y is h i g h .

(9)

D i s a d v a n t a g e s ( a ) T h e r m a l c o n d u c t i v i t y is r e l a t i v e l y h i g h . ( b ) C o s t of corundum a g g r e g a t e i s h i g h .

( 2 d ) High Alumina Cement C o n c r e t e w i t h C h a m o t t e A g g r e g a t e Ad v a n t a g e s ( C h a m o t t e is t h e h i g h e s t q u a l i t y , v e r y d e n s e , c r u s h e d f i r e b r i c k .) ( a ) S i n c e t h i s i s t h e most commonly u s e d r e f r a c t o r y

c o n c r e t e , a l a r g e body o f e x p e r i e n c e is a v a i l a b l e . ( b ) High c o m p r e s s i v e s t r e n g t h c a n be a c h i e v e d t h r o u g h

(c) T h e r m a l c o n d u c t i v i t y is l o w , making t h e ma te r i a l a proper f i r i n g .

good i n s u l a t o r . ( d ) R e s i s t a n c e t o wear damage is g o o d , a l t h o u g h t h i s is

o f n o a d v a n t a g e f o r n u c l e a r reactor s t r u c t u r e s . e

D i s a d v a n t a g e s

( a ) High c o n t e n t o f S i02 (50 to 8 5 % ) m a k e s t h e ma te r i a l v e r y s u s c e p t i b l e to r e a c t i o n w i t h m o l t e n sod ium.

( 2 e ) High Alumina Cement C o n c r e t e w i t h R e f r a c t o r y L i g h t w e i g h t

Agq r eg a t e L i g h t w e i g h t r e f r a c t o r y a g g r e g a t e s i n c l u d e v e r m i c u l i t e ,

p e a r l i t e , c r u s h e d i n s u l a t i n g f i r e b r i c k , and b u b b l e d a l u m i n a . A d v a n t a g e s ( a ) The u s e o f l i g h t w e i g h t a g g r e g a t e s p r o d u c e s i n s u l a t i n g

*

r e f r a c t o r y concretes w i t h t h e r m a l c o n d u c t i v i t i e s 1/3 t o 1 / 1 0 t h a t o f c o m p a r a b l e n o r m a l w e i g h t r e f r a c t o r y c o n c r e t e s .

3 - 6

Disadvantages (a) The use of lightweight aggregate produces concretes

with relatively low compressive strength. (b) Volume stability of the material is lower than for

normal refractory concretes. (c) High porosity of the lightweight aggregate facilitates

molten sodium penetration and chemical attack.

Several types of non-hydraulic binders, in particular the phosphates, magnesia, and waterglass binders will now be considered. These do not chemically combine water in order to harden, although they need water in the mix.

(3) Phosphate-Bonded Concretes (3a) Phosphate-Bound Corundum

Advantages (a) Very good thermal shock resistance. (b) Very high operating temperature limit can be reached,

about 1800OC.

(8000 psi at 6OOOC).

160OOC it is almost zero.

(c) Retains compressive strength at high temperatures

(d) Apparent porosity is very low; after firing at

(e) Shrinkage produced by firing temperature is almost negligible, being generally under 0.4%.

(f) The material can exist in various forms, such as cold-setting concrete, sprayed concrete, and heat- setting tamped concrete.

(9) Abrasion resistance is excellent (40 times higher than normal fire brick).

(h) The material is particularly suited for repair patches particularly in furnace liners or coal gasification reactors.

(i) Resistance to various forms of chemical attack is generally excellent (in particular, high resistance to chloric gases at high temperatures).

Disadvantaqes (a) The initial compressive strength is low when

air-dried (140 to 700 psi at ambient temperature).

3-7

T h e s e ma te r i a l s m u s t be p r e h e a t e d t o 35OoC t o

d e v e l o p a d e q u a t e m e c h a n i c a l s t r e n g t h . S t r e n g t h a c h i e v e d is q u i t e s e n s i t i v e t o t h e h e a t i n g process and mix p r o p o r t i o n s . G a s e o u s e x p a n s i o n c a n sometimes o c c u r d u r i n g t h e d r y i n g or h e a t t r e a t i n g process ( t h i s is a t t r i b u t e d

t o t h e d e v e l o p m e n t o f h y d r o g e n formed by a r e a c t i o n b e t w e e n t h e r e s i d u e s o f metals and a c i d s , and c a n b e c o u n t e r a c t e d by a d d i n g i n h i b i t o r s ) . The h o t t e n s i l e s t r e n g t h (modulus o f r u p t u r e ) d r o p s a p p r e c i a b l y a t t e m p e r a t u r e s o v e r 1000°C. The volume s t a b i l i t y a t temperatures o v e r 120OOC is

poor. Phosphate-Bound Chamo t t e Ad v a n t a g e s (a) This mate r i a l h a s a p a r t i c u l a r l y low d r y i n g s h r i n k a g e

o n h e a t i n g .

D i s a d v a n t a g e s ( a ) The i n i t i a l s t r e n g t h is v e r y l o w , u n l e s s h e a t e d t o

35OoC. ( b ) G a s e o u s e x p a n s i o n is p o s s i b l e .

(c) The h i g h c o n t e n t o f S i O z i n c h a m o t t e a g g r e g a t e i s

l i k e l y t o promote m o l t e n sodium a t t a c k . Phosphate-Bound L i g h t w e i q h t Corundum A d v a n t a g e s ( a ) T h i s i s a v e r y good i n s u l a t i n g ma te r i a l , as a re a l l

l i g h t w e i g h t a g g r e g a t e c o n c r e t e s .

D i s a d v a n t a g e s (a) The c o m p r e s s i v e s t r e n g t h i s g e n e r a l l y l o w , e v e n a f t e r

h e a t i n g or f i r i n g . ( b ) The h i g h p o r o s i t y makes t h e ma te r i a l s u s c e p t i b l e t o

c h e m i c a l a t t a c k by m o l t e n sodium. ( 4 ) Magnesia-Bound Chrome O r e C o n c r e t e

A d v a n t a g e s ( a ) T h i s m a t e r i a l h a s a v e r y good c o m p r e s s i v e s t r e n g t h a t

t e m p e r a t u r e s o v e r 14OOOC.

D i s a d v a n t a g e s ( a ) R e q u i r e s f i r i n g t o t e m p e r a t u r e s o f 1 2 0 0 C to d e v e l o p 0

u s a b l e s t r e n g t h f o r s t r u c t u r a l a p p l i c a t i o n s .

3-8

(b) Initial strength is very low. (c) Heat conductivity and thermal expansion are relatively

high. The high content of Cr203 may be expected to promote molten sodium attack.

(d)

(e) Creep and shrinkage are generally higher. ( 5 ) Waterglass Concretes

The term "waterglass" denotes an aqueous solution or colloidal dispersion of various hydrolized sodium or potassium silicates. vary in practice. Advantages (a) Waterglass is a non-hydraulic binding agent. It does not

The composition and ratio of Si02 to Na20 may

chemically combine water as it hardens. However, water must be present in the mix.

(b) The material hardens and gains strength at relatively low temperatures (100OC) , at which point it loses the mix water, provided that the layer is sufficiently thin to allow migration of the water to the outside. The absence of water in dried concrete ought to be advantageous in case of sodium attack. A l s o , waterglass undergoes benefi- cial bonding reactions with a wide variety of refractory aggregates that lead to retention of strength at tempera- tures greater than 1000 C .

0

(c) Experience with waterglass-bonded concretes in refractory applications indicates that these materials are resistant to sodium vapor and its oxides.

Disadvantages (a) Waterglass concretes have no appreciable strengths unless

heated to above 100°C. heated/dried concrete is not very high when compared to other concrete systems discussed in this report.

space. This suggests heated concrete may be too permeable for sodium containment applications. However, Petzold and Rohrs cite one study where waterglass concretes were pro- duced with low permeability in both unfired and fired states. (Vol. 1, Ref. 253, p. 174).

The compressive strength of

(b) Escape of water during heating leaves a significant pore

3-9

(c) The large porosity and low strength make the performance of the material under sodium attack quite suspect in spite of the probable unreactivity of sodium silicate with sodium. However, hard data are lacking.

(5a) Waterglass - Chamotte Concrete Advantages (a) In addition to general advantages listed previously,

waterglass-chamotte undergoes relatively small drying shrinkage on heating (approximately 0 .5%) .

Disadvantages (a) High silica content of chamotte aggregate is suscep-

tible to attack by sodium. (5b) Waterglass - Chrome Ore Concrete

Advantages (a) Deformation capacity of this concrete is greater than

that of other waterglass-bound refractory concretes. Disadvantages (a) Chrome ore aggregate is susceptible to attack by

sodium. Because of their high porosity ranging from 20 to 25% if unfired, waterglass concretes as well as other refractory concretes do not seem to be ideal for the containment of molten sodium. However, in view of the chemical nature of waterglass, (sodium silicate) it is probable that this binder would be unreactive with molten sodium and its oxides. It seems likely that sodium-resistant waterglass-bound concretes are feasible if sodium-resistant aggregates such as corundum, tabular alumina, and periclase are used. However, no data on waterglass concretes using these resistant aggregates were found in the litera- ture. Additional work on the sodium resistance of waterglass-bound concretes is desirable.

(6) Refractory Brick Linings (Masonry) Although not a concrete, these materials may serve similar functions. Advantages (a) The compressive strength of the lining is almost as high

as the compressive strength of the bricks.

3-10

(b) Refractory bricks can withstand very high temperatures, depending on type of brick. For example, crushing strength of superduty fireclay brick at 126OoC is essentially the same as at room temperature.

(c) The only significant porosity occurs at joints between the bricks. If the width of joints is kept very small, overall porosity of the lining is also small.

(d) If joints are thin, overall thermal expansion coefficient

(e) Shrinkage of bricks on heating is negligible since they of the lining is close to that of the bricks.

are prefired during manufacture. (f) Refractory bricks are in many cases highly resistant to

sodium. They are used as linings in sodium production vesse 1s.

0 (9) Since bricks are fired to over 1000 C during manufacture, they contain no water. No hydrogen is generated by con- tact with sodium.

Disadvantages (a) The tensile strength of brick linings is generally very

low. (b) Joints between bricks constitute a weak part of the system

from the load-carrying ability view-point. As an alterna- tive, dry joints could be contemplated for extremely well fitting bricks or blocks, but dry joints would still represent an open pore space.

(c) If a wet mortar is used in the joints, then water will be released from the joints upon heating. If preheated, the escape of water creates pore space in the joints. Although the bricks may be resistant to the sodium, the pore space in joints caused by loss of water from the binding agent used to mortar the joints can make the lining susceptible to sodium attack.

The fact that bricks can be manufactured to essentially zero open porosity seems favorable with regard to the penetration of molten sodium. The chemical resistance to sodium attack is more difficult to assess. Sodium resistance of bricks that contain large quantities of silica (such as fireclay brick) or chrome ore may be questionable. A recent study showed that materials containing silica or chromite

3-11

are n o t c o m p l e t e l y c h e m i c a l l y s t a b l e i n t h e p r e s e n c e o f m o l t e n s o d i u m ( V o l . 1, R e f . 1 0 ) .

One repor t from a r e f r a c t o r i e s m a n u f a c t u r e r i n d i c a t e s t h a t a d e n s e

f i r e c l a y b r i c k had been u s e d s u c c e s s f u l l y for y e a r s a s a l i n e r i n a v e s s e l f o r p r o d u c i n g sodium e l e c t r o l y t i c a l l y a t t e m p e r a t u r e s up t o 1200°F from m o l t e n sodium c h l o r i d e ( V o l . 1, R e f . 5 7 ) . On t h i s

b a s i s , it appears t h a t h e a v y d u t y h a r d - b u r n e d f i r e c l a y b r i c k s h o u l d s a t i s f a c t o r i l y r e s i s t a sodium s p i l l o f l i m i t e d d u r a t i o n .

Based o n r e s u l t s of t h e SRI s t u d y ( V o l . 1, R e f . 1 0 ) as w e l l a s s t u d i e s by F i n k e t a 1 ( V o l . 1, R e f . 1 1 2 , 113) t h e f o l l o w i n g r e f r a c - t o r y b r i c k s seem t o b e b e s t f o r sodium c o n t a i n m e n t : d e n s e h i g h a l u m i n a b r i c k ( p o r o s i t y u n d e r 2 % ) , m a g n e s i a b r i c k ( p o r o s i t y a b o u t

lo%), and z i r c o n i a b r i c k ( p o r o s i t y 2 8 % ) . T h e s e b r i c k s showed no e v i d e n t d e t e r i o r a t i o n o n h e a t i n g i n m o l t e n sodium a t 85OoC f o r f i v e h o u r s .

O t h e r b r i c k s , which h a v e shown mixed r e s u l t s , some b r i c k s b e i n g a t t a c k e d o t h e r s n o t , i n c l u d e d : v i t r e o u s c a r b o n or g r a p h i t e b r i c k

( p o r o s i t y u n d e r 1%), and d e n s e s u p e r d u t y f i r e c l a y b r i c k ( p o r o s i t y u n d e r 4 % ) .

3.2 IMPROVEMENT OF PERFORMANCE

A s a l r e a d y d i s c u s s e d , some o f t h e d i s a d v a n t a g e s o f c a s t a b l e mater ia l s may be m i t i g a t e d by special m e a s u r e s , which a re d e s c r i b e d i n t h e

f o l l o w i n g s e c t i o n .

R e f r a c t o r y A g g r e g a t e s f o r P o r t l a n d Cement C o n c r e t e

One a rea t h a t d e s e r v e s d e e p e r s t u d y w i t h r e g a r d t o n u c l e a r a p p l i c a -

t i o n s , i s t h e improvement of h e a t - r e s i s t a n t properties of p o r t l a n d c e m e n t c o n c r e t e s . T h i s c a n b e a c c o m p l i s h e d by t h e u s e o f r e f r a c t o r y a g g r e g a t e s and f i n e l y g r o u n d ceramic f i l l e r s , s u c h as c r u s h e d h a r d -

b u r n t f i r e b r i c k , c h a m o t t e , u s e o f b l a s t f u r n a c e s l a g and s l a g p o r t l a n d c e m e n t s , and r e f r a c t o r y l i g h t w e i g h t a g g r e g a t e s .

V a r i o u s s u c c e s s f u l a p p l i c a t i o n s o f p o r t l a n d cement c o n c r e t e s w i t h

s u c h r e f r a c t o r y a g g r e g a t e s h a v e been r e p o r t e d ( V o l . 1, R e f . 253) up

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t o t e m p e r a t u r e s o f llOO°C, however , t h e propert ies o f s u c h c o n c r e t e s s u c h as s t r e n g t h a t h i g h t e m p e r a t u r e , e l a s t i c modulus and creep are n o t w e l l c h a r a c t e r i z e d . I t is n o t a t a l l c lear w h e t h e r s u c c e s s f u l

a p p l i c a t i o n s c a n b e made i n l o a d - b e a r i n g s t r u c t u r e s f o r n u c l e a r reac- tors. I f a l l t h e i m p o r t a n t m e c h a n i c a l p roper t ies a t h i g h t e m p e r a t u r e were s u f f i c i e n t l y r e s e a r c h e d , t h e s e m i g h t be a t t r a c t i v e mater ia l s .

F i b e r R e i n f o r c e m e n t

For v a r i o u s a p p l i c a t i o n s o f r e f r a c t o r y c o n c r e t e s i n c h e m i c a l p r o c e s s i n g t e c h n o l o g y ( V o l . 1, R e f . 251), s t a i n l e s s s t e e l f i b e r s a d d e d t o t h e c o n c r e t e mix c a n s u b s t a n t i a l l y improve i t s p e r f o r m a n c e i n t e n s i o n . The f i b e r s i n h i b i t t h e f o r m a t i o n o f d i s t i n c t c o n t i n u o u s cracks and c a u s e t h e c r a c k i n g t o c o n s i s t o f f i n e l y d i s t r i b u t e d micro- cracks t h a t a r e g e n e r a l l y d i s c o n t i n u o u s . A l t h o u g h t h e a d d i t i o n o f s t e e l f i b e r s h a s h a r d l y a n y e f f e c t o n t e n s i l e s t r e n g t h l i m i t , i t endows t h e ma te r i a l , e v e n w i t h o u t r e i n f o r c i n g bars , w i t h a c o n s i d - e r ab le d u c t i l i t y and e n e r g y a b s o r p t i o n c a p a b i l i t y . F r a c t u r e tough- n e s s is a p p r e c i a b l y improved. I n c o n s e q u e n c e o f t h i s , t h e r e s i s t a n c e t o t h e r m a l s h o c k i s a l so improved . A d d i t i o n o f s t e e l f i b e r s a l so i m p r o v e s c o m p r e s s i v e s t r e n g t h by h o l d i n g t h e ma te r i a l t o g e t h e r l a t e r a l l y .

A s i n d i c a t e d i n t h e p r e c e d i n g volume o f t h i s repor t , t h e e f f i c i e n c y o f s t e e l f i b e r s i s much l e s s documented f o r h i g h t e m p e r a t u r e a p p l i - c a t i o n s t h a n i t is f o r s t r u c t u r e s a t n o r m a l t e m p e r a t u r e . N e v e r t h e - less, it is reasonable to expect that the addition of steel fibers

would lower t h e c h a n c e s o f t h e f o r m a t i o n o f a c a t a s t r o p h i c l eak t h r o u g h a c o n c r e t e c o n t a i n m e n t w a l l . I n case o f a sodium s p i l l , t h e a c t i o n o f t h e s t ee l f i b e r s s h o u l d be t o h o l d t h e f r a c t u r e d pieces o f t h e mater ia l t o g e t h e r , r e d u c e t h e f o r m a t i o n of c o n t i n u o u s c racks , and s i g n i f i c a n t l y d e c r e a s e t h e r a t e o f p e n e t r a t i o n o f sodium and h e a t i n t o t h e c o n c r e t e by d e c r e a s i n g t h e s u r f a c e area t h a t c a n b e a c c e s s e d by t h e sodium.

S i m i l a r l y t o c o n v e n t i o n a l s t e e l bar r e i n f o r c e m e n t , s t e e l f i b e r s may become i n e f f e c t i v e a t t e m p e r a t u r e s o v e r 900°C where t h e creep o f

s t e e l becomes v e r y l a r g e . F o r s u c h s i t u a t i o n s , e x p l o r a t i o n o f t h e

u s e o f ceramic f i b e r s s u c h as g l a s s or b o r o n f i b e r s or c e r t a i n m i n e r a l f i b e r s may b e o f i n t e r e s t . However, t h e s e f i b e r s a r e

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generally less efficient at moderate temperatures due to their lack of ductility .

Reinforcement and Anchors

Steel reinforcement and anchors may be one very effective means of improving the resistance of a refractory concrete wall to heat shock or sodium spill. This is however generally considered as a design measure rather than a material property.

Surface Treatment

The dis'advantages of reactivity of concrete with sodium can be mitigated by certain surface treatments (see Vol. 1). These include impregnation of the surface with a refractory compound that is inert to sodium, such as MgO, A1203, or sodium silicate (waterglass). This treatment reduces the internal surface area over which the compounds reactive with sodium (siliceous compounds) can come in contact with sodium, and reduces surface porosity, thereby limiting physical penetration of sodium into the concrete. However, no data on the effectiveness of these treatments for improving sodium resistance seem to exist at present. Therefore, the effectiveness of such surface treatment for resistance to sodium attack should be investigated.

3.3 RANKING OF MATERIALS FOR VARIOUS PROPERTIES

So far various castable materials have been considered and their advantages and disadvantages for potential use in LMFBR's have been listed. It may be helpful to supplement this by ranking of these materials for different properties. It must be understood, however, that this ranking is crude since some properties are not always strictly comparable. This is due to the lack of data for all mate- rials over the complete range of time, temperature, and environmental conditions of interest.

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(I) S t r e n q t h

( a ) C o l d Compressive S t r e n g t h * E x c e l l e n t

- B r i c k s - i n c l u d i n g hard-burned f i r e c l a y b r i c k (7-11 k s i

compressive s t r e n g t h ) , f i r e d z i rconia b r i c k ( a b o u t t h e same s t r e n g t h ) , and h a r d - b u r n e d h i g h a l u m i n a b r i c k * * (9-16 k s i s t r e n g t h ) ;

- Concretes - h i g h a l u m i n a cement c o n c r e t e s ( a s s u m i n g no

conve r s ion react ion) and phosphate-bound corundum (which

c a n r e a c h 8 , 0 0 0 t o 22,000 p s i s t r e n g t h a f t e r proper p r e h e a t ) .

Good - B r i c k s - h a r d - b u r n e d m a g n e s i a b r i c k ; - Concretes - p o r t l a n d cement concretes (2 t o 8 k s i com-

p r e s s i v e s t r e n g t h ) , w a t e r g l a s s c o n c r e t e s ( 2 t o 3 k s i i f

p r e h e a t e d ) , and magnesia-bound chrome ore c o n c r e t e ( 3 t o 4 k s i i f p r e h e a t e d a b o v e 12OOOC).

Poor - L i g h t w e i g h t i n s u l a t i n g b r i c k s ;

- L i g h t w e i g h t i n s u l a t i n g c o n c r e t e ;

- Magnesia-bound concrete, i f p r e h e a t e d be low 1200OC.

U n c e r t a i n - T h e r e e x i s t s l i t t l e or no d a t a on t h e s t r e n g t h and o t h e r

m e c h a n i c a l p roper t ies a t a m b i e n t or e l e v a t e d tempera- t u re s o f n o n - h y d r a u l i c b i n d e r c o n c r e t e s ( w a t e r g l a s s ,

p h o s p h a t e s , m a g n e s i a ) ; - G r a p h i t e b r i c k .

(b) H o t C o m p r e s s i v e S t r e n g t h ( u p t o 900°F)

Exce 11 e n t - P r o b a b l y most b r i c k s l i s t e d as e x c e l l e n t i n t h e

p r e c e d i n g i t e m , b u t l i t t l e d a t a e x i s t .

*Cold c o m p r e s s i v e s t r e n g t h . S t r e n g t h o f materials a t room t e m p e r a t u r e . Mater ia ls p r e v i o u s l y p r e h e a t e d to d e v e l o p ceramic b o n d s w h e r e i n d i c a t e d .

**High-alumina br ick consists of t a b u l a r a l u m i n a i n a h i g h a l u m i n a cement m a t r i x .

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Good - Portland cement concretes and high alumina cement concretes (of which the latter show a greater strength loss).

Uncertain - Phosphate, waterglass, and magnesia-bound systems at

temperatures under 1000°C.

Information on the strength after long-term heating is unavailable for all these materials, except for some data for portland cement concretes.

(c) Tensile Cracking and Fracture Excellent Although concrete, cold or hot, has low tensile strength and fracture toughness, reinforcement can endow the mate- rial with excellent overall tensile strength that allows distributed tensile cracks to form. Steel reinforcement, even when made of some heat-resistant steel, cannot serve effectively above 9 0 0 C (due to very large creep). The same is true of steel fibers. At temperatures of over 900°C most concretes become ductile and tensile creep is a greater problem than tensile cracking. Good Fired refractory concrete and brick have a good tensile strength, but this is of little use since the joints between blocks or bricks do not. Poor All brick systems as well as prefired concretes have a poor resistance to tensile cracking. The goal is not to eliminate tensile cracking but to live with it and exploit it, through the use of proper reinforcement, for other advantages, especially energy absorption capability. Possible Improvement Addition of steel fibers reduces crack propagation. This makes the material more ductile.

0

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(2) Thermal Deformation and Volume Stability at Elevated Temperature Excellent - Hard-burned zirconia or silicon carbide brick; - Hard-burnt fireclay brick (above 1000°F) ; - High alumina cement concretes of all types (if without MgO);

especially those with corundum aggregates; - Phosphate-bound corundum concretes. Good - High alumina brick; - Most phosphate-bonded concretes; - Most waterglass-bonded concretes. Poor or Uncertain - Portland cement concretes, particularly at temperatures over

50OoC;

- Magnesia-bound concretes (high thermal expansion, unknown creep) ;

- Hard-burned f ire-clay brick (below 1000°F) ; - Magnesia brick (high thermal expansion and unknown creep).

Excellent - Most lightweight aggregates concretes and lightweight bricks,

( 3 ) Thermal Insulation

which generally have thermal conductivities 1/3 to 1/10 that of normal weight materials.

Good - Concretes with non-hydraulic binders (phosphate, waterglass, and magnesia) ;

- Portland cement concrete; - High alumina cement concrete.

(all with no silica or magnesia) Fair or Poor - All high density bricks: superduty fireclay bricks, high

- High alumina cement concretes with tabular alumina aggregate; - A l l concretes with significant amounts of silica or magnesia

density alumina brick, magnesia brick, etc.;

in the aggregate; also those with silicon carbide. (Note that higher density works against thermal insulation capabilities.)

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( 4 ) R e s i s t a n c e t o Sodium E x c e l l e n t ( n o p e n e t r a t i o n , no r e a c t i o n ) - Dense h a r d - b u r n e d h i g h a l u m i n a b r i c k : - Z i r c o n i a b r i c k ( h o w e v e r , o n e case of c r a c k i n g was repor ted) : - Magnes ia b r i c k .

Good ( e i t h e r n o p e n e t r a t i o n or n o r e a c t i o n ) - High a l u m i n a cement c o n c r e t e w i t h corundum a g g r e g a t e ( n o

react i o n ; - W a t e r g l a s s - b o n d e d t a b u l a r a l u m i n a , corundum, or m a g n e s i a

a g g r e g a t e s ( p o s s i b l y - n o d a t a ) . Q u e s t i o n a b l e - G e n e r a l l y c o n c r e t e s o f a l l t y p e s , b e c a u s e o f t h e i r

s i g n i f i c a n t p o r o s i t y : - F i r e c l a y b r i c k , b e c a u s e o f i t s h i g h c o n t e n t o f S i 0 2 , which

- Chrome ore b r i c k as w e l l as c o n c r e t e , t h e y b o t h react w i t h

- P o r t l a n d c e m e n t concretes, a l t h o u g h i t seems t h a t d e l e t e r i o u s e f f e c t s may be r e d u c e d by proper d e s i g n s which do n o t allow deep p e n e t r a t i o n of t h e sodium.

reac ts w i t h sodium;

sodium;

Poss ib le Improvement Improvement o f p e r f o r m a n c e c o u l d be a c h i e v e d by s u r f a c e t r e a t - ment. I m p r e g n a t i o n o f pores n e a r surfaces w i t h a compound i n e r t to s o d i u m , s u c h as MgO or A1203, and a d d i t i o n of g l a s s f i b e r s t h a t t e n d to i n h i b i t f o r m a t i o n o f cracks t h r o u g h which t h e s o d i u m c a n p e n e t r a t e i n t o t h e c o n c r e t e may b e h e l p f u l .

( 5 ) Creep and Decrease o f S t i f f n e s s E x c e l 1 e n t All f i r e d r e f r a c t o r y b r i c k s and most f i r e d r e f r a c t o r y c o n c r e t e s . T h i s is d u e t o t h e s t r o n g , b r i t t l e n a t u r e of ceramic b o n d i n g . Creep o f f i r e d r e f r a c t o r i e s i s g e n e r a l l y less t h a n t h a t o f p o r t l a n d cement c o n c r e t e a t e l e v a t e d t e m p e r a t u r e s ( u p t o 600 C ) .

Above 6 0 0 C creep d e f o r m a t i o n s o f r e f r a c t o r i e s i n c r e a s e . Chrome ore c o n c r e t e s show be t t e r r e s i s t a n c e t o c r e e p deforma- t i o n s t h a n r e f r a c t o r i e s made w i t h o t h e r a g g r e g a t e s .

0

0

R e l a t i v e l y l i t t l e d a t a are a v a i l a b l e on modulus o f e l a s t i c i t y o f r e f r a c t o r y concretes or masonry .

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Fair or Poor - Portland cement and unfired refractory concretes (both

hydraulic and non-hydraulic binders) that exhibit an order of magnitude increase in creep upon heating.

(6) Workability--Possibility to Cast Massive Containment Wall Structures Excellent - Portland cement concretes. Technology well developed. No

special heat-treatment or curing required. Fair or Poor - High alumina cement concretes and practically all other

refractory concretes and brick linings. Their casting in large volumes is limited by hydration heat, by the need of firing at very high temperatures, or by the need for joints between bricks.

3 . 4 SELECTION OF MATERIALS OF MAIN INTEREST

As can be seen from the preceding discussions, there exist many materials of interest for further study with regard to nuclear structures. None of these materials, however, are proven ready for use.

For some, such as the portland cement concretes, a large body of knowledge already exists and relatively small amount of work is needed to evaluate the applicability of these materials for nuclear structures exposed to high temperatures. For others, although existing data are promising, a large amount of additional research would have to be conducted. The use of these materials should obviously be considered as a longer range goal. Besides, not all of these materials and all of their properties would fit within the scope of a reasonably sized research program. Therefore, selection of only a few of the most promising materials for immediate study and design considerations is in order.

Thus, although the risk of uncertainty due to incomplete information exists, it seems desirable to identify several materials of greatest interest. Based on information available, these materials are as follows :

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(1) P o r t l a n d Cement C o n c r e t e s

P o r t l a n d c e m e n t c o n c r e t e s appear t o b e t h e best c a n d i d a t e f o r s t r u c t u r a l a p p l i c a t i o n s f o r s e r v i c e temperatures i n t h e r a n g e from a m b i e n t to 300 or 4OO0C, p r o v i d e d many t e m p e r a t u r e c y c l e s o f l a r g e m a g n i t u d e a re e x c l u d e d . O n l y h e a t r e s i s t a n t , c a r e f u l l y s e l e c t e d a g g r e g a t e s , s u c h as b a s a l t , l i m e s t o n e , or s e r p e n t i n e , s h o u l d b e u s e d f o r t h e s e c o n c r e t e s . F o r a n a c c i d e n t t y p e e x p o s u r e , s u c h as a sod ium s p i l l , t e m p e r a t u r e s u p t o a p p r o x i m a t e l y 6OO0C may w e l l be a c c e p t a b l e f o r l i m i t e d p e r i o d s o f time i f a l i m i t e d damage to t h e s t ruc ture is p e r m i t t e d . I t is i m p e r a t i v e t h a t t h e s t r u c t u r e is p r o p e r l y d e s i g n e d t o g u a r a n t e e t h a t t h e damage would i n d e e d b e l i m i t e d s u c h t h a t n o leak o u t s i d e t h e c o n t a i n m e n t would occur.

F o r t h e u s e o f p o r t l a n d c e m e n t c o n c r e t e s , v a r i o u s t e c h n o l o g i c a l

improvemen t s s h o u l d be c o n s i d e r e d . One i d e a , wh ich h a s b e e n d e v e l o p e d t o some e x t e n t a t Argonne N a t i o n a l L a b o r a t o r y ( V o l . 1, R e f . 31), is t h e h o t - d r i e d p o r t l a n d c e m e n t c o n c r e t e v e s s e l t h a t i s d e p r i v e d by m o d e r a t e h e a t i n g of a l l e v a p o r a b l e water. N o h i g h t e m - perature h e a t i n g t h a t would d r i v e o u t a n y o f t h e c h e m i c a l l y bound water is c o n t e m p l a t e d s i n c e i t wou ld r e d u c e t h e s t r e n g t h . T h i s t r e a t m e n t , which i n c r e a s e s t h e s t r e n g t h of c o n c r e t e i f ca r r ied o u t w i t h o u t c r a c k i n g t h e c o n c r e t e , d i m i n i s h e s t h e creep and reduces t h e amount o f water t h a t c a n reac t w i t h sod ium i n case o f a sod ium s p i l l .

Any u s e o f p o r t l a n d c e m e n t c o n c r e t e s a b o v e a t e m p e r a t u r e of 150°F, t h e maximum t e m p e r a t u r e permit ted by c u r r e n t n u c l e a r reactor con- t a i n m e n t a n d p r e s s u r e v e s s e l c o d e s , r e q u i r e s spec ia l d e s i g n proce- d u r e s and a d d i t i o n a l r e s e a r c h . T h i s w i l l b e f u r t h e r d i s c u s s e d i n t h e n e x t sect i o n .

( 2 ) High Alumina Cement (HAC) C o n c r e t e s

P o r t l a n d c e m e n t c o n c r e t e s c a n be c o n s i d e r e d f o r s u s t a i n e d tempera- t u r e s o v e r 300 or 400 C and f o r s h o r t time e x p o s u r e s o v e r 60OoC. F o r more s e v e r e temperature c o n d i t i o n s a l t e r n a t i v e s t o n o r m a l por t - l a n d c e m e n t c o n c r e t e s m u s t be c o n s i d e r e d . High a l u m i n a c o n c r e t e s r e p r e s e n t o n e a l t e r n a t i v e mater ia l f o r wh ich r e l a t i v e l y e x t e n s i v e

i n f o r m a t i o n and e x p e r i e n c e a l r e a d y e x i s t s . I t is d o u b t f u l , however ,

0

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w h e t h e r a n y t h i n g would be g a i n e d by t h e u s e o f t h e s e mater ia l s i f

t h e y a r e n o t p r e f i r e d .

A l t h o u g h HAC c o n c r e t e s h a v e h i g h e r m o i s t - c u r e d s t r e n g t h s t h a n p o r t l a n d cement c o n c r e t e s , t h i s a d v a n t a g e is l o s t a t t e m p e r a t u r e s a b o v e 5OoC d u e t o d e l e t e r i o u s e f f e c t s of c o n v e r s i o n r e a c t i o n s o f

t h e h y d r a t e d HAC pas te . F o r t h i s r e a s o n , t h e u s e of HAC is n o t p e r m i t t e d i n t h e U . S . f o r n o r m a l s t r u c t u r a l a p p l i c a t i o n s . However, r e s i d u a l s t r e n g t h s o f HAC c o n c r e t e a f t e r c o n v e r s i o n may s t i l l b e s u f f i c i e n t f o r s t r u c t u r a l a p p l i c a t i o n s a t e l e v a t e d t e m p e r a t u r e s .

I f p r e f i r e d t o 1000°C, t h e s e mater ia l s may b e ab le t o s e r v e s t r u c - t u r a l f u n c t i o n s a t t e m p e r a t u r e s g r e a t e r t h a n 60OoC. However, t h e need f o r f i r i n g p r e s e n t s a major t e c h n o l o g i c a l d i f f i c u l t y . I t would

p r o b a b l y be e x t r e m e l y cumbersome and d i f f i c u l t t o f i r e l a r g e mono- l i t h i c s t r u c t u r e s o f h i g h a l u m i n a c o n c r e t e . A more l i k e l y form o f u s e a r e small b l o c k s or p a n e l s t h a t c a n b e f i r e d i n a n o r m a l s i z e f u r n a c e .

A p r o b l e m t h e n a r i ses w i t h r e g a r d t o j o i n t s b e t w e e n t h e b locks .

E i t h e r t h e j o i n t s u s e some s o r t of mortar, as t h i n a l a y e r a s p o s s i b l e , or t h e y a r e l e f t d r y . The u s e o f mortar i n t h e j o i n t s implies t h a t some water w i l l a l w a y s be c o n t a i n e d i n t h e mortar and w i l l b e e v a p o r a t e d upon h e a t i n g , t h u s b e i n g a v a i l a b l e t o react w i t h sodium i n case o f a sodium s p i l l . I f t h e j o i n t is d r y , q u e s t i o n s a r i s e w i t h r e g a r d t o t h e s t r e n g t h o f t h e s t r u c t u r e a s a w h o l e par- t i c u l a r l y for s h e a r l o a d i n g . However, appropriate d e s i g n m e a s u r e s , s u c h a s t h e u s e of s h e a r k e y s and p r e s t r e s s i n g , c o u l d be u s e d to p r o v i d e f o r t r a n s f e r of s h e a r l o a d s . A n o t h e r q u e s t i o n c o n n e c t e d w i t h d r y j o i n t s is t h e p o s s i b i l i t y of access f o r t h e m o l t e n s o d i u m i n t o t h e j o i n t s . The p o t e n t i a l for p e n e t r a t i o n of sodium i n t o s t r u c - t u r a l j o i n t s c o u l d b e d e t e r m i n e d e x p e r i m e n t a l l y by small-scale t e s t i ng . A s f o r t h e mortar t o be u s e d i n j o i n t s b e t w e e n b r i c k s , t h e u s e o f e i t h e r h i g h a l u m i n a c e m e n t mortar or w a t e r g l a s s mortar seems pre- f e r a b l e . The former h a r d e n s a t room t e m p e r a t u r e , w h i l e t h e l a t t e r r e q u i r e s o n l y m o d e r a t e l y h i g h t e m p e r a t u r e s of 1 0 0 t o 2OO0C t o h a r d e n . I n a d d i t i o n , b o t h seem t o be a c c e p t a b l e i n case o f s o d i u m s p i l l .

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( 3 ) N o n - h y d r a u l i c C o n c r e t e s

For temperatures up t o a b o u t 1000°C, n o n - h y d r a u l i c c o n c r e t e s s u c h a s w a t e r g l a s s c o n c r e t e and p h o s p h a t e or m a g n e s i a c o n c r e t e s , d o n o t seem t o o f f e r any p a r t i c u l a r a d v a n t a g e s o v e r a l u m i n o u s cement con- cretes e x c e p t for t h e r e l a t i v e ease t h a t water c a n be removed f rom t h e s e s y s t e m s . They a r e o f t e n f a v o r e d i n c h e m i c a l p r o c e s s i n g t e c h - n o l o g y methods b e c a u s e o f t h e i r e a s y c a s t i n g and c h e m i c a l r e s i s t a n c e . However, t h e f a c t t h a t t h e y h a v e e s s e n t i a l l y n o s t r e n g t h u n l e s s t h e y a r e h e a t e d i s i n c o n v e n i e n t , p a r t i c u l a r l y i n c a s t i n g more m a s s i v e structures. They d o n o t h a v e t o be h e a t e d a s h i g h as a l u m i n a con- cretes t o f o r m a p e r m a n e n t h i g h - t e m p e r a t u r e r e s i s t a n t bond. However, a m b i e n t t e m p e r a t u r e s t r e n g t h s a c h i e v e d w i t h n o n - h y d r a u l i c cement c o n c r e t e s p r e - h e a t e d t o t e m p e r a t u r e s o f a b o u t 2OO0C a r e r e l a t i v e l y l o w compared t o o t h e r r e f r a c t o r y c o n c r e t e s . They a r e less t h a n t h a t of h i g h a l u m i n a cement a c h i e v e d w i t h o u t a n y p r e - h e a t i n g , or pre- h e a t i n g t o 20OoC. However, t h e s t r e n g t h of HAC c o n c r e t e as a f u n c - t i o n o f t e m p e r a t u r e is d i f f i c u l t t o assess, owing t o t h e v a r i a b l e e f f e c t s o f " c o n v e r s i o n " o f HAC s t r e n g t h . For t h i s r e a s o n , s t r i c t c o m p a r i s o n of t h e s e c o n c r e t e s i s n o t a l w a y s possible .

T h e h i g h p o r o s i t y o f these mater ia l s ca s t s d o u b t on t h e i r p e r f o r m a n c e i n case o f sodium a t t a c k , b e c a u s e a much h i g h e r i n t e r n a l s u r f a c e c a n be e x p o s e d t o sodium. T h i s may be a problem e v e n i f t h e i r c h e m i c a l a c t i v i t y i s v e r y small . However, t h e s e a r e o n l y o p i n i o n s based on t h e p r e s e n t l i m i t e d knowledge , and f u r t h e r i n v e s t i g a t i o n s a r e i n order.

One u s e o f n o n - h y d r a u l i c c o n c r e t e s i s f o r t h e case of t e m p e r a t u r e s i n e x c e s s o f 1000°C. N o n - h y d r a u l i c c o n c r e t e s c a n s e r v e s t r u c t u r a l f u n c t i o n s by m a i n t a i n i n g a d e q u a t e s t r e n g t h and l i m i t e d d e f o r m a t i o n a t t e m p e r a t u r e s up t o 18OOOC. c o n t a i n i n g a s t ee l m e l t w i t h o u t a n y s i g n i f i c a n t damage t o t h e wa l l . A v e r y t h i c k p o r t l a n d c e m e n t c o n c r e t e w a l l c a n a l so c o n t a i n a s t e e l m e l t , b u t o n l y if t h e m e l t cools b e f o r e d i s i n t e g r a t i o n o f c o n c r e t e c a n p r o g r e s s t h r o u g h t h e w a l l .

T h e s e c o n d i t i o n s are r e q u i r e d f o r

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( 4 ) R e f r a c t o r y B r i c k S y s t e m s

C o n c r e t e or r e f r a c t o r y c o n c r e t e l i n e d w i t h a l a y e r o f r e f r a c t o r y b r i c k , w i t h or w i t h o u t a s t ee l l i n e r , is o b v i o u s l y o f i n t e r e s t f o r v e r y h i g h t e m p e r a t u r e s , as a n a l t e r n a t i v e to n o n - h y d r a u l i c c o n c r e t e s . A s u f f i c i e n t l y t h i c k l i n i n g o f r e f r a c t o r y b r i c k s is ab le t o c o n t a i n a s t e e l m e l t . C e r t a i n spec ia l b r i c k s , s u c h a s z i r c o n i u m o x i d e or ca rb ide c a n c o n t a i n melts a t t e m p e r a t u r e s o v e r 20OO0C, s u c h as i n t h e case o f m o l t e n u r a n i u m o x i d e .

I n c o n t r a s t t o a l l c o n c r e t e s y s t e m s , d e n s e r e f r a c t o r y b r i c k s h a v e t h e a d v a n t a g e of v e r y l o w a p p a r e n t p o r o s i t y , almost z e r o f o r c e r t a i n b r i c k s . T h i s makes them h i g h l y a t t r a c t i v e f o r c o n t a i n i n g a sodium s p i l l .

R e f r a c t o r y b r i c k s a r e u s u a l l y pre-f i red by t h e m a n u f a c t u r e r . T h u s , n o a d d i t i o n a l h e a t - t r e a t m e n t is r e q u i r e d when b r i c k s are i n s t a l l e d . Dense r e f r a c t o r y b r i c k s u s u a l l y h a v e v e r y h i g h c o l d c r u s h i n g s t r e n g t h s , u p t o 1 6 , 0 0 0 p s i , and r e t a i n c o n s i d e r a b l e s t r e n g t h o n h e a t i n g .

R e s i s t a n c e of r e f r a c t o r y b r i c k t o c h e m i c a l a t t ack f rom m o l t e n s o d i u m is more d i f f i c u l t t o assess t h a n t h e problem o f p h y s i c a l p e n e t r a t i o n o f s o d i u m , a l t h o u g h t h e t w o phenomena are i n t e r - r e l a t e d . Two e x p e r i - m e n t a l s t u d i e s ( V o l . 1, R e f . 9 , 1 1 2 , 1 1 3 ) h a v e i n d i c a t e d t h a t t h e f o l l o w i n g br ick t y p e s show promise f o r m o l t e n s o d i u m c o n t a i n m e n t :

High d e n s i t y h i g h a l u m i n a b r i c k ( o p e n p o r o s i t y 2 % )

High d e n s i t y m a g n e s i a b r i c k ( 1 0 % ) Z i r c o n i a b r i c k ( 30%)

Two o t h e r b r i c k s y s t e m s h a v e shown some l i m i t e d c a p a c i t y f o r r e s i s t i n g c h e m c i a l a t t a c k o f m o l t e n sodium:

V i t r e o u s c a r b o n or g r a p h i t e b r i c k ( o p e n p o r o s i t y 1%)

Dense s u p e r d u t y f i r e c l a y b r i c k ( 4 % )

F i r e c l a y b r i c k h a s r e p o r t e d l y b e e n s u c c e s s f u l l y u s e d t o l i n e v e s s e l s u s e d f o r p r o d u c i n g sodium e l e c t r o l y t i c a l l y from m o l t e n sodium c h l o - r i d e ( V o l . 1, R e f . 5 7 ) . However, l a b o r a t o r y tests o f f i r e c l a y b r i c k

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e x p o s e d t o m o l t e n sodium a t 87OoC h a v e r e s u l t e d i n complete d e s t r u c t i o n o f t h e b r i c k .

S i n c e b r i c k s a re pre- f i red d u r i n g m a n u f a c t u r e , a l l water is removed, and n o a d d i t i o n a l h e a t - t r e a t m e n t is r e q u i r e d . The u s e o f mortar t o b i n d b r i c k s w i l l i n t r o d u c e some water i n t o t h e masonry s y s t e m , b u t amount of moisture p r e s e n t w i l l be much smaller t h a n t h a t p r e s e n t i n a c o n c r e t e l i n e r . A l t e r n a t i v e l y , t h e u s e o f d r y - j o i n t masonry con- s t r u c t i o n methods would e l i m i n a t e t h e need f o r mortar.

3.5 MATERIALS OF INTEREST FOR TYPICAL APPLICATIONS I N LMFBR

Having d e s c r i b e d i n d i v i d u a l mater ia l s and propert ies o f i n t e r e s t , t h i s i n f o r m a t i o n w i l l now be r e l a t e d to p a r t i c u l a r a p p l i c a t i o n s i n LMFBR's . T h e s e are as f o l l o w s :

A. S t r u c t u r a l (Load-Bear ing) S e r v i c e a t T e m p e r a t u r e s U p t o 4OO0C

W i t h i n t h i s t e m p e r a t u r e r a n g e t h e u s e of p o r t l a n d cement c o n c r e t e s seems t o b e p r e f e r a b l e . Complete d e s i g n p r o c e d u r e s f o r temperatures o v e r 150°F ( p r e s e n t code l i m i t ) however , a re n o t a v a i l a b l e a t p r e s e n t and f u r t h e r w o r k n e e d s t o be d o n e . N o r e f r a c t o r y c o n c r e t e or b r i c k s y s t e m seems t o o f f e r any d e f i n i t e a d v a n t a g e s o v e r p o r t l a n d cement c o n c r e t e i n t h i s t e m p e r a t u r e r a n g e . P o r t l a n d cement c o n c r e t e c a n be p r o d u c e d t o h a v e w i t h i n t h i s t e m p e r a t u r e r a n g e a d e q u a t e s t r e n g t h , by c o m p e n s a t i n g f o r losses d u e t o h e a t i n g , as w e l l a s

l i m i t e d d e f o r m a t i o n .

C e r t a i n d e s i g n m e a s u r e s may a l so be c o n s i d e r e d t o improve t h e p e r f o r m a n c e o f p o r t l a n d cement c o n c r e t e i n t h i s t e m p e r a t u r e r a n g e . T h e s e i n c l u d e d r y i n g o f t h e c o n c r e t e by s u s t a i n e d m i l d h e a t i n g t h a t d r i v e s o u t t h e e v a p o r a b l e water b u t n o t t h e c h e m i c a l l y - b o u n d h y d r a - t i o n water. The t h e r m a l e x p a n s i o n , t he rma l s h r i n k a g e , creep, r e s i s t a n c e t o c r a c k i n g , and bond w i t h s t e e l , appear t o be a l l acceptable w i t h i n t h i s r a n g e and n o t v e r y d i f f e r e n t f rom t h e s e p roper t ies a t room t e m p e r a t u r e . T h e o p e n q u e s t i o n i s t h e proper

d e s i g n p r o c e d u r e , t h a t i s t h e need t o c a l c u l a t e s t resses , p red ic t c r a c k i n g and f r a c t u r e b e h a v i o r as w e l l a s c r e e p and s h r i n k a g e d e f o r m a t i o n s , w i t h s u f f i c i e n t a c c u r a c y t o a s s u r e t h e desired

p e r f o r m a n c e .

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B. L i q u i d Sodium C o n t a i n m e n t U p t o 88OoC

Even f o r t h i s a p p l i c a t i o n , p o r t l a n d cement c o n c r e t e s may p r o v e to be

a c c e p t a b l e . A w a l l made o f p o r t l a n d cement c o n c r e t e may u n d e r g o a c o n s i d e r a b l e amount o f damage when e x p o s e d t o a sodium s p i l l . A l a r g e p a r t o f t h e t h i c k n e s s of t h e w a l l may be d e s t r o y e d , and t h e c o n s e q u e n t d e v e l o p m e n t o f h y d r o g e n g a s i n t h e c o n t a i n m e n t must b e h a n d l e d . However, t h e u l t i m a t e g o a l o f c o n t a i n m e n t may s t i l l b e a c h i e v e d w i t h appropriate d e s i g n m e a s u r e s . T h e s e i n c l u d e a l a y o u t o f r e i n f o r c e m e n t t h a t i n h i b i t s c r a c k i n g o f t h e w a l l , s p a l l i n g o f i ts

f r a g m e n t s and p r o v i d e s s u f f i c i e n t r e s t r a i n t ( c o m p r e s s i v e c o n f i n i n g s t r e s ses ) . The u s e of s t e e l f i b e r s to d i s t r i b u t e c r a c k i n g and m i n i m i z e f o r m a t i o n o f f r a g m e n t s may a l so be h e l p f u l .

R e g a r d i n g s o d i u m c o n t a i n m e n t o t h e r mater ia l s a re preferab le t o

p o r t l a n d cement c o n c r e t e s . High a l u m i n a cement c o n c r e t e s would p r o b a b l y p e r f o r m be t t e r i f o n l y b e c a u s e t h e y d o n o t c o n t a i n l a r g e a m o u n t s of s i l i c a . The bes t p e r f o r m a n c e would be p r o v i d e d by d e n s e r e f r a c t o r y b r i c k l i n i n g s , owing t o t h e i r l o w p o r o s i t y as compared t o c o n c r e t e s . T h i s is c o n f i r m e d by t h e f a c t t h a t s u c h a l i n i n g is used i n v e s s e l s f o r p r o d u c t i o n o f sodium, and v e r i f i e d i n l a b o r a t o r y t es t s o f v a r i o u s r e f r a c t o r y b r i c k s y s t e m s e x p o s e d t o m o l t e n sodium. Such a l i n i n g c a n be d e s i g n e d to s u f f e r no damage i n e x p o s u r e t o sodium. The d e s i g n e r must weigh t h e cost and o t h e r c o m p l e x i t i e s o f d o i n g t h i s a g a i n s t t h e c o n s e q u e n c e s or a c c e p t a b i l i t y o f a s e v e r e b u t

l i m i t e d damage t o a p o r t l a n d cement c o n c r e t e s t r u c t u r e .

W a t e r g l a s s c o n c r e t e is q u e s t i o n a b l e i n t h i s app l i ca t ion because o f i t s h i g h p o r o s i t y and presumed h i g h p e r m e a b i l i t y and l o w s t r e n g t h , e v e n though t h e c h e m i c a l r e a c t i v i t y of w a t e r g l a s s w i t h l i q u i d sodium may be n e g l i g i b l e . F u r t h e r i n f o r m a t i o n is n e e d e d o n p e r m e a b i l i t y o f w a t e r g l a s s c o n c r e t e s t o m o l t e n sodium.

For c o n c r e t e e x p o s e d t o l i q u i d s o d i u m , t h e c h o i c e o f a g g r e g a t e is e x t r e m e l y i m p o r t a n t . As a g e n e r a l r u l e , a g g r e g a t e s c o n t a i n i n g s i l i c a or chrome ore s h o u l d b e a v o i d e d where r e s i s t a n c e t o sodium is r e q u i r e d . The a g g r e g a t e s of b e s t p e r f o r m a n c e a re as f o l l o w s : r e f r a c t o r y a g g r e g a t e s - t a b u l a r a l u m i n a , corundum, z i r c o n i a , s i l i c o n c a r b i d e , and m a g n e s i a ; n o r m a l a g g r e g a t e s - b a s a l t , l i m e s t o n e .

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C. R a d i a t i o n S h i e l d i n g a t T e m p e r a t u r e s f rom Ambient t o 65OoC

Inasmuch as n e u t r o n r a d i a t i o n s h i e l d i n g d e p e n d s on t h e c o n t e n t of h y d r o g e n m o l e c u l e s , it s h o u l d be r e c o g n i z e d t h a t p o r t l a n d cement c o n c r e t e s as w e l l a s u n f i r e d h i g h a l u m i n a c o n c r e t e s a l w a y s c o n t a i n a l a r g e amount o f h y d r o g e n . Even when t h e i r e v a p o r a b l e water is d r i v e n o u t d u e t o h e a t i n g , t h e r e s t i l l r e m a i n s i n t h e mater ia l a l a r g e

amount o f c h e m i c a l l y - b o u n d water. The w a l l t h i c k n e s s r e q u i r e d f o r s t r u c t u r a l r e a s o n s i s u s u a l l y a d e q u a t e t o p r o v i d e a d e q u a t e s h i e l d i n g w i t h a p p r o p r i a t e c h o i c e o f a g g r e g a t e s .

The r a d i a t i o n s h i e l d i n g c h a r a c t e r i s t i c s of c o n c r e t e h a v e b e e n d i s - c u s s e d e x t e n s i v e l y e l s e w h e r e i n t h e l i t e r a t u r e . B i o l o g i c a l s h i e l d i n g of n e u t r o n s and gamma r a y s h a s b e e n a c h i e v e d by i n c o r p o r a t i n g a g g r e - g a t e s t h a t e f f i c i e n t l y a t t e n u a t e t h e s e t y p e s o f r a d i a t i o n . S i n c e

n e u t r o n s a r e h e a v i l y a t t e n u a t e d by h y d r o g e n , u s u a l l y i n t h e form o f wa te r , or b o r o n a t o m s , a g g r e g a t e s c o n t a i n i n g e i t h e r e l e m e n t h a v e

b e e n u s e d . T h e s e i n c l u d e : s e r p e n t i n e , b a u x i t e , l i m o n i t e , b o r o n c a r b i d e , or b o r o n f r i t . S e r p e n t i n e and b a u x i t e w i l l r e t a i n t h e i r c h e m i c a l l y - b o u n d water when h e a t e d t o t e m p e r a t u r e s a b o v e 1 0 0 C ,

b u t w i l l lose a l l water when h e a t e d t o 1000°C. a r o u n d 2000°C, w h i l e b o r o n c a r b i d e h a s a m e l t i n g p o i n t o f a b o u t 245OOC. With s u c h h i g h m e l t i n g p o i n t s , b o r o n and i t s c a r b i d e c a n

be c o n s i d e r e d " r e f r a c t o r y " , however , n o d a t a were found o n t h e i r u s e i n r e f r a c t o r y c o n c r e t e s . By c o n t r a s t , a g g r e g a t e s commonly u s e d i n r e f r a c t o r y c o n c r e t e s d o n o t show a n y p r o n o u n c e d c a p a b i l i t y f o r

s h i e l d i n g n e u t r o n r a d i a t i o n . *

0

Boron melts a t

Gamma r a d i a t i o n i s e f f e c t i v e l y a t t e n u a t e d by i n c o r p o r a t i n g a g g r e g a t e s

o f h i g h d e n s i t y i n c o n c r e t e u s e d f o r b i o l o g i c a l s h i e l d i n g . T h e s e i n c l u d e : m a g n e t i t e , h e m a t i t e , b a r i t e , or e v e n s t e e l p u n c h i n g s . A

number o f d e n s e r e f r a c t o r y a g g r e g a t e s u s e d i n c o n c r e t e or b r i c k s h a v e h i g h s p e c i f i c g r a v i t i e s which may m a k e them s u i t a b l e f o r gamma

r a d i a t i o n s h i e l d i n g . Among t h e s e are: z i r c o n i a ( 6 . 5 ) **, chrome ore

* B o n i l l a , C.F., N u c l e a r E n g i n e e r i n g , M c G r a w - H i l l , 1 9 5 7 , pp.863-841.

**Numbers i n p a r e n t h e s e s a r e m i n e r a l ' s s p e c i f i c g r a v i t y . F o r com- a r i s o n , Sp . G r . of water = 1. Sp. G r . n o r m a l c o n c r e t e = 2.3 t o 2 . 4 . Sp . G r . s t e e l = 7.6 t o 8 .0 .

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( 4 . 5 ) , corundum and t a b u l a r a l u m i n a ( 4 . 0 ) , m a g n e s i a ( 3 . 6 ) , and

s i l i c o n c a r b i d e ( 3 . 2 ) .

N o r m a l w e i g h t r e f r a c t o r y c o n c r e t e s i n c o r p o r a t i n g t h e s e a g g r e g a t e s

h a v e b u l k s p e c i f i c g r a v i t i e s lower t h a n t h a t o f t h e a g g r e g a t e , d u e t o t h e h i g h p o r o s i t y ( a p p r o x i m a t e l y 1 5 to 2 5 % ) and lower d e n s i t y o f t h e cement b i n d e r p h a s e . T y p i c a l l y , s p e c i f i c g r a v i t y o f t h e s e c o n c r e t e s r a n g e s f r o m a b o u t 2.0 to 3 . 0 . Dense r e f r a c t o r y b r i c k s r e a c h somewhat h i g h e r v a l u e s d u e t o t h e i r lower p o r o s i t y . L i g h t - w e i g h t r e f r a c t o r y c o n c r e t e s h a v e s p e c i f i c g r a v i t i e s below 2 . 0 , and would n o t appear t o b e s u i t a b l e for gamma r a d i a t i o n s h i e l d i n g .

F o r LMFBR's , t h e b i o l o g i c a l s h i e l d i n g must be a b l e t o w i t h s t a n d s u s t a i n e d e l e v a t e d t e m p e r a t u r e s i n t h e r a n g e o f 100 t o 3OO0C, and possible t r a n s i e n t s u p t o 84OoC ( u p s e t c o n d i t i o n s ) , p r e E e r a b l y w i t h o u t r e l e a s i n g water or r e a c t i n g e x o t h e r m i c a l l y w i t h m o l t e n sodium. With t h e s e r e q u i r e m e n t s , s e v e r a l p o s s i b l e a l t e r n a t i v e s y s t e m s show promise f o r s h i e l d i n g .

Dense r e f r a c t o r y b r i c k s made f rom t a b u l a r a l u m i n a , z i r c o n i a , or o t h e r d e n s e r e f r a c t o r y a g g r e g a t e s l i s t e d p r e v i o u s l y , most c l o s e l y meet r e q u i r e m e n t s for a n h y d r o u s s h i e l d i n g i n LMFBR's. T h e i r den- s i t ies a p p r o a c h t h a t of h e a v y w e i g h t c o n c r e t e s u s e d i n s h i e l d i n g f o r c o n v e n t i o n a l l i g h t - w a t e r n u c l e a r reactors. T h e i r r e s i s t a n c e t o e l e v a t e d t e m p e r a t u r e s and m o l t e n sodium h a s b e e n d e m o n s t r a t e d i n

tests a n d i n s e r v i c e . I f no mortar is u s e d , a r e f r a c t o r y b r i c k masonry w a l l w i l l c o n t a i n n o water, s i n c e t h e b r i c k s a re p r e - f i r e d t o a t l ea s t 1000°C d u r i n g m a n u f a c t u r e .

C o n c r e t e s made w i t h w a t e r g l a s s or p h o s p h a t e b i n d e r s and d e n s e r e f r a c t o r y a g g r e g a t e s may a l so m a k e s u i t a b l e s h i e l d wal l s i n LMFBR's, i f p r e h e a t e d t o remove a l l water. P r a c t i c a l l y , t h i s means

h e a t i n g t o 1 0 0 t o 35OoC. lower t h a n t h a t of d e n s e s t b r i c k s , d u e t o h i g h e r porosi t ies o f cas t ab le s .

D e n s i t i e s of t h e s e c o n c r e t e s a r e

P o r t l a n d cement or HAC c o n c r e t e s u s i n g d e n s e r e f r a c t o r y a g g r e g a t e s

may a l so be s u i t a b l e f o r s h i e l d i n g i n LMFBR's, i f s t r o n g h e a t i n g ( 60OoC) u n d e r l o a d or c o n t a c t w i t h s o d i u m c a n b e m i n i m i z e d or

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a v o i d e d . A l t e r n a t i v e l y , t h e s e c o n c r e t e s c o u l d be p r e - h e a t e d t o 8 0 0 C t o remove t h e i r water and meet t h e r e q u i r e m e n t o f a n " a n h y d r o u s " s h i e l d . However, e v e n if h e a t e d t o remove a l l water , p o r t l a n d cement c o n c r e t e w i l l s t i l l be s u s c e p t i b l e t o sodium a t t a c k , owing t o t h e h i g h s i l i c a c o n t e n t o f cement paste .

F i n a l l y , t h e e f f e c t s o f gamma r a d i a t i o n o n t h e materials d i s c u s s e d a re n o t w e l l - c h a r a c t e r i z e d , w i t h t h e poss ib le e x c e p t i o n s o f o r d i n a r y

p o r t l a n d cement c o n c r e t e and z i r c o n i a . E f f e c t s o f gamma r a d i a t i o n o n m e c h a n i c a l and t h e r m a l p rope r t i e s would h a v e t o be d e t e r m i n e d b e f o r e t h e i r s u i t a b i l i t y a s s h i e l d i n g m a t e r i a l s i n LMFBR's c a n be

e v a l u a t e d .

D. Core M e l t I s o l a t i o n ( a b o v e 87OoC)

F o r t o t a l m e l t i s o l a t i o n w i t h minor or no s t r u c t u r a l damage t o t h e w a l l , t h e only poss ib i l i t i e s a re d e n s e r e f r a c t o r y b r i c k l i n i n g s and n o n - h y d r a u l i c c o n c r e t e s , e s p e c i a l l y t h e p h o s p h a t e or m a g n e s i a con- cretes . T h e s e mater ia l s h a v e b e e n p r o v e n i n i n d u s t r y for c o n t a i n i n g m o l t e n meta ls , m o l t e n s t e e l i n p a r t i c u l a r . Materials w i t h p r o v e n r e s i s t a n c e t o m o l t e n sodium i n c l u d e :

High d e n s i t y a l u m i n a b r i c k High d e n s i t y m a g n e s i a b r i c k

2 i r c o n i a b r i c k

I n a d d i t i o n , some s o d i u m r e s i s t a n c e is r e p o r t e d f o r t h e f o l l o w i n g :

V i t r e o u s c a r b o n or g r a p h i t e b r i c k

High d e n s i t y f i r e c l a y b r i c k ( 600OC) F i r e d w a t e r g l a s s - b o n d e d corundum or t a b u l a r a l u m i n a c o n c r e t e

For melts t h a t d o n o t e x c e e d t h e t e m p e r a t u r e o f a b o u t llOO°C, h i g h a l u m i n a c e m e n t c o n c r e t e w i t h h i g h q u a l i t y r e f r a c t o r y a g g r e g a t e s (corundum or t a b u l a r a l u m i n a ) , i f p r e v i o u s l y f i r e d , may a l so s e r v e

w i t h o u t u n d e r g o i n g s i g n i f i c a n t c h e m i c a l a t t a c k .

The d e g r e e o f damage and d i s i n t e g r a t i o n o f t h e w a l l by t h e a c t i o n o f

t h e m e l t is however a d e s i g n parameter of p a r a m o u n t i m p o r t a n c e . N o d o u b t , a c e r t a i n , p e r h a p s v e r y l a r g e , amount o f damage may b e a l l o w e d , p o s s i b l y i n v o l v i n g t h e d i s i n t e g r a t i o n of a l a y e r of s e v e r a l

3-28

f e e t o n t h e i n s i d e of t h e reactor c o n t a i n m e n t w a l l or f l o o r . Dur ing t h e d i s i n t e g r a t i o n process, t h e metal l ic m e l t g r a d u a l l y cools down. I f it s o l i d i f i e s b e f o r e t h e damage a d v a n c e s a l l t h e way t h r o u g h t h e w a l l , t h e g o a l o f s a f e c o n t a i n m e n t of t h e t o t a l m e l t i n g of n u c l e a r reactor core is a c h i e v e d .

F o r t h i s t y p e o f p e r f o r m a n c e , n o r m a l p o r t l a n d cement c o n c r e t e s may

p r o v e t o be s a t i s f a c t o r y . T h i s o f c o u r s e r e q u i r e s a c a r e f u l d e t e r - m i n a t i o n o f t h e d e p t h t o which t h e w a l l d i s i n t e g r a t e s , as w e l l a s v a r i o u s d e s i g n m e a s u r e s a l r e a d y m e n t i o n e d , i n c l u d i n g proper r e i n - f o r c e m e n t , a g g r e g a t e s e l e c t i o n , s t e e l f i b e r s , l a t e r a l r e s t r a i n t s , and t r a n s v e r s e t i e s or a n c h o r s .

An i m p o r t a n t c o n s i d e r a t i o n is t h e l i m i t i n g o f t h e re lease o f

h y d r o g e n r e s u l t i n g f rom r e a c t i o n o f sodium w i t h water f r o m t h e c o n c r e t e . From t h i s p o i n t o f v i e w , s e v e r a l m e a s u r e s may b e t a k e n to r e d u c e water loss f rom t h e c o n c r e t e . One method is t o p r e - d r y n o r m a l p o r t l a n d cement c o n c r e t e t o remove f r ee water, b u t n o t water o f h y d r a t i o n w i t h i n t h e c o n c r e t e . A l s o many o f t h e s t r a t e g i e s c i t e d s h o u l d b e u s e d t o c o n t r o l c r a c k i n g . C r a c k i n g is u n d e s i r a b l e s i n c e it p r o v i d e s a d d i t i o n a l s u r f a c e area for c h e m i c a l a t t a c k o f sodium, and creates p a t h w a y s f o r m o i s t u r e w i t h i n t h e c o n c r e t e t o escape.

The u s e o f s u r f a c e t r e a t m e n t s ( w a t e r g l a s s or m a g n e s i a ) may a l s o be

u s e f u l i n l i m i t i n g h y d r o g e n - p r o d u c i n g s o d i u m - c o n c r e t e i n t e r a c t i o n s

by p r o v i d i n g a c h e m i c a l l y - i n e r t , impermeable l a y e r o f mater ia l b e t w e e n c o n c r e t e and sodium. T h i s may r e d u c e p h y s i c a l p e n e t r a t i o n , h e a t i n g , and b r e a k u p o f c o n c r e t e , and a l s o r e d u c e t h e r a t e of ( e x o t h e r m i c ) c h e m i c a l r e a c t i o n b e t w e e n c o n c r e t e and m o l t e n sodium. T h i s p o s s i b i l i t y s h o u l d be e x t e n s i v e l y i n v e s t i g a t e d .

The u s e of f i r e d r e f r a c t o r y c o n c r e t e s , made w i t h a l u m i n o u s c e m e n t s

or n o n - h y d r a u l i c w a t e r g l a s s or p h o s p h a t e b i n d e r s , would e l i m i n a t e t h e need for c o n c e r n a b o u t water g e n e r a t i o n o n h e a t i n g from a s o d i u m s p i l l . However, t h i s ra ises t h e unanswered q u e s t i o n o f how t o p r a c t i c a b l y u n i f o r m l y h e a t a m o n o l i t h i c reactor s t r u c t u r e f rom

temperatures o n t h e o r d e r of s e v e r a l h u n d r e d up to 1000°C.

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E. Summary

In suggesting the foregoing selection, primary attention has been given to material properties important for nuclear reactor struc- tures; in particular good compressive and tensile strength, small thermal expansion, low creep and shrinkage, and, in certain situa- tions, low susceptibility to sodium attack. Other properties, which may be of importance for other applications, have not been emphasized in this selection. These include resistance to large and numerous thermal cycles, abrasion resistance, and resistance to temperatures over ~ O O O ~ C .

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Section 4

PRINCIPAL GAPS IN KNOWLEDGE

Principal gaps in knowledge of material behavior needed to produce a reliable design of nuclear reactor containment structures are briefly outlined. For this purpose the basic materials already discussed are considered.

4.1 PORTLAND CEMENT CONCRETES

Present knowledge is incomplete in regard to the thermal and mechan- ical behavior of concrete exposed to sustained heat for moderate to long duration. Important properties data needed include: creep, the effect of moisture conditions and moisture migration upon creep, calculation of moisture migration, along with associated pore pres- sures and shrinkage deformations, the multiaxial properties of creep, and stress-strain relations in general. Also, very little is known about the fracture characteristics of concrete structures at elevated temperature, as well as the strain rate effects needed to evaluate the consequences of rapid loading. In addition, the thermodynamic relations for the equilibrium of pore water, and equilibrium pore pressures at elevated temperatures and various moisture contents, need to be studied. In addition, some sources suggest that tempera- ture resistance of portland cement concrete can be greatly improved by using refractory aggregates. Very little material properties data exist for heat-resistant portland cement concretes.

When sufficient material properties data become available, identifi- cation and selection of promising materials for LMFBR applications can be made. Subsequently, testing of model or full-scale structural elements, made from selected candidate materials, should be conducted under conditions of load, restraint, and temperature comparable to those found in LMFBR's. Tests should be conducted under both service and accident conditions, where appropriate. Testing of this type will be needed both for preliminary evaluation of structural per- formance of selected materials, and also to serve as a "benchmark"

4-1

against which results of any future numerical structural analyses can be compared and evaluated.

Successful design will also require development of more realistic models for the prediction of thermal stresses in reinforced and prestressed concrete structures, especially those subjected to pro- longed periods at elevated temperatures. The analytical model should account for effects of pore pressures, moisture migration as affected by the concrete thickness (size) and surface sealing con- ditions, as well as the effects of cyclic heating and very rapid heating (thermal shock).

The validity of such analyses could be determined by comparison with data from preliminary structural tests recommended previously.

The ultimate goal of the needed research should be the development of precise rules for the analysis and design of concrete structures for temperatures which exceed those currently permitted by codes for nuclear containments and vessels. Far higher service temperatures than the 150°F limit currently allowed by these codes are possible, but a better knowledge of mechanical properties coupled with much more realistic design and analysis procedures based on structural test data will be needed to allow higher temperature exposures.

4 . 2 HIGH ALUMINA CEMENT (HAC) CONCRETES

The knowledge of mechanical properties of HAC concretes at high tem- peratures is very limited. For some of these materials, not even the compressive and tensile strengths are known over the entire tem- perature range. Strength behavior of HAC concretes is complicated by the deleterious effects of conversion reactions (see Volume 1, Section 3 . 2 . 3 ) . Because HAC loses much of its strength as a result of conversion reactions, it is not permitted as a substitute for portland cement in normal construction in the U . S . However, its residual strength after conversion may be sufficient for structural applications at elevated temperatures. Data on degree of strength loss due to conversion under conditions similar to those in LMFBR's are needed for material evaluation.

4-2

Furthermore, little is known of the effects of temperature on elastic modulus. Almost no data are reported on creep of these materials. Nothing is found in the literature on the migration of water in the pores of these materials, on pore pressures, or sorption isotherms. These are, however, not needed in case of a fired concrete where water has been driven out.

For high alumina concretes with tabular alumina aggregates, no data are available for hot compressive strengths, cold compressive strengths, or porosity before and after firing. For high alumina concretes with corundum aggregates no data exist on hot compressive strengths, on creep and shrinkage, and on porosity changes due to firing. For one promising candidate, namely the high alumina con- crete with chamotte aggregate, sodium-concrete reaction tests appar- ently have not been carried out. Nothing is available on creep and shrinkage as well. For high alumina concretes with refractory light- weight aggregates, data are lacking on hot compressive strengths, on creep and shrinkage, porosity change due to firing, on Poisson's ratio, and on the resistance to thermal shock and spalling.

Mathematical models that have recently been developed for the defor- mation of normal concretes at high temperatures have at present no parallel in the case of refractory HAC concretes. Realistic designs would call for eventual development of models based on preliminary structural test data.

4 . 3 NON-HYDRAULIC CONCRETES

4.3.1 Phosphate-Bound Concretes

Knowledge of the mechanical properties of these refractory concretes is even more limited than that of high alumina cement concretes. Data on hot compressive as well as tensile strengths are lacking. A l s o , data on thermal conductivity, heat capacity, porosity or permeability changes due to firing, sorption isotherms at various temperatures, and moisture permeabilities are all unavailable in the literature. Further, at present no information on sodium-concrete reactivity is available. Information on volume stability of phosphate-bound aggregates is also limited. No creep data are reported in the literature, and for phosphate-bound lightweight

4-3

corundum aggregate concrete the coefficient of thermal expansion is also not given.

Non-hydraulic concretes generally have not been used for load- bearing structural applications. Data on structural behavior of these materials at room and elevated temperatures will be needed before their suitability for LMFBR applications can be assessed or modeled.

With regard to mathematical models needed for prediction of struc- tural performance, the same comments can be made as for high alumina cement concretes.

4 . 3 . 2 Magnesia-Bound and Waterqlass Concretes

Essentially the same comments can be made about these concretes as were made about phosphate-bound concretes.

4 . 4 REFRACTORY BRICK LININGS (MASONRY)

For these materials problems of design are somewhat different. The mechanical properties of refractory bricks are reasonably well known. The inelastic deformations, creep, and moisture sorption effects are relatively insignificant at temperatures below 60OoC. problem in design lies in determining the behavior of masonry struc- tures. This behavior is strongly influenced by inelastic behavior of joints, the moisture absorption and diffusion properties, and possible chemical reaction of any joint binder material with sodium in the event of a sodium spill. Measurements of structural behavior

The chief

obviously require relatively large specimens consisting of many bricks or blocks, which is an expensive proposition.

Nearly all construction practice in chemical processing industry is directly based on experience with refractory brick linings in vessels under service conditions. This experience cannot, however, be easily transplanted to the completely different situations in nuclear reactor structures. Some tests of large specimens consisting of many bricks would be highly desirable to determine irreversible deformations, response to multiaxial loading, creep, shrinkage, and thermal expansion.

4-4

However, many aspects could probably be determined using small-scale masonry models with bricks of a much reduced size. To some extent it should also be possible, and certainly much cheaper, to develop mathematical models for this behavior once the joint material behavior is well understood.

4.5 MATERIALS FOR RADIATION SHIELDING

On the subject of gamma radiation shielding, it should be noted that information on shielding characteristics of refractory concretes or brick systems is very scarce. Some data may be found for certain concrete components, such as zirconia or alumina minerals, however, shielding capability of most refractories has not been measured.

This does not mean refractories have no shielding capability. It may be that certain refractory systems incorporating dense aggre- gates are effective as primary gamma radiation shields. However, this will have to be established by future experimental evidence.

4 . 6 ADMIXTURES AND SURFACE TREATMENTS

Improvements in performance of castable materials for LMFBR applica- tions may be realized by the use of various fiber admixtures or sur- face treatments. For instance, steel fibers are often added to refractory concretes used in furnace linings to improve resistance to structural cracking and increase liner durability. The use of steel or ceramic fibers may also offer improvements in performance of concretes for various LMFBR applications. T h e effectiveness of fiber reinforcement s h o u l d be investigated as an adjunct to tests of concrete materials suggested previously.

Surface treatments are commonly used in concrete construction for severe chemical environments. Surface treatments are used to seal concrete surfaces. This reduces permeability and protect against attack from deleterious chemical reagents. It may be possible to improve resistance of portland cement or other concretes to sodium attack by using one of these surface treatments.

Materials used in surface treatment of portland cement concrete include sodium silicate (waterglass), magnesium oxide (MgO) , carbon

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d i o x i d e , and v a r i o u s f l u o r i d e s . ( V o l . 1, Ref . 2 4 0 ) . T h e i r e f f e c - t i v e n e s s as b a r r i e r s t o mol ten s o d i u m a re unknown, b u t s h o u l d be

i n v e s t i g a t e d as par t of s t u d i e s o f sodium r e s i s t a n c e o f c o n c r e t e s 1 is t e d pr ev i o u s l y .

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Section 5

RECOMMENDATION FOR FURTHER RESEARCH

Based on the preceding considerations an outline is given of some recommended further research. This work is needed to obtain suffi- cient additional material properties and structural behavior data to evaluate selected promising materials for LMFBR's, or other nuclear applications.

5.1 EXPERIMENTAL WORK

Phase A - Material Properties Basic data on strength and deformation properties as a function of temperature are needed for most refractory concrete materials, par- ticularly for non-hydraulic concretes. Effects of conversion reac- tions on strength and elastic properties of HAC concretes needs to be quantified. Values for residual strength at elevated temperatures need to be determined for assessment of suitability for structural applications in LMFBR's. Strength, fracture behavior, and deforma- tion properties as a function of temperature are also needed for portland cement concretes incorporating refractory aggregates. In addition, effects of fiber reinforcement on concrete mechanical properties at elevated temperatures needs to be examined.

Further testing is needed on compressive, tensile, and triaxial strengths as well as fracture behavior under various thermal condi- tions (hot, cold, reheated, cyclic heating) and various moisture conditions (free moisture escape, both during and after the moisture escape, and where moisture is prevented from escaping). This needs to be supplemented by measurements of thermal expansion, shrinkage, and thermal properties during various typical temperature histories. This data is generally unavailable for refractory concretes, and very limited for portland cement concretes.

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For nuclear structural applications, the triaxial stress-strain relations for selected materials, and their time and temperature dependence need to be known. The influence of temperature on strain-rate sensitivity of concrete stiffness also needs to be determined. Effects of moisture content on thermal and mechanical behavior and its variation, must also be determined. The knowledge in this area is incomplete for portland cement concretes, and completely lacking for high alumina cement concretes and other refractory systems.

It is therefore recommended that appropriate tests be conducted to determine deformation under constant uniaxial and triaxial stress states at elevated temperatures, measuring creep under sustained load. These tests should be carried out on a concrete that has lost its water due to heating and on concrete that was prevented from losing water (as in the case for concrete in the interior of a mas- sive wall at the beginning of heating). Finally, tests should be made on concretes where water content is varied. These tests should be carried out in such a way that the effect of microcracking on the measured deformations can be eliminated and constitutive properties of these materials can be identified.

Refractory concrete or masonry, or portland cement concrete with refractory aggregates, have not reportedly been used for biological shielding from gamma radiation. Based on their high specific gravi- ties, however, a number of dense refractory aggregates may prove effective in attenuating gamma radiation.

Dense refractory bricks may be the most effective system for "anhydrous" radiation shielding, but dried concretes made with water- glass or phosphate binders and dense refractory aggregates also look promising. Portland cement and HAC concretes may also be adequate if free water is removed by heating. Shielding capabilities of these concretes made from dense refractory aggregates should be investigated in comparison tests with normal weight and heavyweight portland cement concretes of the type currently being used for bio- logical shielding in nuclear facilities. Promising refractory aggregates include: zirconia, silicon carbide, magnesia, chrome ore, and dense alumina (corundum or tabular alumina).

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Molten sodium and molten core containment present the most severe environmental conditions to be faced by materials used in LMFBR's. Some data exists for molten sodium reactivity of various refractory concrete aggregates and masonry. However, data are needed on sodium resistance of liners made with refractory bricks using various mortars as binders, and also using dry joint construction. Also, very little data are available for refractory concretes. Tests of selected refractory concretes exposed to molten sodium at most severe temperature conditions (87OOC) are needed. Tests should evaluate the effects on permeability of physical penetration by molten sodium. A l s o the resistance to chemical attack of the exposed material should be investigated.

Further, the possibility exists that sodium resistance of normal portland cement concrete or susceptible refractory concretes may be improved by use of surface treatments of concrete wall/floor assem- blies. A number of surface treatments for concrete are currently used in severe chemical environments applications. These include: impregnation with waterglass, magnesia, or alumina, and treating concrete with carbon dioxide or various fluorides to harden and reduce permeability of the concrete surface. Effects of these sur- face treatments in limiting sodium-concrete interaction should be carefully studied.

Any improvement in sodium resistance of castable materials would provide greater flexibility in choice of containment materials and increase structural integrity under hypothetical accident conditions. Thus, surface treatments may provide a low-cost alternative to use of refractory masonry for adequate sodium containment and protection of containment structure. Tests could be conducted on portland cement concretes with and without surface treatments. Comparison tests could be made using refractory masonry materials.

Use of surface treatments for fired and unfired refractories may also be effective for enhancing resistance to sodium spills. However, no data is available on the effectiveness of these surface treatments for improvement of chemical resistance of refractory concretes. Treated and untreated refractory concretes should be tested under

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sodium exposure conditions similar to those recommended above for portland cement concretes.

Phase 3 - Structural Performance After material properties data are determined, it should be possible to identify and select promising materials for various LMFBR

applications. It will then be necessary to evaluate the structural performance of selected materials under conditions simulating those found in LMFBR's , including load, restraint, and temperature.

Depending on the materials selected, the amount of structural testing needed will vary. Since much data already exist on ordinary portland cement concretes, structural testing needed would not be as extensive as for refractory concretes.

Tests should examine structural load capacity, deformation, shrink- age, expansion, and thermal properties of structural elements repre- sentative of those found in LMFBR's . Tests should be conducted under normal service conditions (sustained elevated temperatures), and also under upset conditions (transient heating or cooling), where appropriate.

Also, as mentioned earlier, the use of metal, glass, or ceramic fibers may be a useful method to improve the toughness and crack resistance of concrete for high-temperature applications. Testing of structures made from promising castable materials under high temperature exposures should include an assessment of the addition of fiber-reinforcement to improve high-temperature performance.

The purpose of structural testing is two-fold. First, it will be needed for preliminary evaluation of structural performance of selected materials. That is, structural testing is needed to deter- mine if the promising material can be used to build a structure capable of performing adequately in an LMFBR environment. Second, results of structural testing, together with material properties data, can serve as the basis for numerical modeling of the response of the structure in an LMFBR. Structural tests data can be used as a "benchmark" against which results of numerical models can be compared and evaluated.

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5 . 2 THEORETICAL WORK

P h a s e A

The t h e o r e t i c a l w o r k s h o u l d b e g e a r e d t o d e t e r m i n i n g t h e p h y s i c a l mechanisms o f mater ia l d e f o r m a t i o n , c r a c k i n g , f a i l u r e , and f r a c t u r e . A l s o t h e d e t e r m i n a t i o n o f a s t r e s s - s t r a i n r e l a t i o n s h i p t h a t c a n be

u s e d i n n u m e r i c a l a n a l y s i s o f n u c l e a r s t r u c t u r e s or s u p p o r t s is r e q u i r e d . C o n s i d e r a b l e e f f o r t i n t h i s d i r e c t i o n h a s a l r e a d y b e e n made f o r p o r t l a n d cement c o n c r e t e s , b u t a d d i t i o n a l w o r k is s t i l l n e e d e d . For e x a m p l e , a s u i t a b l e t i m e - d e p e n d e n t s t r e s s - s t r a i n re la- t i o n (creep) u n d e r t h e c o n d i t i o n s of v a r i a b l e m o i s t u r e c o n t e n t and v a r i a b l e t e m p e r a t u r e s need t o be d e t e r m i n e d . E x i s t i n g t e s t s and t h e o r e t i c a l w o r k a re s u f f i c i e n t o n l y for d e s c r i b i n g t h e b e h a v i o r of d r i e d c o n c r e t e t h a t h a s b e e n a l l o w e d to f r e e l y lose i t s m o i s t u r e upon h e a t i n g .

For h i g h a l u m i n a and o t h e r r e f r a c t o r y c o n c r e t e s , w o r k n e e d e d is much more e x t e n s i v e . E s s e n t i a l l y a l l aspects o f s t r e s s - s t r a i n and f r a c - t u r e b e h a v i o r u n d e r s h o r t - t i m e and s u s t a i n e d l o a d s a t h i g h tempera- t u r e and v a r i o u s moisture c o n d i t i o n s h a v e t o be d e v e l o p e d .

S u b s e q u e n t l y , it is n e c e s s a r y t o f o r m u l a t e a n a l y t i c a l methods f o r t h e a n a l y s i s o f t h e e f f e c t s of a n t i c i p a t e d l o a d i n g s and a c c i d e n t c o n d i t i o n s o n t h e s t r u c t u r e . Such n u m e r i c a l or a n a l y t i c a l methods would b e based o n s t r e s s - s t r a i n r e l a t i o n s d e t e r m i n e d e x p e r i m e n t a l l y f o r p o r t l a n d c e m e n t or h i g h a l u m i n a cement c o n c r e t e . The methods s h o u l d a l so a c c o u n t f o r t h e d i f f u s i o n o f moisture caused by h e a t i n g of t h e s t r u c t u r e .

P h a s e B

Of g r e a t e s t p r a c t i c a l i m p o r t a n c e is t h e d e v e l o p m e n t o f c l ea r and precise g u i d e l i n e s u n d e r which p o r t l a n d cement c o n c r e t e s t r u c t u r e s c a n be allowed to be h e a t e d a b o v e 150°F. T h i s l i m i t is s p e c i f i e d by t h e c u r r e n t ASME 1 1 1 - 2 c o d e ( V o l . 1, Ref. 2 1 ) . T h i s c o d e does n o t p r o h i b i t t e m p e r a t u r e e x p o s u r e s beyond 150°F, b u t s t a t e s t h a t t h e d e s i g n e r m u s t when e x c e e d i n g a t e m p e r a t u r e of 150°F, p r o v e h i s

d e s i g n by a d e q u a t e tes t d a t a and special a n a l y s i s . The r u l e s f o r s u c h a n a n a l y s i s s h o u l d b e d e v e l o p e d s u c h t h a t t h e r e q u i r e d i n d i v i d u a l s t eps f o r h a n d l i n g d e f o r m a t i o n s , c r a c k i n g or f r a c t u r e ,

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spalling, effects of reinforcement, restraint, pore pressure and moisture movement, and other significant variables are specified.

The alternative use of refractory concretes or masonry for nuclear applications would require development of entirely different design procedures for high temperature exposure conditions. For these materials, it is unlikely that requirements of the current ASME code would be applicable.

Any proposed use of refractory systems in place of portland cement concrete in nuclear containment applications will require thorough experimental determination of material properties, and detailed analysis of structural behavior. Proposed design procedures developed for refractory systems would have to be verified by testing of structural components under conditions simulating the service environment. These steps would also be required for any design procedures for portland cement concrete containments operat- ing above temperature limits imposed by the current nuclear code.

Comments on Research Direction

The preceding exposition involves a great amount of work to be done. Since the research funds are always limited, the choice of the best research direction is, therefore, extremely important.

In this regard, we should recognize that experimental work is nor- mally much more expensive than theoretical work. For this reason, one should not embark on expensive testing where data could be col- lected from the literature, even if those data are not as perfect as one would desire. This is particularly important to keep in mind for portland cement concretes, for which rather extensive data already exists in the literature. Most of them are, however, not being used for designing structures for high temperatures, since the experimental results are not understood and have not been adequately explained with a coherent theory. In this area, the theoretical research might be more cost-effective. The first attempt should be to develop physically realistic and experimentally justified mathe- matical models of material behavior on the basis of the existing test data, before acquiring extensive additional test data.

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With regard to model building, it should be remembered that only those models which explain the experimental evidence over its complete range and which are based on a rational and consistent concept of the physical mechanisms involved in the microstructure, are useful. Much too often, mathematical models and numerical simulations are being developed to explain only some data, dis- regarding others and neglecting to base the model on physical processes in the microstructure. There is little need for such models, even less than for duplicative material testing.

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Section 6

SUMMARY

The present report analyzes existing knowledge on the use of con- cretes, especial refractory concretes, for nuclear reactor structures exposed to high temperatures, with particular reference to liquid metal cooled fast breeder reactors (LMFBR's). First, some basic properties such as strength, deformation capacity, thermal expansion, and insulating properties, as well as resistance to sodium attack, where available, were analyzed and compared for various concretes. Subsequently, advantages and disadvantages of these materials under various types of thermal exposure were summarized and discussed. Where comparative data were available these materials were ranked according to their performance in various situations. This led to identification of several materials that appear to be of greatest interest and potential for LMFBR applications.

From this analysis, which draws from information compiled in Volume 1, it was found that, for moderately high temperatures up to about 4OO0C, normal portland cement concretes appears to be a suitable material, provided that appropriate analysis and design procedures are developed. For higher temperatures, the use of high alumina cement concretes with certain types of aggregates appears to be a promising alternative material, provided that these materials can be prefired to develop ceramic bonds. The structure would best take the form of an assembly of fired blocks, plates or bricks fired above 1000°C, with joints that are either dry or made with a thin layer of refractory mortar. For very high temperature exposures, as well as for the conditions of sodium spill, use of refractory brick layers seems to be the most promising system.

(I

The consequence of this analysis, and the principal gaps in existing knowledge, were identified and briefly outlined. The report was concluded by a recommendation for immediate research efforts.

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In summary, it may be stated that normal portland cement concretes can serve at moderately high temperatures provided that proper design methods are developed. At very high temperatures or under conditions of sodium attack the use of refractory concretes as well as refractory bricks, and to some extent also portland cement con- cretes with refractory aggregates, appears to have a great potential.

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