HW-69234 UC-25, Metals, Ceramics and Materials (TID-4500, 24th Ed.) IRRADIATION EFFECTS ON URANIUM DIOXIDE MELTING By J. A. Christensen Ceramics Research and Development Reactor and Fuels Laboratory Hanford Laboratories March 1962 ft»T UiiESTRICTED | « . | „ »r» »iS1!?BWB.»f W9E J i l l 0 0' HANFORD ATOMIC PRODUCTS OPERATION RICHLAND, WASHINGTON Work performed under Contract No. AT(45-1)-1350 between the Atomic Energy Commission and General Electric Company Printed by/for the U. S. Atomic Energy Commission Printed in USA. Price $0-75 Available from the Office of Technical Services Department of Commerce Washington 25, D.C.
Transcript
HW-69234
UC-25, Meta ls , C e r a m i c s and Mater ia l s
(TID-4500, 24th Ed . )
IRRADIATION EFFECTS ON URANIUM DIOXIDE MELTING
By
J . A. Chr i s tensen
C e r a m i c s R e s e a r c h and Development Reactor and Fue ls Labora tory
Hanford Labora tor ies
March 1962
ft»T UiiESTRICTED | « . | „ »r» »iS1!?BWB.»f W9E J i l l 0 0 '
HANFORD ATOMIC PRODUCTS OPERATION RICHLAND, WASHINGTON
Work performed under Contract No. AT(45-1)-1350 between the Atomic Energy Commiss ion and Genera l E lec t r i c Company
Pr in ted by/ for the U. S. Atomic Energy Commiss ion
Pr in ted in USA. P r i c e $ 0 - 7 5 Available from the Office of Technica l Serv ices Department of Commerce Washington 25, D . C .
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.
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ABSTRACT
Irradiated uraniura dioxide with exposures to 11. 25 at.% uranium burnup has been examined microscopically at temperatures to 3000 C. Melting temperatures were measured and mass vaporization rates compared for UO„ with 14 different exposures.
I r radia t ion Effect on Melting Point . . . . . . . 9
Vaporizat ion and Crysta l Growth of UO„ . • . . . 11
I r radia t ion Effect on Volatili ty and Crys t a l Growth . . 11
DISCUSSION . . . . . . . . . . . . . . . . 15
Chemical Stabilization by I r radia t ion . . . . . . 15
Location of the Liquidus Maximum . . . . . . ' 15
Hypothetical High Tempera tu re Phase Relat ionships 17
I r radia t ion Effects on the Melting P r o c e s s . . . . . 18
APPENDIX . . . . . . . . . . . . . . . . 21
BIBLIOGRAPHY . . . . . . . . . . . . . . . 23
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IRRADIATION EFFECTS ON URANIUM DIOXIDE MELTING
INTRODUCTION
Uranium dioxide is frequently exposed to t e m p e r a t u r e s approaching
the melt ing point in r eac to r fuel a s s e m b l i e s . The melt ing t empera tu re and
vaporizat ion cha rac t e r i s t i c s of UO„ must be known to in te rp re t the a l t e r a
tion in fuel s t ruc tu re caused by i r rad ia t ion . The p r i m a r y objective of
this work was to m e a s u r e the melt ing point of UO„ after exposure to a
var ie ty of neutron doses . In addition, vaporization r a t e s and the appea r
ance and growth r a t e s of vapor deposited UO„ c rys t a l s were studied.
Hanford 's study of UO„ vaporizat ion and c r y s t a l growth has i ts
origin in the work of Bates and Newkirk who descr ibed the rapid growth (1 2) of UO„ c rys t a l s from the vapor . ' Modifications of thei r equipment and
techniques were used to m e a s u r e the melt ing t empe ra tu r e of UO„ with
i n - r eac to r exposures from 0. 005 to 11.3 at . % uranium fissioned. Mel t
ing point r esu l t s and observat ions of i r rad ia t ion effects on vaporizat ion
and c rys t a l growth a r e repor ted here-
SUMMARY
The melt ing point of UO„ i n c r e a s e s with i r rad ia t ion exposure to a
maximum of 2915 C at 0. 09 at .% burnup. This i s 125 C above the mel t ing
point of nonir radia ted UO^. A gradual dec rease to 2800 C at 1. 2 at . %
burnup and to 2660 C at 11. 25 at. % burnup follows the maximum. Mass
vaporizat ion ra t e s seem to vary inversely with mel t ing point a s a function
of neutron dose . The change in vaporizat ion ra te is a reflect ion of the
melt ing point change, and, perhaps , a dependence of vapor p r e s s u r e upon
i r radia t ion .
Apparatus developed for these m e a s u r e m e n t s is general ly applicable
to microscopic examination of any hazardous m a t e r i a l at t e m p e r a t u r e s to
3400 C and magnifications to lOOX.
HW-69234
APPARATUS
Specimens were heated in a water -cooled r e s i s t ance furnace (3) (Figure 1) by d i rec t contact with the tungsten element- The power
supply was a manually controlled 4 kva, 5 to 30 v, step-down t r a n s
fo rmer . A flowing, 15 psia helium a tmosphere was maintained during
each heating cycle . Purging the furnace for 15 min with a 3 l i t e r / m i n
flow of helium provided an a tmosphere with less than 0 .01% each of
0 „ , N_, and HpO, . . The effluent gas passed through a charcoa l fil ter
submerged in liquid nitrogen to remove radioact ive contamination.
The furnace was installed on a un iversa l mount in a glove box 3 made from ^ in. Lucite and plywood (Figure 1). Lead impregnated,
0.030 in. thick neoprene gloves were used for operat ions within the
glove box. Twenty to 100 mg specimens of i r rad ia ted UO„ with unit
gamma act ivi t ies to 2500 r / h r - g were examined in this facility with no
excess ive personnel exposure .
Specimens were observed through a microsope located outside
the glove box. This ins t rument , which is commerc ia l ly available as a
complete unit, has a working distance of 21. 3 cm and a numer i ca l a p e r
ture of 0. 10. A t r i p l e - sp l i t - beam eyepiece permit ted s imultaneous
viewing, photography, and photometry of incandescent spec imens .
T e m p e r a t u r e s were measu red with an optical py romete r ca l i
bra ted against the known melt ing points of high purity copper, nickel ,
a lumina, molybdenum, and tantalum. A prec is ion of ± 30° (95% confi
dence) for an individual measu remen t was achieved between 1000 C and
3000 C.
MATERIALS
I r rad ia ted UO„ was obtained from test capsules with fuel exposures
from 0. 005 at. % to 11.3 at .% uranium burnup. Brief i r rad ia t ion and fab
r icat ion h is tor ies a r e given in Table I.
ammm^
mmm €ftl»ERA
TO PMQTOMETEH /
OPTienL ptmmEnn
mm FOCAL immn ' mmsmm Mtm
iWRASM
45* FIIONT SURFACE ftOB
EMHAUST 3«*l.«CfTE SiO¥E BOM
/ FWSNACe
*., .̂ l ^ f ^ v M l
Tf. 4 KVA
.f! ' > y ^ i
\ ", . y
FURNACE EXHf tUST
CHARCOAL TRAP
^ ,/* ^ I
,F
UWIVERSAL FtJftSACE MOUNT
MOVfttiE CASllASt
CA3 I
rUBNACE COOiAiHT
ATMOSPHERE SUPPLt
COO
PjgMACg i g m i L \
e EXHAUST
FIGURE 1
Apparatus F o r High Tempera tu re Examination of Radioactive Mater ia ls
I
CD to
HW-69234
T \BLE I
UO^ IRRADIATION i\ND FABRICATION HISTORIES
Irradiat ion 0 005 at % L burnup at tempera ture less than 100 C
0 014 at % U burnup at tempera ture lebfa than 100 C
Fabricat ion
94% dense extruded, hydrostatically pressed , and sintered, 1/4 m diam rods micronized, natural , UO„ powder
91% dense, extruded, hydrostatically pressed , and sintered, 1/4 m diam rods micronized, natural , UO„ powder
0 021 at % U burnup at tempera ture less than 100 C
87% dense, extruded, hydrostatically pressed , and sintered, 1/4 m diam rods, micronized, natural UO„ powder
0 051 at % U burnup at temperature less than lOOC
92% dense, extruded, hydrostatically pressed , and sintered 1/4 m diam rods, micronized, natural , UO„ powder
0 09 at % U burnup with core tempera tures to 1750 C
93% dense, p ressed , and sintered pellets PWR type, natural , UO„ powder
0 11 at % U burnup with core tempera tures to 2600 C
80% dense, loosely packed, fused, natural UO„ O U ratio uncertain
0 17 at % U burnup with core tempera tures to 2800 C
0 20 at % U burnup with core tempera tures to 2100 C
80% dense, p ressed , s in tered, and crushed, loosely packed PWR type, 1 6% enriched UO„ powder
97% dense, pressed , and sintered pellets, PWR type, powder
natural , UOr
0 38 at % U burnup with core tempera tures to 1800 C
86% dense, p ressed , s in tered, and crushed PWR type, natural , UO„ powder
1 2 at % U burnup with core t empera tures to 2800 C
84% dense, pressed , s intered, and crushed PWR type 2 44% enriched powder
8 4 at % U burnup with core t empera tures to 1200 C
P re s sed and s intered plates, 37% enriched UO„
11 3 at % U burnup with core tempera tures to 1200 C
Pressed and s intered plates, 53% enricbed UO„
•
UO„ with 8 4 and 11 3 at % burnup were supplied by Bettis Atomic Power Laboratory
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Oxides with burnups to 1. 2 a t .% were s in tered in hydrogen for 12 hr
at 1750 C and had 0:U ra t ios between 2.00 and 2 .01 before i r rad ia t ion .
Ten g r a m s of each i r rad ia ted m a t e r i a l (as -10 -t-30 mesh par t ic les) were
obtained for melt ing point de terminat ions and high t empera tu re photo
micrography .
PROCEDURE
(4)
Mendenhall V type tungsten f i laments were used for melt ing
point de terminat ions . These w e r e - j in. wide, 0.005 in. thick s t r i p s with
a 15° "V" in the cen te r . The s tandard procedure for measur ing a mel t ing
point was to; (1) place the specimen in the filament V and purge the furnace with
he Hum
(2) r a i s e the t empera tu re at a ra te of approximately 50° / s ec until the specimen, as seen through the mic roscope , melted
(3) with the pyromete r , m e a s u r e the t empera tu re of the tungsten V at which the specimen began to mel t at the tungs ten-spec imen in ter face .
Two ope ra to r s were required—one to inc rease the t e m p e r a t u r e and observe
the spec imen and the other to simultaneously m e a s u r e the t e m p e r a t u r e . A
minimum of t h r ee , and usually at least five, mel t ing point m e a s u r e m e n t s
were made for each m a t e r i a l examined.
Motion or s t i l l p ic tu res were taken through the mic roscope as
each spec imen was heated to melt ing. These p ic tures r eco rd the vapor i za
tion and c r y s t a l growth cha rac t e r i s t i c s of UO„ with var ious i r rad ia t ion
exposures . Still p ic tu res were taken at each of th ree t h e r m a l a r r e s t s
made in the course of heating one specimen from each sample of i r r a
diated oxide. These p ic tures were at 2400 C, 2750 C, and the mel t ing
point with holding t imes of 1 min at each t e m p e r a t u r e .
I r rad ia ted and nonir radia ted UO^ were compared di rect ly by s imu l
taneously heating a specimen of each placed side by side on a common
tungsten filament. This technique provided the most p rec i se indications of
i r rad ia t ion effects on both mel t ing point and m a s s vaporizat ion r a t e s .
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EXPERIMENTAL VARIABLES
Effects of var ia t ion in oxide composition and heating ra te on
mel t ing and vaporizat ion cha ra c t e r i s t i c s were studied to es tabl i sh
exper imenta l p rocedures which sepa ra t e i r rad ia t ion effects on mel t ing
behavior from extraneous effects.
Oxide Pur i ty
Varia t ions in UO„ composit ion can cause significant differences
in mel t ing behavior . F o r example, the addition of 0. 4 wt% TiO^ or
NbpOp. to UO„ causes the shape of vapor deposited c rys t a l s to change from
well defined te t rahedrons to i r r e g u l a r globules and also resu l t s in a me l t -(5)
ing range r a the r than a d i sc ree t melt ing point. Poss ib le sources of
impur i t i es which could influence the observat ions include (1) the annealing
a tmosphere , (2) the tungsten filament, and (3) var iab le ini t ial oxide com
position.
1. Atmosphere Pur i ty
Varying the oxygen and nitrogen content of the helium a tmosphere
from <0. 01% to 0. 18% and <0. 01% to 0.67% for O^ and N respect ive ly
did not affect e i ther the melt ing point or the vaporizat ion and r e d e -
position cha rac t e r i s t i c s of UO„. An a tmosphere with 0. 18% Op caused
excess ive oxidation of the tungsten fi lament.
2. Tungsten-UO^ Reactions
UO„ and tungsten r eac t slightly at 2800 C in time per iods of a (6) week or l e s s . The react ions which occur red between f i laments
and UO„ specimens yielded a thin, re f rac tory product which formed
between the UO„ and the tungsten and seemed to inhibit further at tack.
Nei ther the p resence of the react ion product nor of WO^ formed by
react ion between the tungsten fi lament with oxygen in the furnace
a tmosphere had an effect upon UO„ melt ing behavior .
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3. Initial Oxide Composit ion
UO„ composit ions between UO^ Q„ and UO„ „_ behaved s imi l a r ly
when heated at 50 C / s e c to the UO„ melt ing point. During heating,
hypers to ichiometr ic oxides d issocia ted to UO„ which remained stable
to the melt ing point. The substoichiometr ic oxides were two-phase
at room tempera tu re with uranium me ta l inclusions in a UO„ ^.n.
matr ix . They appeared to mel t at the same t empe ra tu r e as UO„ „„.
Since the same a tmosphere composit ion was used for a l l heating,
it is reasonable to a s sume that the UO„ e i ther dissociated or oxidized
to the equil ibrium composit ion corresponding to the par t ia l p r e s s u r e
of oxygen in the a tmosphere and the melt ing t e m p e r a t u r e . The com
position of the oxide as it mel t s is then dictated, not by the ini t ial
composit ion, but by the oxygen p r e s s u r e in the furnace a tmosphe re .
Heating Rate
A heating ra te of approximately 50 C / s e c was chosen for mel t ing
point m e a s u r e m e n t s of both i r rad ia ted and uni r rad ia ted UO„. The me l t
ing point of composit ions between UO„ „^ and UO„ „ . was constant and
independent of heating ra te in the range 1000 C / s e c to 1000 C / h r . These
heating r a t e s provide sufficient t ime for the UO„ to dispropor t ionate ^ T" X.
to an oxygen-r ich vapor and a UO^ res idue which subsequently me l t s at
the UO„ melt ing point.
RESULTS
UO^ Melting Point
A mel t ing point of 2790 ± 20 C (95% confidence) was m e a s u r e d
for UO„. The p r i m a r y s tandard was 95% dense UO„ p r e s s e d and s in tered
in hydrogen for 12 hr at 1750 C Composit ion of the s tandard before and
C r <.2 < 2 Cu 2 2 F e 10 15 Ge <,0.5 < 0 . 5 Mg < 1 <. 1 Mn < 1 < 1 Ni <5 <5 P .-,100 <100
Pb 2 5 Si -^10 14 Sn < 0 . 5 - 0. 5 T l . .200 <200 V <;50 <50
Zn . 5 0 ..,50
0 : U r a t i o s w e r e d e t e r m i n e d by c o n t r o l l e d po t en t i a l c o u l o m e t r y which d o e s not p e r m i t m e a s u r e m e n t s below 2. 00.
A p p r o x i m a t e l y 50% of the 100 m g UO„ s p e c i m e n s was los t a s v a p o r
d u r i n g fus ion . T h i s r a p i d v a p o r i z a t i o n o b s c u r e s the m e l t i n g point and i n t r o
d u c e s d i f f icu l t ies m i t s m e a s u r e m e n t not e n c o u n t e r e d with l e s s vo l a t i l e
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c o m p o u n d s . R e p o r t e d v a l u e s for the UO„ m e l t i n g point r a n g e f r o m 2200 (7 12)
to 2900 C ' wi th the l ower f i gu re p r o b a b l y r e s u l t i n g f r o m m i s t a k e n
i n t e r p r e t a t i o n of r a p i d v a p o r i z a t i o n a s m e l t i n g . R e c e n t r e s u l t s a r e c o n
c e n t r a t e d a r o u n d 2800 C which is g e n e r a l l y a c c e p t e d a s the m e l t i n g point
UO m e l t e d a t 2400 ± 100 C when hea ted a t a r a t e of 1200 C / s e c
A 4 s e c h e a t i n g cyc l e ( r o o m t e m p e r a t u r e to 2400 C to r o o m t e m p e r a t u r e )
d e c r e a s e d the O U r a t i o f r o m 2. 24 to 2. 07 . When UO„ „ . w a s r a p i d l y
b r o u g h t to 2400 C, it fused and then r e s o l i d i f i e d a s l o s s of oxygen y i e lded
the m o r e r e f r a c t o r y UO„ „.
I r r a d i a t i o n Effec t on Mel t ing P o i n t
Low i r r a d i a t i o n e x p o s u r e s c a u s e d a UO„ m e l t i n g point i n c r e a s e
of up to 130 C ( F i g u r e 2). A r a p i d i n c r e a s e f r o m 2790 C a t z e r o b u r n u p
to 2920 C at 0. 05 a t . % b u r n u p o c c u r r e d fol lowed by an e s t i m a t e d m a x i
m u m of 29 30 C at 0. 09 a t . % b u r n u p . F o l l o w i n g the m a x i m u m , a g r a d u a l
d e c r e a s e m m e l t i n g t e m p e r a t u r e wi th i n c r e a s i n g e x p o s u r e o c c u r r e d . At
1. 75 a t . % b u r n u p , the c u r v e is n e a r l y h o r i z o n t a l and h a s d e c r e a s e d to
2800 C .
2700
2600 0 8 1 0
Percent Lranmm 1 ibbioned
FIGURE 2
Melting Point vs In-Reactor Exposure for I r rad ia ted UO,
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The melt ing point of UO decreased to 2760 and 2660 C during
i r radia t ion to 8. 4 and 11. 3 at. % burnup, respec t ive ly . Plott ing these
values in F igure 2 resul ted in the point of inflection at approximately
7. 5 at . % burnup followed by a rapidly increas ing r a t e of melt ing point
dec rease with additional i r rad ia t ion .
Melting point data for the oxides with 0. 005, 0. 014, 0. 021, and
0.051 at .% burnup a r e mos t significant. The samples were al l p repa red
from the same lot of powder, fabricated by identical methods , and i r r a
diated at t empera tu re s l ess than 100 C in the sanae r eac to r for varying
lengths of t ime . Calculated and measu red exposures for these m a t e r i a l s
agreed to within l ess than 10%. The melt ing points of these samples
es tabl ish the order ly , increas ing pat tern leading to the maximum at
0. 09 at .% burnup.
UO„ with 0. 09 and 0 .11 at . % burnup had melt ing points well below
the t rend establ ished by the lower burnup oxides. The oxide with 0. 11 at .%
burnup was the only sample of fused UO„ examined. The 0:U ra t io of this
m a t e r i a l before i r rad ia t ion var ied from 1. 95 to 2. 05 with mos t r e su l t s
below 2.00. If the i r rad ia ted specimens were subs to ich iomet r ic , they
might be expected to mel t a t the same t e m p e r a t u r e s a s noni r radia ted UO„
as explained in the appendix. The oxide with 0. 09 at .% burnup should
have mel ted at 2930 C if the extrapolat ion of low burnup r e su l t s is c o r r e c t .
Instead, it mel ted at 2840 C, 50 C above the mel t ing point of noni r radia ted
UO„. No explanation for this depar ture is available, except a possible
e r r o r in burnup m e a s u r e m e n t .
An ana lys is of var iance of a l l data showed that, at the 99% level
of significance, a definite change in melt ing point resu l ted from i r rad ia t ion .
A s imi l a r analysis consider ing only data for nonir radia ted UO„ and for
UO„ with 0. 051 at .% burnup showed a significant mel t ing point inc rease at
the 99% level. This enhanced t he rma l stabil i ty was not influenced by the
ra te at which the melt ing point was approached. Also, s eve ra l high me l t
ing samples were heated repeatedly to the t e m p e r a t u r e at which the UO„ in
contact with the hot filament fused with no noticeable change in mel t ing
point.
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Vaporization and C r y s t a l Growth of UO„
Because of its high vapor p r e s s u r e , UO„ subl imes rapidly at
t empera tu re s near the melt ing point. The vapor p r e s s u r e at 2750 C is (13) es t imated to be 50 m m of m e r c u r y . The vapor from a UO„ specimen
heated in a tungsten wedge is rapidly chilled by the cool furnace a tmosphe re ,
thus a supersa tura ted UO„ vapor is formed above the spec imen. P a r t of
this vapor redepos i t s on cooler port ions of the specimen and filament and
the balance plates out on the furnace wal l s . The t h e r m a l gradient in the
specimen is toward the port ions far thest from the hot filament, thus mos t
of the redeposi ted vapor solidifies on the re la t ively cool, ou termost s u r
faces of the UO„ specimen. A cyclical p rocess occurs in which UO„
vapor izes rapidly from the spec imen surface in contact with the hot fi la
ment and part ia l ly redepos i t s on cooler specimen su r faces . During this
p r o c e s s , convex port ions of the cooler UO„ tend to become more convex
because of a g rea t e r heat t r ans fe r efficiency. As the convex port ions
expand, they as sume regu la r , pointed shapes , the points growing at the
expense of flat s ides because UO„ molecules can a r r i v e at a point from (14) any direct ion. The resul t ing vapor deposited c rys t a l s have r egu la r
dendrit ic shapes and a r e t rue single c rys ta l s with a pronounced p re fe r r ed
orientat ion. Crys t a l s grown from UO„ vapor have the i r (111) direct ion i 1 !^\ 1
para l le l to the growth direct ion. C rys t a l growth r a t e s of- j in. /min
a r e common from UO„ at 2750 C. Typical vapor deposited UO„ c ry s t a l s
a r e shown in F igure 3.
I r radia t ion Effect on Volatility and C r y s t a l Growth
Mass vaporizat ion r a t e s of i r rad ia ted UO„ appeared to vary
inverse ly as the mel t ing point as a function of neutron dose . Neither the
m a s s nor the surface a r e a of each specimen was measu red , thus quan
titative vapor p r e s s u r e data cannot be formulated. The spec imens were ,
however, approximately the same size and shape so that any g ro s s dif
ferences in volatili ty were d i sce rnab le . Specimens with exposures l ess
12- HW-69234
^ ^ . ^^'
-'^;»p-.: - i j -M
• - • f t ' • '
'A^-
•••• r<S -vt^^^Jv^Sfes^^
is.̂ f"
. / , f •
FIGURE 3
Uranium Dioxide Crys t a l s Deposited F r o m the Vapor at 2750 C
- 1 3 - HW-69234
than 1. 75 at .% burnup exhibited increased melt ing points and vaporized
more slowly than nonir radia ted UO„ during anneals at 2400 C, 2750 C,
and the melt ing point. UO„ with higher exposures had melt ing points
below 2800 C and vaporized more rapidly than e i ther un i r rad ia ted UO„
or low burnup UO„ when held at the same t e m p e r a t u r e s .
Di rec t compar isons between i r rad ia ted and un i r rad ia ted UO„
made by simultaneously heating spec imens of each on a common filament
c lear ly i l lus t ra te the changes in both mel t ing point and volati l i ty which
occur during i r rad ia t ion as shown by F igure 4. The two spec imens in
F igure 4 were s in tered UO„ prepared from the same type of powder.
They were identical in a l l r e s p e c t s (aside from slight differences in s ize
and shape) except i r rad ia t ion exposure . The specimen on the left was
i r rad ia ted to 0. 20 at. % burnup while the one on the right was not i r r ad ia t ed .
The specimens were placed side by side in the center of a box-shaped
tungsten filament. P ic tu re 4A shows the two at a t e m p e r a t u r e of 2400 C
before any melt ing or appreciable vaporizat ion occur red . P i c tu r e 4B was
taken 5 sec after r a i s ing the t e m p e r a t u r e to 2790 C The un i r rad ia ted
UO„ fused and was about 50% vaporized while the i r r ad ia ted UO„ did not
mel t and vaporized only slightly. P ic tu re 4C shows the two spec imens
after 20 sec at 2790 C. The uni r rad ia ted oxide continued to vapor ize at
a fas te r r a t e . P i c tu re 4D was taken at 2880 C where the i r r ad ia ted UO„
melted and began to vapor ize more rapidly. P i c tu re s 4E, F , and G were
taken at t ime intervals of 2, 3, and 5 min after ra i s ing the t e m p e r a t u r e
to 2880 C. At this t empe ra tu re , both m a t e r i a l s were molten at the
f i lament-specimen interface, and both vaporized rapidly. P i c tu re 4H
shows the re f rac to ry res idue which remained solid to 3400 C where the
tungsten filament mel ted.
At both 2790 and 2880 C, the i r rad ia ted UO„ vaporized more slowly
with the g r e a t e r difference at 2790 C This was because at 2790 C, the
i r rad ia ted specimen was solid while the nonir radia ted specimen mel ted .
The melt ing m a t e r i a l had an increased sur face- to-volume ra t io and a
• ->
; • !
• ^ . ^ - 1 ^ ^
J.^i'^-v-'-kSk^l
hf̂
FIGURE 4
Melting and Volatility Comparison, 25X (Left Unirradiated Right 0. 20% Burnup)
ffi
0 3 CD CO CO 4^
- 1 5 - HW-69234
higher average t empera tu re because , a s it mel ted, it dec reased in height
causing a dec rease in AT a c r o s s the specimen. The difference in vapor iza
tion ra t e at 2790 C thus ref lec ts , p r imar i ly , the higher mel t ing point of
the i r rad ia ted UO„. At 2800 C, both spec imens melted; and the slightly
lower volatility of the i r rad ia ted m a t e r i a l indicates a possible suppress ion
of the vapor p r e s s u r e by i r rad ia t ion .
All i r rad ia ted specimens vaporized and formed typical dendrit ic
c rys t a l s at t e m p e r a t u r e s near the melt ing point. When the t empera tu re
of these vapor deposited c rys ta l s was dec reased to 2750 C, vaporizat ion
seemed to occur at lower r a t e s than from specimens of un i r rad ia ted
oxide at the same t e m p e r a t u r e .
DISCUSSION
Implicat ions of the r e su l t s can be d iscussed e i ther in t e r m s of
chemical s tabil izat ion, consider ing probable effects of i r rad ia t ion upon
phase re la t ionships in the uranium-oxygen sys tem, or from a physical
point of view which es t ima tes changes in the mel t ing p r o c e s s which might
be affected by i r rad ia t ion . The f i rs t approach is amenable to a s e m i
quantitative analysis and will be d iscussed in more detai l than the la t te r .
The discuss ions a r e speculative because of insufficient data to formulate
a r igorous quantitative theory .
Chemical Stabilization by I r rad ia t ion
Location of the Liquidus Maximum
The maximum in the liquidus of UO„ need not occur at the s to i
chiometr ic composit ion. The g rea t e r solubility of oxygen than of uran ium
in UO„ „„ plus the oxygen-l iberat ing dissociat ion which occurs indicates
the existence of a liquidus maximum at some composit ion UO„ where ^ ~T JL
X can be e i ther positive or negative. The inc rease in melt ing point of
i r rad ia ted UO„ may reflect s tabil izat ion of an anion deficient defect
lattice by meta l fission fragment ions with valences < -i- 4. The oxygen
vacancies in the e lec t r ica l ly neut ra l , s tabi l ized s t ruc tu re may hold
dissolved oxygen a toms , thus a UO„ solid solution, which would
-16 - HW-69234
normally d issocia te yeilding free oxygen, may be stabi l ized at t e m
pera tu res to the melt ing point. Some other mechan i sm would have to
be responsible for stabil izing lower 0:U ra t ios if x is negat ive.
If the liquidus maximum is shifted to UO„ a s expected, the ^ "Y jL
i r rad ia ted UO^ , as a stabil ized solid solution, me l t s at a higher 2 +x ' ' ^ t empera tu re than the nonstabilized uni r rad ia ted oxide. As higher
(or lower) 0:U ra t ios become stable with increas ing fission fragment
concentrat ion ( i . e . , with increas ing i r radia t ion exposure) , the liquidus
maximum is reached and passed and the melt ing point dec r ea se s with
fur ther i r radia t ion t ime . The bulk concentrat ion of f ission fragments
requi red to stabil ize the latt ice i s re la t ively low (about 0. 1 at .%);
however, it has been shown that these fragments a r e not uniformly
dis tr ibuted in a polycrystal l ine specimen. Instead, they a r e highly
concentrated in the grain boundar ies ; and the concentrat ion d e c r e a s e s
exponentially as the m a s s gradient is t r a v e r s e d toward the center of
each individual grain .
The concentrat ion of fission f ragments in the gra in boundar ies i s
s eve ra l t imes the bulk concentrat ion and may be near that normal ly
required for significant lattice stabil ization. The grain boundar ies
may thus be more re f rac tory than the in te r io r of the gra ins and may
se rve as a solid, supporting la t t ice-work at t e m p e r a t u r e s where the
grain in te r io rs a r e molten. This is the inverse of the si tuation normal ly
encountered in mel t ing pure polycrystal l ine bodies, in which melt ing is
initiated at the grain boundaries or other in ternal su r f aces .
Stabilization of an anion deficient defect lattice occurs in many
oxide-additive s y s t e m s . In at least one sys t em, the addition of a
second component to UO„ resu l t s in an increased liquidus t e m p e r a t u r e ,
this being the UO„-Y„0„ sys tem in which an addition of 4 mole % (17) Y„0„ yields a melt ing point inc rease of 36 C
-17 - HW-69234
Hypothetical High Tempera tu re Phase Relat ionships
Two features of the uranium-oxygen phase d iagram tentatively
established by this work a re :
(1) liquidus maximum of 2930 C at an 0:U ra t io of 2. 006
(2) melt ing t empe ra tu r e of 2400 C for oxide with an 0:U ra t io s o m e where between 2. 07 and 2. 24.
A third considerat ion is the solubility of uran ium in UO„ at t e m p e r a -(18) tu res near the melt ing point. Anderson and others re ta ined UO. „_
1.97
by cooling molten UO„ in argon, but some doubt is cast on this figure
by recent latt ice p a r a m e t e r m e a s u r e m e n t s . The approach was to
extrapolate the then accepted plot of lat t ice p a r a m e t e r v e r s u s 0;U
ra t io to the composition corresponding to the inc reased p a r a m e t e r
observed in the fused oxide. MacEwan has identified uranium meta l
inclusions formed in UO„ pellets by heating to > 2000 C in an 0 „ p a r --7 (19)
t ial p r e s s u r e of 10 a tmosphe re s . His calculat ions show that
the most feasible path by which the oxide could be reduced under these
conditions is by an oxygen l iberat ing dissociat ion reac t ion . Since the
oxide was substoichiometr ic af ter heating, the net dissociat ion products
probably were oxygen and homogeneous UO„ _ from which meta l l ic
uranium precipi ta ted on cooling. Rothwell drew para l le l conclusions
from his study of fission gas r e l ea se r a t e s for i r r ad ia ted UO„.
Simi lar observat ions have been made at Hanford and other s i t e s . The m i c r o s t r u c t u r e of commerc ia l ly fused UO„, which is often
subs to ichiometr ic , provides fur ther evidence for the exis tence of UO„ 2 - X
The excess uranium is p resen t a s meta l , probably because of the low
cooling ra t e after fusion, but it has the appearance of a p rec ip i ta te .
The me ta l inclusions form a fine d ispers ion and often appear to be
preferent ia l ly oriented much as the second phase in an overaged p r e c i
The dec rea se in latt ice p a r a m e t e r during annealing is sma l l but s ig -(21) nificant. Extrapolat ing Schaner ' s plot of lat t ice p a r a m e t e r v e r s u s
0:U ra t io yields an 0:U ra t io of 1.997 ± 0.002 for the annealed oxide. No meta l inclusions were found in the annealed oxide at a magnification of 500X. Longer t ime at t empera tu re and /o r a more rapid quench should yield lower 0:U r a t i o s . The heating ra te was g r e a t e r than that used for measur ing melting points, thus uni r rad ia ted UO„ specimens were probably substoichiometr ic when the melt ing point was reached. The measu red melt ing point, 2790 C, i s mos t likely that of homogeneous UO„ r a the r than UO„ ^nr,-^ 2 - X 2. 000
A tentative phase d iagram for uranium-oxygen showing the features
discussed above is presented in F igure 5.
I r rad ia t ion Effects on the Melting P r o c e s s
The observed increase in melt ing t empera tu re with i r rad ia t ion
dose can a lso be theoret ical ly examined by consider ing possible i r r a d i a
tion effects on the mel t ing mechan i sm. Hoffman has re la ted subcrys ta l l i t e
size and vacancy concentrat ion to the en t ropy-energy d iagram for a solid
-19- HW-69234
3000
2500
^ X -- 0 ^
Signifu .ifit Pomts .Measured MfUiiiK Fcniit tit UO2. 27'H) t 20 C 2700 C O-U = l.')U7 29 30 C O U = 2.008 2400 C 2.07 < O-U < 2. 24
FIGURE 5
Tentative Uranium-Oxygen Phase Diagram
(22) by considering only the entropy of mixing of a toms and vacanc ies .
This simplified t rea tment yields energy-entropy curves s imi la r to those
in F igure 6. F o r smal l subcrys ta l l i t es , these curves have positive
curvature throughout; but for l a rge r subscrys ta l l i t es (the usual case) ,
two points of inflection occur . Since the slope of the ent ropy-energy
curves equals the absolute t empera tu re (•TO~ = T ) , the double tangent
denotes a t empera tu re at which two phases exist in equi l ibr ium. The
higher entropy phase is the liquid, the lower entropy phase is the solid,
and the t empera tu re at which they coexist is the melt ing point. Hoffman
concludes that, if a high, metas table concentration of vacancies can be
produced in the solid at a t empera tu re less than the melt ing point, the
position. A, on the diagram can be reached. A subsequent r i s e in t em
pera ture at a sufficiently high ra te to prevent extensive recombinat ion of
- 2 0 - HW-69234
Entropy
FIGURE 6 Energy-Ent ropy Curves for Various Subcrystal l i te Sizes
(Ng > N 2 >N^)
metas table vacancies should permit ra i s ing the energy and entropy to equi
l ibrium values corresponding to the normally "forbidden" portion of the
curve between the points of common tangency (Point B in F igure 6). Since
the slope of the energy-entropy curve in this region is g r ea t e r than at the
melt ing point, position B r ep resen t s a superheated solid which should be
stable indefinitely at this t empe ra tu r e .
The argument above may apply to the UO„ examined in these exper
iments since it was subjected to high neutron, beta, and gamma fluxes
which undoubtedly produced a concentrat ion of metas table vacancies .
Heating r a t e s during melt ing point m e a s u r e m e n t s , however, seem too low
to prevent recombination of most of the vacancies formed.
ACKNOWLEDGEMENTS
The author is indebted to J . L. Bates whose initial guidance in the
exper imental techniques used made this work possible and to S. J . Wal ter
for her ass i s tance in gathering and compiling data . Special thanks a r e due
M. G. Bowman and N. H. Krikorian of Los Alamos Scientific Labora tory
for their helpful d iscuss ions regard ing phase equil ibr ia .
- 2 1 - HW-69234
APPENDIX A
Calculation of the Liquidus Maximum
(2 3) An express ion for the composition at the liquidus maximum is
£aa 2 ""'^ .p + q) n^/(p + q) '
where p and q a r e the subscr ip t s in the general dissociat ion reaction- A B = pA-i-qB
p q m is moles of B which must be added to one mole of A B
P q to achieve the composition with the maximum melting t empera ture
a is degree of dissociat ion
n is rat io of the slopes of the two liquidus curves at their point of in tersect ion, i-e- at the melting maximum-
The react ion for the dissociat ion of UO may be writ ten as
UO„ = UO„ + xO , 2 2 - X
(18)
where Anderson and others have determined the l imiting value for x
to be 0. 14 for initially UO„ „„ held in the molten state- Thus, a = 0- 07,
and the general dissociat ion react ion for UO„ -̂.̂ may be wri t ten as UO^ > U + O ^ ;
thus,
p = 1, q = 1.
The slope of the liquidus on e i ther side of the maximum can be
est imated from the data in F igure 2 which gives the variat ion in UO„
melt ing point with increas ing i r radia t ion exposure . It is f i rs t neces sa ry
to assume that the ra te at which high oxygen concentrat ions a re stabil ized
with respec t to i r radia t ion dose is constant for exposures extending
slightly to e i ther side of the maximum. If this is t rue , a l inear c o r r e s
pondence exis ts between i r radiat ion dose and oxygen concentrat ion; and
-22 - HW-69234
the ra t io of the liquidus slopes when oxygen concentrat ion is plotted a s
absc i s sa is the same as the rat io of the slopes on e i ther side of the max i
mum when i r rad ia t ion dose is plotted a s a b s c i s s a .
The ra t io of the s lopes, n, was es t imated by fitting an empi r i ca l
equation to the data on e i ther side of the maximum and taking the rat io
of the f i rs t der iva t ives of the two equations at their point of in tersec t ion .
The rat io obtained was 1. 28. The slope of the curve fitting the data to the
left of the maximum had the g rea t e r absolute value, thus
n = 1. 28.
Using the values of p, q, a, and n derived above, m was calculated.
Thus , the addition of 0. 004 moles of oxygen to one mole of UO„ „ ,
yielding an 0:U ra t io of 2. 004 should give the composit ion with the max i
mum melt ing point, 29 30 C. 0:U ra t ios of the i r rad ia ted UO„ examined
were typically between 2.005 and 2.010 before i r rad ia t ion . Oxide which
was initially substoichimetr ic would not be expected to have a higher
pos t i r radia t ion melt ing point since the excess oxygen requ i red to achieve
higher mel t ing composit ions would not be avai lable . P re sumab ly , this is
why the fused UO„ with 0. 11 at . % burnup melted at the same t empera tu re
as nonir radia ted UO„.
-23 - HW-69234
BIBLIOGRAPHY
1. J . L. Bates and H. W- Newkirk. Photographic Observat ions of the Growth of Uranium Dioxide Crys t a l s by Vapor Deposit ion, HW-59575-March 1959.
2. J . L. Bates a n d W . E. Roake. I r rad ia t ion of Fue l E lements Containing UO^ Powder, HW-60578. June 1959. (Nuclear Congres s , Engineers ?oint Council , Apr i l 6-9, 1959. P r ep r in t V-90)
3. H. W. Newkirk and J . L. Ba tes . "Fi lament Furnace for Microscopy Studies at High T e m p e r a t u r e s , " Review of Scientific Ins t rumen t s , vol. 30, pp. 645-646. August 1959.
4. C E. Mendenhall. "The Emiss ive Power of Wedge-Shaped Cavi t ies and The i r Use in T e m p e r a t u r e Measu remen t s , " The Ast rophys ica l Journa l , vol. 33, pp. 91-97. 1911.
5. J . L. Bates and H. W. Newkirk. Growth of Uranium Dioxide Crys t a l s by Vapor Deposit ion, HW-57113. August 1958- (16 mm movie)
6. D. Esch . Unpublished Data. August 1961. (Pe r sona l Communication)
7. E . F r i e d e r i c h and L. Sittig. "Hers te l lung und Eigenschaften Hoch-schmelzender N iede re r Oxyde, " Z . anorg . u. a l lgem. Chem. , vol. 145, pp. 127-140. 1925. (Lower Oxides of High Melting Point . German)
8. W. A. Lamber t son and M. H- Muel ler . "Uranium Oxide Phase Equi l ibr ium Systems- I -U0„-A1„0„ , " J . Am. C e r a m . S o c , vo l .36 , pp. 329-331. 1953.
9. O. Ruff and O. Goecke. "Uber das Schmelzen und Verdampfen U n s e r e r Sogenannten Hochfeuerfesten Stoffe, " Z . angew. Chem. vol. 24, pp. 1459-1465- 1911. (Fusion and Evaporizat ion of So-cal led Refractory Bodies . German)
10. L. G. Wisnyi and S. Pijanowski. T h e r m a l Stability of Uranium Dioxide, KAPL-1702. Knolls Atomic Power Labora tory , November 1957.
11. H. W. Newkirk and J . L. Bates. Melting Points of Uranium Dioxide, Uranium Monocarbide, and Uranium Mononitr ide, HW-59468. March 1959.
12. M. G. Bowman. Unpublished Data. August 1961. (Pe r sona l Communication)
13- R. J . Ackermann, P . W. Gi l les , and R. J . Thorne . "High T e m p e r a tu re Thermodynamics P r o p e r t i e s of Uranium Dioxide, " J . of Chem. Phys i c s , vol. 25, pp. 1089-1097. 1956.
14. D. D. Saratovkin. Dendri t ic Crys ta l l iza t ion , Consultants Bureau, I n c . , New York, 1959. 2nd ed.
-24 - HW-69234
BIBLIOGRAPHY (cont'd)
15. V. B. Law son and J . R. MacEwan. T h e r m a l Simulation Exper iments with a UO2 Fue l Rod Assembly , AECL-994. Atomic Energy of Canada Limited, Chalk River , March I960.
16. Westinghouse E lec t r i c Corporat ion. P r e s s u r i z e d Water Reac to r Pro jec t , WAPD-MRP-92. Bet t is Atomic Power Labora tory , P i t t sburgh, June , 1961. pp 53-54. (Technical P r o g r e s s Report)
17. R. M. P o w e r s . "UO2 Additives, " Nucleonics, vol- 18, p. 6. October 1960. (Le t te r to the Editor)
18. J . S. Anderson, J . O. Sawyer, H. W. Worne r , G. M. Wil l is , and M . J . Bannis te r . "Decomposition of Uranium Dioxide at its Melting P o i n t , " Nature , vol. 185, pp. 915-916. March 1960.
19. J . R. MacEwan. Grain Growth in Sintered Uranium Dioxide, AECL-1184. Atomic Energy of Canada Limited, Chalk River , January 1961.
85 20. E . Rothwell. The Release of Krypton from I r rad ia ted Uranium
Dioxide on Pos t - I r r ad ia t ion Annealing, AERE-R-3672 . Atomic Energy R e s e a r c h Es tab l i shment , Harwel l , England, March 1961.
21. B. E . Schaner . "Metallographic Determinat ion of the UO2-U4O9 Phase Diagram, " J . of N u c Mat. , 2, pp. 110-120. 1960.
22. T . A. Hoffmann. "Possibi l i ty of Attaining the Superheated Solid State in Reac to r s , " Proceedings of the Second United Nations Internat ional Conference on the Peaceful Uses of Atomic Energy , Bas ic Metal lurgy and Fabr ica t ion of F u e l s . United Nations, Geneva, 1958. vol. 6, pp. 240-244.
23. J . Zern ike . Chemical Phase Theory, N. V. Ui tgevers -Maatschappi j , A . E . E . Kluwer, Deventer -Antwerp-Djakar ta , 1955- pp. 228-230 (Pakistan)
F . W. Albaugh H. J . Anderson W. J . Bailey J . L. Bates J . B. Burnham S. H. Bush J . J . Cadwell D. F . C a r r o l l J . L. Daniel K. Drumhel le r E . A. Evans W. J . F laher ty M. D. F r e s h l e y S. Goldsmith D. R. de Halas W. M. Harty C. A. Hinman G. R. Horn J . J . Houth W. J . Lackey, J r . C. E . McNeilly J . O. McPart land M. K. Millhollen L. E . Mills R. E . Olson L. A. P e m b e r W. E. Roake R. E. Skavdahl O. J . Wick R. D. Widrig T. W. Woodfield Technical Publications 300 F i l e s Record Center Ex t ra
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FRANKFORD ARSENAL
FRANKLIN INSTITUTE OF PENNSYLVANIA
FUNDAMENTAL METHODS ASSOCIATION
GENERAL ATOMIC DIVISION
GENERAL DYNAMICS/ASTRONAUTICS (NASA)
GENERAL DYHAMICS/FORT WORTH
GENERAL ELECTRIC COMPANY, CINCINNATI
GENERAL ELECTRIC COMPANY, PLEASANTON
GENERAL ELECTRIC COMPANY, SAN JOSE
GENERAL NUCLEAR ENGIHEERIMG
CORPORATION
GENERAL TELEPHONE AND ELECTRONICS
LABORATORIES, IMC.
GOODYEAR ATOMIC CORPORATION
IIT RESEARCH INSTITUTE
IOWA STATE UNIVERSITY
JET PROPULSION LABORATORY
KNOLLS ATOMIC POWER LABORATORY
LOCKHEED-GEORGIA COMPANY
LOCKHEED MISSILES AND SPACE COMPANY
(NASA)
LOS ALAMOS SCIENTIFIC LABORATORY
M & C NUCLEAR, INC.
TID-4500
Ptd. Standard Distribution
MALLINCKRODT CHEMICAL WORKS
MARITIME ADMINISTRATION
MARTIN-MARIETTA CORPORATION
MATERIALS RESEARCH CORPORATION
MOUND LABORATORY
NASA LEWIS RESEARCH CENTER
NASA SCIENTIFIC AND TECHNICAL
INFORMATION FACILITY
NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS (LIBRARY)
NATIONAL LEAD COMPANY OF OHIO
NAVAL POSTGRADUATE SCHOOL
NAVAL RESEARCH LABORATORY
NEW BRUNSWICK AREA OFFICE
NEW YORK OPERATIONS OFFICE
NUCLEAR MATERIALS AND EQUIPMENT
CORPORATION
NUCLEAR METALS, INC.
OFFICE OF ASSISTANT GENERAL COUNSEL
FOR PATENTS (AEC)
OFFICE OF NAVAL RESEARCH
OFFICE OF NAVAL RESEARCH (CODE 422)
ORDNANCE MATERIALS RESEARCH OFFICE
ORDNANCE TANK-AUTOMOTIVE COMMAND
•PETROLEUM CONSULTANTS
PHILLIPS PETROLEUM COMPANY (NRTS)
PICATINNY ARSENAL
POWER REACTOR DEVELOPMENT COMPANY
PRATT AND WHITNEY AIRCRAFT DIVISION
PURDUE UNIVERSITY
RAND CORPORATION
RENSSELAER POLYTECHNIC INSTITUTE
RESEARCH ANALYSIS CORPORATION
SAN FRANCISCO OPERATIONS OFFICE
SANDIA CORPORATION, ALBUQUERQUE
SANDIA CORPORATION, LIYERMORE
Ptd
325
75t
UC-25 METALS, CERAMICS, AND MATERIALS
Standard Distribution
SPACE TECHNOLOGY LABORATORIES, INC.
(NASA)
STANFORD UNIVERSITY (SLAC)
SYLVANIA ELECTRIC PRODUCTS, INC.
TECHNICAL RESEARCH GROUP
TENNESSEE VALLEY AUTHORITY
UNION CARBIDE NUCLEAR COMPANY (ORGDP)
UNION CARBIDE NUCLEAR COMPANY (ORNL)
UNION CARBIDE NUCLEAR COMPANY (ORNL-
Y-12)
UNION CARBIDE NUCLEAR COMPANY (PADUCAH PLANT)
•UNITED NUCLEAR CORPORATION (NDA)
U. S. GEOLOGICAL SURVEY, DENVER
U. S. GEOLOGICAL SURVEY, MENLO PARK
U. S. GEOLOGICAL SURVEY, WASHINGTON
U. S. PATENT OFFICE
UNIVERSITY OF CALIFORNIA, BERKELEY
UNIVERSITY OF CALIFORNIA, LIVERMORE
UNIVERSITY OF PUERTO RICO
WATERTOWN ARSENAL
WESTERN RESERVE UNIVERSITY (MAJOR)
WESTINGHOUSE BETTIS ATOMIC POWER
LABORATORY
WESTINGHOUSE ELECTRIC CORPORATION
WESTINGHOUSE ELECTRIC CORPORATION
(NASA) YANKEE ATOMIC ELECTRIC COMPANY
DIVISION OF TECHNICAL INFORMATION EXTENSION
OFFICE OF TECHNICAL SERVICES, WASHINGTON
*New listing or change in old listing. tThese copies should be shipped directly to the Office of Technical
Services, Department of Commerce, Washington, D. C.
