their contractors, subcontractors, or thrk emploYees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com- pleteness or usefulness of any information, apparatus, This document is PUBLICLY RELEASABLE 252Cf SHIELQIFJG GUIDE D. H. Stoddard H. E. Hootrnan Approved by C. M. Patterson, Research Manager Radio1ogi cal Sciences Division and P. L. Roggenkamp, Res8earchManager Theoreti cal Phys i c:s Di vi si on March 1371 DP-1246 Health and Safety (TID-4500, UC-41) E. I. DU PONT DE NEMOURS COMPANY SAVANNAH RIVER L.ABORATORY AIKEN, S. C. 29801 CONTRACT AT(07 - 2)- I WITH THE UNITED STATES ATOMIC ENERGY COMMISSION
Transcript
their contractors, subcontractors, or t h r k emploYees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com- pleteness or usefulness of any information, apparatus,
This document is PUBLICLY RELEASABLE
252Cf SHIELQIFJG GUIDE
D. H. Stoddard H . E. Hootrnan
Approved by
C. M. P a t t e r s o n , Research Manager Radio1 o g i c a l Sciences D i v i s i o n
and
P . L . Roggenkamp, Res8earch Manager T h e o r e t i c a l Phys i c:s D i v i s i on
March 1371
DP-1246
H e a l t h and S a f e t y (TID-4500, UC-41)
E. I . DU PONT DE NEMOURS COMPANY SAVANNAH RIVER L.ABORATORY
AIKEN, S. C. 29801
CONTRACT AT(07 - 2)- I WITH THE
UNITED STATES ATOMIC ENERGY COMMISSION
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.
ABSTRACT
Neutron and gamma r a y dose r a t e s have been c a l c u l a t e d f o r 252Cf source i n t e r a c t i o n with most common s h i e l d i n g m a t e r i a l s . Information has been compiled t o ass is t p r o s p e c t i v e u s e r s i n determining:
5 2 C f e v a l u a t o r s and
0 Radia t ions involved with t h e use of 252Cf .
0 Personnel dose guides and shipping dose r a t e l e v e l s f o r 2 5 2 ~ f .
0 Comparative th i cknesses of d i f f e r e n t s h i e l d m a t e r i a l s r e q u i r e d f o r a s p e c i f i c 2 5 2 C f dose r educ t ion .
Approximate f l u x l e v e l s f o r neutron a c t i v a t i o n and back- ground r a d i a t i o n l e v e l s i n a p p l i c a t i o n s .
0
- 2 -
n
CONTENTS
. . . . . . . . . . . . . . . . . . I n t r o d u c t i o n 5
The Radiat ion P r o p e r t i e s o f 2 5 2 C f . . . . . . . 5
. . . . . . . . . . . Gamma A c t i v i t y of 2 5 2 C f 6
General Nuclear P r o p e r t i e s of Californium Isotopes
Gamma Rays from 252Cf Alpha Decay , , . . . . . . Gamma Rays from Spontaneous F i s s i o n of 252Cf . . . Neutrons from Spontaneous F i s s i o n o f 2 5 2 C f . . . . Twenty-Two Group Source D i s t r i b u t i o n f o r 2 5 2 C f . . Atomic Composition of t h e "Standard Man" . . . . . Dose Rate Conversion Fac to r s . . . . . . . . . . . Composition of Window M a t e r i a l s . . . . . . . . . Calcu la t ed E f f e c t i v e n e s s of P a r a f f i n - F i l l e d Drums
Page
. 6
. 6
. 7
. 8
. 14
. 15
. 15
. 25
. 27
- 4 -
INTRODUCTION
A neu t ron -emi t t i ng r a d i o i s o t o p e such a s 2 5 2 C f poses new and unique s h i e l d i n g problems both wi th r e s p e c t t o t h e n a t u r e of t h e r a d i a t i o n s t o be s h i e l d e d and t h e o r t a b i l i t y requirements i n some o f i t s proposed a p p l i c a t i o n s ? Californium-252 s h i e l d i n g i s similar t o n u c l e a r r e a c t o r s h i e l d i n g i n t h a t t h e spontaneously emi t t ed neut rons react wi th t h e s h i e l d t o prDduce secondary r a d i a t i o n sources d i s t r i b u t e d throughout t h e s h i e l d . In many cases t h i s secondary r a d i a t i o n i s t h e dominant c o n t r i b u t o r t o t h e p e n e t r a t i n g dose r a t e .
Modern methods of s h i e l d i n g c a l c u l a t i o n use l a r g e , high-speed computers t o s o l v e f o r neut ron and gamma dose r a t e s . mul t i -energy t r a n s p o r t equa t ions used r e q u i r z d e t a i l e d coupled neutron-gamma c r o s s s e c t i o n s e t s t o ensure b 2 t t e r than a f a c t o r of two accuracy f o r t he requirccl s h i e l d dose relrluctions of l o 4 t o l o 5 .
The complex
An aim o f t h i s r e p o r t i s t o supply t h e 2 5 2 C f u s e r a s e r i e s o f a t t e n u a t i o n curves with which p a r t i c u l a r s h i 3 l d i n g needs may be eva lua ted .
THE RADIATION PROPERTIES OF 2s2Cf
Californium-252 decays 97% by t h e emissj-on of a lpha p a r t i c l e s , and 3% by t h e spontaneous f i s s i o n process whj.ch causes t h e emission of 2 . 4 x 10l2 neu t rons / sec from one gram of t h i s m a t e r i a l .
Although i s o t o p i c compositions w i l l var) ' somewhat depending on t h e mode of product ion and decay t ime, t h e pene t r a t ing* r a d i a t i o n from ca l i fo rn ium sources w i l l p r i m a r i l y be neut rons and gamma rays from t h e i s o t o p e 2 5 2 C f and from i t s f i s s i o n products .
The genera l n u c l e a r p r o p e r t i e s o f t h e ca l i forn ium i so topes The p e n e t r a t i n g r a d i a t i o n s o f 252Cf a r e p re sen ted i n Table I . 2
are l i s t e d i n t h e fo l lowing s e c t i o n s .
* P e n e t r a t i n g r a d i a t i o n i s t h a t which w i l l p e n e t r a t e t h e horny l a y e r o f t h e s k i n .
- 5 -
TABLE I
General Nuclear Properties o f Californium Isotopes
Neutrons Per Alpha or Specific Activity Spontaneous Fission Fission of Beta Decay o f Pure Isotope,
Isotope Half - Li f e Pure Isotope Half-Life Ci/g
2"Cf 1.5 x io9 3 360 y 4
2 5 0 C f (1.73 20.06) x l o 4 y 3 1 1 Y 2 5 1 C f - a1500 y 0.95
2 5 3 C f 18 ?r3 d 2.87 i o 4
1.31 x lo2
2 5 2 C f 85.5 k 0 . 5 y 3.80 iO.035 2.646 iO.004 y 5.37 x l o 2
2 5 4 C f 61.9 +1.1 d 3.9 20.14
GAMMA ACTIVITY OF 2 5 2 C f
The gamma a c t i v i t y of 252Cf c o n s i s t s of gamma r a y s from:
0 The a lpha decay p r o c e s s .
0 Prompt spontaneous f i s s i o n .
0 F i s s i o n p roduc t s cont inuously produced by spontaneous f i s s i o n .
Gamma r a d i a t i o n s from each of t h e s e sources a r e desc r ibed i n t h e fol lowing s e c t i o n s .
Alpha Decay
Gamma r a y s r e p o r t e d f o r t h e a lpha decay of 252Cf a r e l i s t e d i n Table 11.
T A B L E I 1
Gamma Rays f r o m 2 5 2 C f A lpha Decay
Energy, Abundance, Mev photons/ (sec) (g of nuc l ide )
0 .043 2 . 7 8 i o 9 0 .100 2 . 0 i o 9
- 6 -
Spontaneous F i s s i o n 63
The energy and abundance of prompt gamma rays from spontaneous f i s s i o n a r e l i s t e d i n Table 111.
E q u i l i b r i u m F i s s i o n Products
The f i s s i o n p roduc t s formed from spontaneous f i s s i o n approach equ i l ib r ium wi th in a few hours a f t e r s e p a r a t i o n . product gamma ac t iv i t i e s are l i s t e d i n Table 111.
Equi l ibr ium f i s s i o n
TABLE I11
Gamma Rays from Spontaneous F i s s i o n o f 2 5 2 C f
0 - 0 .5
0.5 - 1.0
1 .0 - 1 . 5
1 . 5 - 2.0
2.0 - 2.5
2.5 - 3.0
3 .0 - 3.5
3.5 - 4.0
4 . 0 - 4.5
4 . 5 - 5.0
5.0 - 5.5
5 . 5 - 6.0
6.0 - 6 . 5
Abundance, photons/ (sec) (g of n u c l i d e ) Equi l ibr ium F i s s i o n
P romp t G ammas Product Gammas T o t a l
3 . 3 x 1 0 l 2 1 .3 x 1 0 l 2 4.6 x lo’*
1 . 7 x 1 0 l 2 4.0 x 10’* 5 .7 x 10’2
4 . 2 x 10” 3.5 x 10” 7.7 x 10”
2 . 2 x 10” 2 . 2 x 1 o I 1
7.7 x 10” 9 .1 x 10’’ 1 . 7 x 1 0 l 2
1.1 x l o 1 ’
5.6 x 10”
3.0 x lo1’ 1 . 7 x 10”
8.2 x i o 9
1.1 x 10”
5.6 x 10”
3.0 x 10” 1 .7 x 10”
8.2 x i o 9 4 . 9 x i o 9 4.9 i o 9 i , 8 x i o 9 1 . 8 x i o 9 1.0 x i o 9 1.0 x i o 9
- 7 -
X-rays
Although X-rays a r e r e l a t i v e l y high i n abundance, t hose produced a r e very low i n energy (most are (40 keV) and are n o t included i n t h i s r e p o r t .
BETA ACTIVITY OF 2 5 2 C f
No b e t a r a d i a t i o n has been r e p o r t e d from t h e decay p rocess . The b e t a r a d i a t i o n a s s o c i a t e d with t h e equ i l ib r ium f i s s i o n products during spontaneous f i s s i o n i s e a s i l y absorbed and i s no t included i n t h i s r e p o r t .
NEUTRON ACTIVITY OF 252Cf
The neutron r a d i a t i o n from 2 5 2 C f c o n s i s t s p r i n c i p a l l y of neutrons from spontaneous f i s s i o n . i s t h e a , n neutrons from t h e r e a c t i o n of a lpha p a r t i c l e s with l i g h t elements. Table I V l i s t s t h e neutrons present from spontaneous f i s s i o n .
A second source of neutrons
TABLE I V
Neutrons from Spontaneous Fission of 252Cf
Energy 3
Mev Neutrons/ ( sec) (E of nuc l ide )
0 - 0.5
0 .5 - 1 . 0
1 . 0 - 2 .o 2.0 - 3.0
3.0 - 4.0
4 . 0 - 5 .0
5 . 0 - 6 . 0
6 .0 - 7 .0
7.0 - 8.0
8.0 -10.0
10.0 -13.0
To ta l
2.8 x 10”
3 . 7 x l o l l
7 .6 x 10”
4.6 x lo1’
2 . 8 x 10”
1.6 x l o 1 ’
5.6 x l o l o
4 .0 x 10”
1 .3 x 10”
9.9 x i o 9 2.2 x i o 9 2.4 x 10l2
- a -
Neutrons a s s o c i a t e d with 2 5 2 C f can a l s o be r ep resen ted by t h e 3 Watt f ormu 1 a.
N(E) = 0.373 exp(-0.88E) sinh(2.0E)’
where N(E) i s t h e f r a c t i o n of neutrons p e r u n i t energy range, and E i s t h e neutron energy i n MeV.
UNSHIELDED DOSE RATE
The neutron and gamma dose r a t e i n a i r from 2 5 2 C f may be determined by mul t ip ly ing t h e unshielded neutron f l u x by t h e 4nr2D(r) f a c t o r s l i s t e d below:
Dose Rate 4m2D(r )
) cm2 TYP e ( mrad-hr-I )cm2 ( mrem-hr-’ Radiat ion neutron-sec-1 neutron-sec-1
Neutron 1.43 x 1 . 2 2 x 10-1
Gamma 8.58 1 0 - ~ 8.58 x 1 0 - ~
T o t a l 2.29 x 1.31 x 10-1
For example, t h e t o t a l dose r a t e (DT) a t 100 cm from a 100-mg source of 2 5 2 ~ f i s
- 1 x I O 2 mg x 2 . 4 x l o 9 neutrons/sec-mg
4 ~ ( 1 0 0 ) ~ cm2 DT -
m r e m - h r - )cm2 = 2.5 x l o 5 mrem/hr 1.31 lo-’ neutron/sec- l
- 9 -
A REGULATIONS
PERSONNEL DOSE G U I D E S "
Federal r e g u l a t i o n s , e f f e c t i v e January 1, 1961, and i n e f f e c t when t h i s document was pub l i shed , permit no more than t h e following r a d i a t i o n doses t o any person 18 y e a r s o f age o r o l d e r :
0 5 rems m u l t i p l i e d by t h e number of yea r s beyond age 18, and t h e dose i n any consecut ive 13 weeks no t t o exceed 3 rems f o r t h e c r i t i c a l organs (whole body, head and t runk , a c t i v e blood forming organs, eyes , o r gonads).
0 30 rems/yr t o t h e s k i n (not t o exceed 7.5 rems i n any 13 consecut ive weeks).
0 75 rems/yr and t h e dose i n any 13 consecut ive weeks s h a l l no t exceed 18.75 rems f o r t h e hands, forearms, f e e t , and ank le s .
Any person less than 18 y e a r s o l d may r e c e i v e no more than 10% o f t h e above doses .
Federal r e g u l a t i o n s urge t h a t every reasonable e f f o r t be made t o maintain r a d i a t i o n exposures as f a r below t h e s e l i m i t s as p rac - t i c a b l e , t h a t i s , "as low as i s p r a c t i c a b l y achievable t a k i n g i n t o account t h e s t a t e of technology, and t h e economics o f improvements i n r e l a t i o n t o b e n e f i t s t o t h e p u b l i c h e a l t h and s a f e t y and i n r e l a t i o n t o t h e u t i l i z a t i o n of atomic energy i n t h e p u b l i c i n t e r e s t . I f A t Savannah River , cont inuously occupied a reas are s h i e l d e d t o <1 mrem/hr dose r a t e . This a s s u r e s personnel exposures of < 2 rem/yr when working 250 days p e r y e a r . s h i e l d t o 2 . 5 mrem/hr, which a s s u r e s doses wi th in t h e r e g u l a t i o n s .
Other AEC s i t e s
S H I P P I N G REGULATIONS'
A t t h e time of p u b l i c a t i o n o f t h i s document, s h i e l d e d sh ipp ing casks f o r gene ra l use r e q u i r e s h i e l d i n g t o dose ra te l e v e l s o f <ZOO mrem/hr a t t h e a c c e s s i b l e s u r f a c e of t h e c o n t a i n e r and/or <10 mrem/hr a t 3 f e e t from t h e a c c e s s i b l e e x t e r n a l s u r f a c e .
Packages exceeding these l e v e l s may be t r a n s p o r t e d by exclu- s i v e use of t h e v e h i c l e (except a i r c r a f t ) i f t h e fo l lowing r egu la - t i o n s a r e n o t exceeded:
0 1,000 mrem/hr a t 3 f t from t h e e x t e r n a l s u r f a c e o f t h e package (closed t r a n s p o r t v e h i c l e o n l y ) .
0 200 mrem/hr a t any p o i n t on t h e e x t e r n a l s u r f a c e o f t h e car o r v e h i c l e (c losed t r a n s p o r t v e h i c l e o n l y ) .
- 10 -
Q
0 10 mrem/hr a t 6 f t from t h e e x t e r n a l s u r f a c e of t h e car o r v e h i c l e .
0 2 mrem/hr o r equ iva len t i n any normally occupied p o s i t i o n i n t h e c a r o r v e h i c l e except t h i s does n o t apply t o p r i v a t e motor c a r r i e r s .
SHIELD MATERIAL CHARACTERISTICS
ATTENUATION CALCULATIONS
i n F igu res 4 through 48 p rov ides a handy and compact s h i e l d r e f e r e n c e f o r 2 5 2 C f u s e r s and p o t e n t i a l e v a l u a t o r s . f i g u r e s dose ra tes are normalized t o 1 f i s s i o n neu t ron / sec , and m u l t i p l i e d by 4 m 2 t o remove t h e e f f e c t o f s p h e r i c a l divergence.
The dose r a t e l i s t e d i s t h e s u r f a c e dose r a t e t o t i s s u e surrounding t h e s h i e l d i n g m a t e r i a l . t h i s dose r a t e i s almost t h e same as i f it were i n an i n f i n i t e t h i c k n e s s of t h e s h i e l d i n g material . Some suggested uses a re :
The c o l l e c t i o n of dose a t t e n u a t i o n c a l c u l a t i o n s p re sen ted
I n t h e s e
In most hydrogenous materials,
0 Comparisons of S h i e l d Ma te r i a l s
R e l a t i v e e f f e c t i v e n e s s of d i f f e r e n t s h i e l d m a t e r i a l s may be eva lua ted by summing t h e dose c o n s t i t u e n t s a t a common th i ckness f o r t h e m a t e r i a l s being compared. For example, such a comparison between "Benelex"* 70 and water i n d i c a t e s t h e former material t o be f o u r times as e f f e c t i v e i n dose r e d u c t i o n a t t h i c k n e s s e s of 120 cm, b u t e s s e n t i a l l y t h e same a t t h i c k n e s s e s l e s s t han 30 cm.
0 Estimations o f S h i e l d Thickness
2 5 2 C f may u s u a l l y be assumed t o be a p o i n t sou rce ; however, t h e cask o r s h i e l d may be any of a number o f shapes. The e f f e c t o f s h i e l d c o n f i g u r a t i o n on t h e p re sen ted dose d a t a fo l lows
Applications Dose Rates a t the Surface of a SphericaZ ShieZd T o t a l dose ra te a t t h e s u r f a c e of a s p h e r i c a l s h i e l d (Figure 1)
may be determined by adding t h e neu t ron dose r a t e s and t h e gamma dose r a t e s a t t h e common s h i e l d t h i c k n e s s and m u l t i p l y i n g t h e 2 5 2 C f s o u r c e weight i n grams times 1/4.rrr2 x 2 . 4 x l o 1 * neu t rons / sec-g
DT (mrem/hr) = (Dn + D ) x 4 x 2.4 x 1 0 l 2 x grams T YT 4.rrr
* Regi s t e red trademark of Masonite Corp.
- 11 -
8 Dose Rates a t the Surface of a Cylindrical Shield
T o t a l dose r a t e a t t h e s u r f a c e of a c y l i n d r i c a l s h i e l d (Figure 2) w i l l always be l e s s than a t t h e s u r f a c e of a sphe re t h a t can be i n s c r i b e d w i t h i n t h a t c y l i n d e r . dose r a t e curves p re sen ted f o r c y l i n d r i c a l s h i e l d s may be expected t o y i e l d s l i g h t l y conse rva t ive r e s u l t s (or a h i g h e r dose r a t e ) .
D i r e c t u se of t h e s p h e r i c a l s h i e l d
Dose Rates for a Slab sh ie ld
The dose a t t e n u a t i o n curves used f o r a s l a b s h i e l d (Figure 3) w i l l y i e l d a conse rva t ive dose r a t e (%2x depending on t h e s h i e l d i n g m a t e r i a l ) a t s h o r t ranges. The d a t a become l e s s conse rva t ive , and w i l l approach t h e s p h e r i c a l dose r a t e as t h e d i s t a n c e between t h e . s h i e l d and r e c e p t o r i n c r e a s e s .
Dose Rates for Laminated ShieZds
The curves p r e s e n t e d may be used t o approximate 6 t h e e f f e c t s The of u s ing d i f f e r e n t m a t e r i a l s t o make a composite s h i e l d .
l i m i t a t i o n on such u s e i n c l u d e s t h e assumption t h a t t h e neutron and gamma energy spectrum does n o t vary s i g n i f i c a n t l y where t h e materials j o i n .
A t y p i c a l cask might be a 40-in.-OD (20- in . - r ad ius ) p a r a f f i n sphe re with 10 mg/cm3 n a t u r a l l i t h i u m mixed uniformly i n t h e p a r a f f i n . c a v i t y . f a c e o f t h e cask i s e s t ima ted by t h e fo l lowing :
A 3-in.-OD i n n e r l ead s h i e l d surrounds a 1-in.-OD source Assuming a 5O-ug 2 5 2 C f s o u r c e , t h e dose r a t e a t t h e s u r -
T o t a l Dose Rate , mrem-hr-' cm2
Dose C o n s t i t u e n t s 4 m 2 D Cor rec t ion neutron-sec-1
Neutron Fast 9 - 9 x 1 0 - ~
Thermal 1 - 1 x
Primary 1.6 1 0 - ~ 0. 17b 2 . 7 i o - ' + Secondary 1 . 2 1 0 - ~ 0.24c 2.9 x
G a m m a
6 . 6 x l o d 4
a. From p a r a f f i n curve Figure 7. b. From i r o n - l e a d curve Figure 5 . e. From l i t h i u m curve F igu re 35.
- 1 2 -
@ There fo re , t h e t o t a l dose ra te DT i s :
1 2 neutron-sec-’ cm2 50 x 10 g 2 . 4 x 10 m r e m - h r - ’ neutron- s ec- ’ g
DT = 6 . 6 x 4F(20 x 2.5)2cm2
A
= 2 . 4 mrem/hr
Other Appl icat ions
Data p l o t t e d on curves f o r d i f f e r e n t m a t e r i a l s may be used t o e s t i m a t e unperturbed f l u x l e v e l s f o r a c t i v a t i o n and t o e s t i m a t e background r a d i a t i o n l e v e l s i n d e t e c t o r a p p l i c a t i o n s . For example, t h e thermal neutron f l u x may be c a l c u l a t e d from t h e thermal neutron dose r a t e by d i v i d i n g t h e thermal neutron dose r a t e by t h e thermal dose r a t e conversion f a c t o r 0.00373 from Table VII.
Calculations
Dose r a t e a t t e n u a t i o n curves shown i n t h i s r e p o r t have been c a l c u l a t e d u s i n g t h e ANISN7code.
ANISN i s an acronym f o r t h e d i s c r e t e o r d i n a t e s o r Sn method of approximating t h e s o l u t i o n of t h e energy dependent l i n e a r Boltzmann t r a n s p o r t equat ion with a n i s o t r o p i c s c a t t e r i n g . The one-dimensional equat ion may be app l i ed t o s l a b , s p h e r i c a l , o r c y l i n d r i c a l geometry.
In every case, t h e sou rce was assumed t o be i n an aluminum mat r ix , s p h e r i c a l i n shape, having a volume of 1 cm3, and a r a d i u s of 0 .62 c m .
The angu la r f l u x was approximated by a s i x t e e n t h o r d e r quadra tu re (S16) and t h e an i so t ropy of e l a s t i c a l l y s c a t t e r e d neutrons and of Compton-scattered photons was approximated by a t h i r d o r d e r Legendre expansion ( P 3 ) .
Energy Group Structure
The energy group s t r u c t u r e has been de r ived us ing t h e mul t i - group theo ry of neutron and gamma-ray t r ansmiss ion . The groups have been d iv ided i n t o 13 neu t ron energy groups and 9 gamma energy groups. t o c a l c u l a t e t h e i n e l a s t i c and c a p t u r e gamma a c t i v i t y t h a t arises from neu t ron s c a t t e r o r abso rp t ion .
The neutron groups have been coupled with t h e gamma groups
- 1 3 -
Source The source used i n t h e c a l c u l a t i o n s inc ludes t h e gamma rays
shown i n t h e p rev ious s e c t i o n d i s t r i b u t e d wi th in t h e i r energy i n t e r v a l s and t h e neutrons as d i s t r i b u t e d by t h e Watt formula. Low energy X-rays and b e t a r a d i a t i o n a r e n o t s i g n i f i c a n t f o r personnel s h i e l d i n g because of t h e i r i n a b i l i t y t o p e n e t r a t e m a t e r i a l s i n which sources a r e u s u a l l y encapsulated.
Table V l i s t s t h e d i s t r i b u t i o n of neutrons and gamma rays a s a f u n c t i o n of energy f o r each neutron emit ted from 2 5 2 C f .
TABLE V
Twenty-Two Group Source D i s t r i b u t i o n f o r 2 5 2 C f
Cross s e c t i o n s f o r neutron-induced r e a c t i o n s and t r a n s i t i o n s between groups were taken from a 100-group E N D F / B l i b r a r y . ANISN code was used t o c o l l a p s e t h e c r o s s s e c t i o n s from 100 t o 13 neutron energy groups. s e c t i o n s were generated by t h e MUG e l e c t r i c and p a i r p roduc t ion abso rp t ion as wel l as Compton e f f e c t s . Nine gamma-ray energy goups were used. Cross s e c t i o n s f o r neutron- induced secondary and i n e l a s t i c s c a t t e r i n g gamma r a y s were coupled i n t h e manner of POPOP49 between t h e neutron and photon downscatter m a t r i c e s .
The
Gamma-ray $roup-to-group t r a n s f e r c r o s s - code which inc ludes photo-
- 14 -
Flux-to-Dose Convemion Dose r a t e s are given i n terms of t h e b i o l o g i c a l dose r a t e
( m u l t i p l i e d by 4m2) computed a t t h e s u r f a c e of t i s s u e a t some p o i n t , This i s accomplished by immediately surrounding t h i s p o i n t with a material of composition approximating t h a t of man. The dose r a t e then a t t h a t p o i n t con ta ins t h e c o n t r i b u t i o n s from t i s s u e b a c k s c a t t e r . man i s shown i n Table V I . Conversion f a c t o r s used t o convert t o b i o l o g i c a l dose r a t e are given i n Table V I I .
The composition o f t i s s u e used f o r t h e s t anda rd
a. Dose r a t e c o n v e r s i o n f a c t o r s f o r Groups 1-13 were d e t e r m i n e d from d a t a i n R e f e r e n c e 1 0 ; t h o s e f o r Groups 14-22 were d e t e r m i n e d from d a t a i n R e f e r e n c e 11
- 1s -
Shie ld O p t i m i z a t i o n
Many a p p l i c a t i o n s f o r 2 5 2 C f u s e w i l l r e q u i r e s h i e l d s which are h igh ly mobile; t h e r e f o r e , weight i s l i k e l y t o be t h e most sens i t ive des ign parameter f o r o p t i m i z a t i o n . a p p l i c a t i o n s of 2 5 2 C f , such as geo log ica l wel l - logging, t h e source i s t r a n s p o r t e d over rough t e r r a i n t o remote l o c a t i o n s f o r u se . In medical o r educa t iona l uses t h e source i s s t o r e d nea r i t s p l a c e of use. Obviously, t h e weight and s i z e of s h i e l d s f o r t h e s e d i f f e r e n t a p p l i c a t i o n s have d i f f e r e n t p r i o r i t y . General ly , t h e more dense gamma s h i e l d i n g m a t e r i a l s should be kept c l o s e t o t h e sou rce where they occupy l e s s volume f o r a given th i ckness of absorber . complicate t h e des ign and u s u a l l y r e q u i r e laminat ion of t h e dense and l i g h t e r s h i e l d m a t e r i a l s . Addit ives such as boron and l i t h i u m can be used t o suppress t h e secondary gamma dose from hydrogenous s h i e l d s . (A more mathematical approach t o s p h e r i c a l s h i e l d weight op t imiza t ion may be found i n Reference 1 2 2 ,
I n many o f t h e
Secondary g a k a r a d i a t i o n s o r ig ina tyng i n t h e s h i e l d
Sh ie ld Hea t ing
252Cf g e n e r a t e s about 38 watts of energy p e r gram o f pu re i s o t o p e : remainder, from spontaneous f i s s i o n . 86% i s depos i t ed i n t h e sou rce capsu le ; t h e remainder, i n t h e form of neu t rons , gamma r a y s , and n e u t r i n o s , i s d i s s i p a t e d i n t h e s h i e l d and beyond. Thus, 93% of t h e 38 w a t t s / g , o r 35 w a t t s / g , appears as s e n s i b l e h e a t a t t h e sou rce .
h a l f o f which comes from an a lpha decay r e a c t i o n ; t h e O f t h e f i s s i o n energy r e l e a s e d ,
An approximation can be made by assuming t h a t a l l t h e hea t i s generated a t t h e sou rce , then one can use a simple hea t conduction equat ion t o e s t i m a t e maximum s h i e l d i n t e r n a l temperatures . example, assume a 1-mg 2 5 2 C f sou rce i n a p a r a f f i n s p h e r i c a l cask of 40-in. OD and 4 - in . ID. The temperature d i f f e r e n c e through t h e p a r a f f i n s h i e l d would be 0.4"F from
For
r - r1
K 4 ~ r rorl 3.41 Q o AT =
where
K = thermal conduc t iv i ty ( 0 . 1 4 BTU-ft/hr-ft*-'F)
Q = thermal power, 2 5 2 C f source h e a t = 0.038 wa t t s
r = o u t e r r a d i u s , 20 inches = 1.67 f t
rl = i n n e r r a d i u s , 2 inches = 0.167 f t 0
- 16 -
This r e s u l t i s conse rva t ive because i t ignores t h e e f f e c t of h e a t leakage along t h e source rod and shea th .
The above example i l l u s t r a t e s t h a t h e a t i n g i s no t a major design concern f o r moderately s i z e d 2 5 2 C f sources . d e t a i l e d a n a l y s i s of t h e thermal cons ide ra t ions i n both f inned and unfinned casks i s given i n Reference 5A.
A more
S h i e l d Damage f r o m R a d i a t i o n
Extended exposure t o neutron and gamma r a d i a t i o n w i l l p o s s i b l y a l t e r t h e s t r u c t u r e and p h y s i c a l p r o p e r t i e s of s h i e l d m a t e r i a l s . This i s caused i n p a r t by c o l l i s i o n of f a s t neutrons with s h i e l d n u c l e i , d i s p l a c i n g them from m e t a l l i c l a t t i c e s o r from molecules. Ion formation o r e x c i t e d atoms a l s o c o n t r i b u t e t o s h i e l d damage. Fragments of d i s r u p t e d molecules i n some m a t e r i a l s may form new chemical compounds,some of which may be gaseous ( i . e . , C H b , H2).
Radiat ion damage from 2 5 2 C f sources i n casks and s h i e l d s would be a n t i c i p a t e d t o be much lower than r e p o r t e d e f f e c t s i n n u c l e a r r e a c t o r s because of t h e lower doses and l e s s c r i t i c a l o p e r a t i n g cond i t ions . Nevertheless , f o r l a r g e 5 2 C f source containment, p r o v i s i o n f o r gas vent ing and cask component replacement may be deemed necessary. There has been a l a r g e amount of work done on r a d i a i i v n damage e f f e c t s , 13,14
Induced A c t i v i t i e s
Neutron i r r a d i a t i o n l eads t o induced a c t i v i t y which can be est imated using t h e equat ion:
A = Na+(l - e -AT )
where N = number of atoms
CJ = c ross s e c t i o n , cm2/atom
4 = f l u x , neutron/cm2-sec
T = t ime, s e c
A = decay cons t an t of a c t i v a t e d material
- 17 -
n
S i z e and Weight
For many applications, both the physical size and the weight of the source shield may be important considerations.
Since very few shields are constructed of only on material, and becuse their shape may vary as well, the shield designer must compute separately the various volumes and weights of the shields. Figures 49 and 50 showing spherical and cylindrical shield volumes and Figures 51 and 52 showing some spherical and cylindrical shield weights for some common material densities are included and may be used as a guide for first approximation data. for spherical geometries can usually be used for cylindrical shields as well.
Shield data
S H I E L D MATER I AL SELECT I O N
Several factors must be considered when selecting a shield They are cost, safety, reliability, weight, and main-
Properties to be considered are density, material. tenance requirements. fabricability, durability, and heat transfer properties. following sections of this report are designed to provide only neutron and gamma attenuation properties calculated for a 252Cf source spectrum. costs, merits, properties, or construction methods.
The
No attempt has been made to catalogue material
S i ngl e Component S h i e l d s
"Bene lex "
"Benelex" (Figure 4) is made from pulverized wood chips. Unwanted elements are driven off, leaving only cellulose fibers and lignin, wood's natural bonding agent. rewelded by pressing to form panels of controlled densities, thicknesses, and sizes.
Fibers and lignin are
This material is a good neutron shielding material because of its relatively high hydrogen atom concentration. density of "Benelex" is 1.4 g/cm3.
The average
"Benelex" has good structural strength and rigidity, is easily machined.
- 18 -
Iron and Lead
I r o n and/or l ead by i t s e l f provide almost no fast neutron a t t e n u a t i o n (ppb = 11.3 g/Cm3, PFe = 7.9 g/cm3). m a t e r i a l s do provide gamma a t t e n u a t i o n , however, and when used with hydrogenous materials become an i m o r t a n t p a r t o f c a l i - fornium s h i e l d s . Figure 5 shows t h e 259Cf primary gamma a t t e n u - a t i o n f o r i r o n , l e a d , and normal conc re t e . I ron and lead w i l l a t t e n u a t e most c a p t u r e gammas i n a s i m i l a r manner. For compari- son of t h e gamma a t t e n u a t i o n p r o p e r t i e s , normal conc re t e (p = 2.35 g/cm3) i s a l s o shown.
Both o f t h e s e
"Lucite" ( C 5 H 8 0 ~ ) ~ with a d e n s i t y o f Q1.18 i s a good neutron s h i e l d l i k e almost a l l o rgan ic m a t e r i a l s (Figure 6 ) . I t s low d e n s i t y i n d i c a t e s i t must ( i n most ca ses ) be combined with a more dense m a t e r i a l t o s h i e l d t h e gamma r a y s . lead laminat ions w i l l perform s i m i l a r l y i n s h i e l d i n g e f f e c t i v e n e s s t o i r o n o r l ead and water.
"Lucite" and i r o n o r
Paraffin
Because of i t s high hydrogen d e n s i t y , p a r a f f i n ( C 3 0 H 6 2 )
(densi ty Q0.952 g/cm3) provides an e x c e l l e n t neutron s h i e l d (Figure 7 ) . P a r a f f i n , i n combination with a thermal neutron abso rbe r , such as boron o r l i t h i u m , and with a more dense gamma a t t e n u a t o r , such as i r o n o r l e a d , provides an e x c e l l e n t s h i e l d f o r ca l i fo rn ium. I t i s e a s i l y formed, bu t because of i t s low melt ing and f l a s h p o i n t s , i t must be used with c a r e .
Po 2 ye thy 2 ene
Polyethylene (p = 0.961 g/cm3) has a hydrogen d e n s i t y g r e a t e r t han t h a t of water; t h e r e f o r e , i t s neutron a t t e n u a t i o n p r o p e r t i e s are b e t t e r t h a n t h o s e of water (Figure 8 ) . t o shape, and can be purchased i n many preformed shapes. I t s low d e n s i t y , however, r e q u i r e s t h a t i t be used i n con junc t ion with some gamma a t t e n u a t i o n m a t e r i a l s . Because it s o f t e n s a t about 220'F and burns r e a d i l y , i t r e q u i r e s c a r e f u l u se . adding boron ca rb ide (B4C).
I t i s e a s i l y formed
I t can be borated by
* Registered trademark of Du Pont.
- 19 -
Water
Because of i t s high d e n s i t y of hydrogen atoms, c o s t , and a v a i l a b i l i t y , water Is o f t e n used as a s h i e l d i n g m a t e r i a l (Figure 9 ) . Because t h e mass d e n s i t y i s low, however, i t i s a poor gamma s h i e l d . Capture gamma r a d i a t i o n produced i n t h i c k s h i e l d s a l s o may be t h e most dominant t ype of r a d i a t i o n c o n t r i b u t i n g t o t h e dose r a t e . The a d d i t i o n of a thermal neutron absorber such as boron w i l l h e lp reduce t h e c a p t u r e gammas. Boron-containing compounds, however, a r e known t o corrode carbon s t e e l , p r i m a r i l y a t t h e a i r s u r f a c e i n t e r f a c e . For t h i s reason, s t a i n l e s s s t e e l should be considered f o r a t least t h e a i r - l i q u i d i n t e r f a c e .
The primary c o s t of water s h i e l d s i s containment. P l a s t i c water-containing t anks appear f e a s i b l e i n conjunct ion with some gamma s h i e l d i n g m a t e r i a l s .
Water a l s o has t h e disadvantages of evapora t ing o r l eak ing which could r ende r t h e s h i e l d u s e l e s s .
Water-Extended Po Zyester
A water-extended p o l y e s t e r resin used as a s h i e l d by G . D . O l i v e r , Jr . and E . Bailey Moore15 of t h e M . D . Anderson Hosp i t a l and Tumor I n s t i t u t e has t h e appeal ing q u a l i t i e s of easy p r e p a r a t i o n , high water con ten t , s o l i d form, and r e s i s t a n c e t o f i r e (Figure 10) .
The water-extended p o l y e s t e r with 65% water ( c a l l e d WEP-65) has a d e n s i t y o f 1.1 g/cm3.
Lithium- 6 Stearate
Lithium-6 S t e a r a t e , with a me l t ing p o i n t around 2 2 O o C , a t t e n u a t e s neutrons s i m i l a r l y t o water (Figure 1 1 ) . I t can be c a s t and r e c a s t i n t o shape, carved, machined, o r d r i l l e d . When used wi th a more dense material , such as i r o n o r l ead t o a t t e n u a t e t h e rimary gamma r a y s , i t appears t o be a good s h i e l d i n g m a t e r i a l f o r '52cf.
G Z y c e ~ n
Glycerin (C3H~03) with a d e n s i t y of Q1.26 g/cm3 i s a good neutron s h i e l d (Figure 1 2 ) . I t s low d e n s i t y i n d i c a t e s i t may r e q u i r e t h e a d d i t i o n o f a more dense m a t e r i a l t o s h i e l d t h e gamma r a y s . t h e cap tu re gamma dose r a t e .
Glycerin w i l l mix e a s i l y with b o r i c ac id s o l u t i o n t o reduce
- 20 -
Q
Mu1 ti component Shields
Concrete
Concrete i s a commonly used s h i e l d i n g m a t e r i a l t h a t i s inexpensive, s t r u c t u r a l l y u s e f u l , and e a s i l y formed. Concrete can vary widely i n i t s elemental composition, and because of t h i s w i l l v a ry cons ide rab ly i n s h i e l d i n g e f f e c t i v e n e s s f o r ca l i fo rn ium. F igu res 13 through 16 show t h e e f f e c t of varying t h e elemental composition of conc re t e s and t h e densf.y. cu r ing time and temperature , and t h e aggregate used a f f e c t t h e water con ten t of t h e conc re t e . be important t o ensure t h a t t h e mixture i s f r e e from l a r g e vo ids and i s of s u f f i c i e n t l y high d e n s i t y .
The most s eve re ly
Aggregate s ize a l s o may
2 5 2 C f s h i e l d i n g i s similar t o nuc lea r r e a c t o r s h i e l d i n g i n t h a t both neutron and gamma r a y s must be a t t e n u a t e d . Concrete has been widely used as an inexpensive r e a c t o r s h i e l d because i t i s a mixture of l i g h t n u c l i d e s , which slow down neu t rons , and heavy n u c l i d e s , which absorb gamma r a y s . Although not a s e f f i c i e n t a neutron s h i e l d as p a r a f f i n , polyethylene, o r wa te r , conc re t e casks provide a s a f e t y f a c t o r because they w i l l no t me l t , burn, o r l e a k .
The conc re t e s chosen r e p r e s e n t both t h e b e s t and worst a t t e n u a t i o n t o be expected from any conc re t e s h i e l d . The conc re t e types 01, 03, and 02A a r e c l a s s i f i e d a s o rd ina ry conc re t e s con- s i s t i n g of a water-cured cement and an aggregate y i e l d i n g a d e n s i t y of 2 .3 g/cm3. Heavy conc re t e s [ e . g . , LS-a (Limonite and S t e e l Punchings, t ype a] a r e made by a d d i n t i r o n punchings t o t h e aggre- g a t e t o o b t a i n a d e n s i t y of 4 . 6 g/cm . gamma-ray a t t e n u a t i o n (Figure 16 ) .
A d e n s i t y i n c r e a s e improves
Water con ten t p l a y s a s i g n i f i c a n t r o l e i n t h e e f f e c t i v e n e s s of conc re t e as a neutron s h i e l d because hydrogen i s t h e most e f f e c t i v e l i g h t element f o r slowing down neutrons. A f a c t o r of 8 i n t h e d i f f e r e n c e i n hydrogen con ten t between conc re t e types 01 and LS-a i s r e f l e c t e d i n t h e i r r e l a t i v e neutron s h i e l d i n g a b i l i t i e s .
s o i z The major c o n s t i t u e n t s of Nevada Test S i t e (N.T.S.) s o i l with
varying degrees of water content a r e given i n Reference 18. S o i l may vary cons ide rab ly i n i t s elemental composition and i t s water con ten t f o r d i f f e r e n t l o c a t i o n s as we l l as climates around t h e world. This d a t a may be u s e f u l , however, f o r t hose cons ide r ing underground s t o r a g e of ca l i fo rn ium sources , o r f o r t hose u s e r s making t e s t s such as a c t i v a t i o n ana lyses o r well logging i n t h e s o i l (Figures 17-19.)
- 2 1 -
Iron- Water and Lead- Water Combinations
Combinations o f water and dense m a t e r i a l s such a s i r o n o r The i r o n o r lead are good o v e r a l l s h i e l d materials f o r 2 5 2 C f .
l ead and water combinations can be made l i k e laminat ions o r l i k e a homogeneous s o l u t i o n , Water s h i e l d s con ta in ing 20 vo l % l ead and 60 v o l % i r o n , uniformly d i s t r i b u t e d throughout t h e s h i e l d , w i l l a t t e n u a t e neut rons a t about t h e same r a t e a s t h e gamma rays . This n e a r l y homogeneous s h i e l d can be obta ined by us ing t h i n m u l t i p l e s l a b s , packed lead o r s t e e l wool, o r b a l l s ( s h o t ) . The a d d i t i o n o f a h i g h l y absorb ing thermal neut ron absorber , such a s boron o r l i t h ium, w i l l reduce t h e cap tu re gammas so t h a t t h e primary dose r a t e w i l l b e from t h e f a s t neut rons even f o r s h i e l d s 100 cm t h i c k i n a 40 v o l % F e - H20 s h i e l d . Although laminat ions do cause i n c r e a s i n g and decreas ing dose r a t e s from t h e var ious energy groups, an approximate f i n a l dose r a t e can be es t imated wi th t h e assumed homogeneous mixtures shown i n F igures 20-25.
If t h e r e i s some q u e s t i o n as t o what arrangement may be cons idered homogeneous, t hen t h e s h i e l d should be eva lua ted on t h e b a s i s of i t s a c t u a l materials arrangement. A l l t h e i r o n o r l e a d cannot be p l aced c l o s e t o t h e source f o r t h i s i n t e r p r e - t a t i o n , because t h e cap tu re gamma r a d i a t i o n produced i n t h e s h i e l d w i l l e a s i l y p e n e t r a t e t h e remaining water. Placement o f a l l t h e l ead o r i r o n on t h e o u t s i d e of t h e s h i e l d may n o t p rov ide t h e neut ron a t t e n u a t i o n necessa ry , and w i l l i n c r e a s e t h e o v e r a l l weight o f t h e s h i e l d s i g n i f i c a n t l y .
Boron Addition to Hydrogenous ShieZds
Because most hydrogenous s h i e l d s have a d e n s i t y ~1 g/cm3, they are poor gamma r a d i a t i o n s h i e l d s . d e n s i t y i s h igh they are e x c e l l e n t s h i e l d s f o r f a s t neut rons . A s u b s t a n t i a l r educ t ion i n cap tu re $amma dose r a t e s can be obta ined by adding boron t o t h e hydrogenous 5 2 C f s h i e l d . This a d d i t i o n of boron can e i t h e r reduce t h e s h i e l d weight and s i z e , o r i n c r e a s e t h e 2 5 2 C f c apac i ty o f t h e s h i e l d .
However, because t h e hydrogen
Boron has a h igh c r o s s s e c t i o n (3840 ba rns ) f o r t h e neut ron abso rp t ion r e a c t i o n "B(n,a) ' L i , which i s accompanied by a 0.48-Mev gamma r a y from t h e e x c i t e d 7 L i nuc l ide . hydrogen (0.33 barn) produces a 2.23-Mev cap tu re gamma r a y t h a t i s more d i f f i c u l t t o a t t e n u a t e than t h e 0.48-Mev boron r e a c t i o n gamma r a y . hydrogen i n p ropor t ion t o t h e reduced macroscopic c r o s s s e c t i o n f o r hydrogen i n t h e mixture and produces a change i n t h e s p e c t r a l d i s t r i b u t i o n o f t h e neut rons and cap tu re gamma r a y s .
Neutron abso rp t ion i n
The a d d i t i o n of boron reduces neutron cap tu res i n
- 22 -
Thermal neutron dose r a t e s a r e reduced by $90% with t h e a d d i t i o n of boron; however, t h e t o t a l neutron dose r a t e s i n t h e s h i e l d remain r e l a t i v e l y una f fec t ed because thermal neu t rons normally c o n s t i t u t e l e s s than 10% of t h e t o t a l neutron dose r a t e .
F igu re 26 shows t h e e f f e c t of ' O B on t h e c a p t u r e gamma dose r a t e f o r a s p h e r i c a l water s h i e l d . dec reases r a p i d l y and then l e v e l s o f f a t low 'OB c o n c e n t r a t i o n s .
F igu re 27 shows t h e e f f e c t of n a t u r a l boron a d d i t i o n t o s p h e r i c a l p a r a f f i n s h i e l d s ,
boron c a r b i d e ( B 4 C ) t o p a r a f f i n . A t t h i c k n e s s e s up t o about 30 cm, p a r a f f i n by i t s e l f r a p i d l y reduces t h e t o t a l dose r a t e because of i t s a t t e n u a t i o n of t h e dominant f a s t neutron r a d i a t i o n (Figure 7 ) . A t t h i c k n e s s e s g r e a t e r than 30 cm, t h e gamma r a d i a t i o n i s t h e predominant c o n t r i b u t o r t o t h e dose r a t e . This gamma r a d i a t i o n i s from two s o u r c e s ,
The cap tu re gamma dose r a t e
Figures 28 through 34 show t h e e f f e c t of t h e a d d i t i o n of
1) Primary r a d i a t i o n which can be s h i e l d e d by surrounding t h e source with a dense m a t e r i a l .
2 ) Secondary o r c a p t u r e gamma r a d i a t i o n r e s u l t i n g i n t h e s h i e l d m a t e r i a l .
Two methods a r e used t o reduce t h e c a p t u r e gamma dose r a t e : one i s t o provide s u f f i c i e n t dense m a t e r i a l i n t h e outermost p a r t of t h e s h i e l d , and t h e o t h e r i s t o provide a thermal neu t ron abso rbe r such as ' O B . I t i s sometimes necessary t o u s e both methods. Care should be t a k e n , however, t o a s s u r e t h a t boron a d d i t i v e s do not con ta in l a r g e q u a n t i t i e s of elements t h a t r e s u l t i n s i g n i f i c a n t l y more e n e r g e t i c gamma r a y s than t h o s e produced by hydrogen.
Figure 34 shows t h e t o t a l neutron and gamma dose r a t e ( l e s s t h e primary gamma r a d i a t i o n ) i n p a r a f f i n w i t h va ry ing q u a n t i t i e s of B 4 C added. For s h i e l d s g r e a t e r t han 30 cm i n t h i c k n e s s , tf.e a d d i t i o n of boron c a r b i d e s i g n i f i c a n t l y reduces t h e t o t a l dose r a t e i n t h e s h i e l d . Reference 19 l i s t s many p o t e n t i a l s h i e l d a d d i t i v e s con ta in ing boron. The e f f e c t of boron a d d i t i o n t o water f o r windows i s d i scussed on page 25.
Li th ium A d d i t i o n t o H3drogenous ShieZds
S u b s t a n t i a l r e d u c t i o n i n c a p t u r e gamma dose r a t e s can be ob ta ined by adding n a t u r a l l i t h i u m t o hydrogenous 2 5 2 C f s h i e l d s . Th i s r e d u c t i o n i s caused by t h e 6 L i ( n , a ) T r eac t ion ,wh ich has a 945-barn c r o s s s e c t i o n f o r thermal neu t rons and produces no c a p t u r e gamma r a d i a t i o n . The l i t h i u m a b s o r p t i o n o v e r r i d e s t h e H(n,y)D r e a c t i o n i n t h e hydrogenous materia1,which i s t h e predominant sou rce of t h e secondary gamma. l i t h i u m compounds used f o r t h i s purpose do no t r e s u l t i n s i g n i f i - c a n t l y more e n e r g e t i c gamma r a y s than t h o s e produced by t h e
Care should be taken t o a s s u r e t h a t
hydrogen. - 2 3 -
The r e s u l t s shown i n Figure 35 were c a l c u l a t e d with t h e ANISN code f o r d i f f e r e n t concen t r a t ions of n a t u r a l (7.65% 6Li) l i t h ium and s h i e l d t h i c k n e s s e s from 14 t o 50 i n . The cap tu re gamma dose de,creases r a p i d l y on a d d i t i o n of r e l a t i v e l y small amounts of l i t h i u m .
~
A
*Contamination i s r e f e r r e d t o he re as t h e presence of an unwanted m a t e r i a l , r a d i o a c t i v e o r otherwise.
Previous s e c t i o n s have d i scussed t h e similar e f f e c t of boron a d d i t i o n t o reduce c a p t u r e gamma a c t i v i t y . f i n d l i t h i u m more a t t r a c t i v e than boron as a moderator a d d i t i o n because i t i s e a s i l y d i s t r i b u t e d uniformly i n t h e s h i e l d m a t e r i a l .
Cask des igne r s may
To estimate t h e e f fec t of l i t h ium a d d i t i o n on a s p e c i f i c cask d e s i g n , t h e cap tu re gamma dose determined from curves i s m u l t i p l i e d by t h e a p p r o p r i a t e r e d u c t i o n f a c t o r f o r t h e d e s i r e d l i t h i u m concen t r a t ion . Neutron dose r a t e i nc reased only 10% over t h e range of l i t h i u m concen t r a t ions shown.
Window A t tenua t ion
Large sources t h a t must be handled r o u t i n e l y a t s h o r t d i s t a n c e s o f t e n r e q u i r e t h e use of a s h i e l d i n g window. a d d i t i o n t o i t s s h i e l d i n g e f f e c t i v e n e s s , windows must have good t r anspa rency f o r viewing, allow s u f f i c i e n t viewing a r e a , and have a r easonab le l i f e t i m e .
In
Although i n some i n s t a n c e s hydrogenous m a t e r i a l s such as water o r o i l , contained by g l a s s o r "Lucitei ' may be warranted, t h e s e low d e n s i t y materials o f f e r such poor gamma a t t e n u a t i o n by themselves, i t i s g e n e r a l l y necessary t o make t h e window much l a r g e r t han t h e s h i e l d i n g wall . For t h i s r eason , t h e s e m a t e r i a l s a r e o f t e n combined wi th reasonably t r a n s p a r e n t m a t e r i a l s w i t h h i g h e r densi ty , such as l ead -con ta in ing g l a s s .
The a t t e n u a t i o n p r o p e r t i e s f o r "Lucite" o r water windows a r e shown i n F igu res 6 and 9 .
The a t t e n u a t i o n p r o p e r t i e s of laminat ions o f va r ious types Figure 36
The s i d e of t h e window
of m a t e r i a l s , however, become much more complicated. shows t h e a t t e n u a t i o n p r o p e r t i e s f o r one s p e c i f i c design with t h r e e types of g l a s s combined with o i l . n e a r e s t t h e source has a double th i ckness of nonbrowning g l a s s s e p a r a t e d by an a i r space , which a c t s a s a p r o t e c t i v e b a r r i e r t o t h e window from r a d i a t i o n damage, chemical damage, shock, and contamination*. The g l a s s pane n e a r e s t t h e source i s u s u a l l y
8
n
designed s o t h a t i t might be changed i f damaged without a f f e c t i n g t h e res t of t h e window.
Because s p e c t r a l changes occur as t h e t h i c k n e s s of t h e s h i e l d changes, each window design with t h e s e laminat ions should be s e p a r a t e l y c a l c u l a t e d f o r t h e s e changes.
The compositions of t h e m a t e r i a l s used i n t h e c a l c u l a t i o n s a r e l i s t e d i n Table VIII.
Zinc bromide (ZnBr2) s o l u t i o n s can be used as s h i e l d i n g windows f o r 2 5 2 C f sou rces . e f f e c t of t h i c k n e s s on dose r a t e f o r t h r e e d i f f e r e n t concen t r a t ions of Z n B r 2 . r e s i s t a n c e t o t h e e f f e c t s of r a d i a t i o n . Reported d a t a suggest t h e a d d i t i o n of hydroxylamine hydrochlor ide (H2NOH*HC1) t o i n h i b i t d i s c o l o r a t i o n . Venting and c a r e f u l s e l e c t i o n o f m a t e r i a l s (because of co r ros ion ) may be r equ i r ed .
Figures 37 , 38, and 39 show the
These s o l u t i o n s have good t r anspa rency and good
Boric a c i d (HJBO~) s o l u t i o n s can be used i n s h i e l d i n g windows. Figure 40 shows t h e e f f e c t of t h i ckness on dose r a t e f o r s a t u r a t e d b o r i c a c i d .
TABLE VI11
Composition o f Window Materials Elemental Density,
Material Component g (component)/cm3 (compound)
Oi 1 t l
C Glass (p=2.7 g/cm3) B 2 0 3
NazO
Si02
0.125
0.751
0.081
0.176
1.750
Glass (p=3.39 g /cm3) S i 0 2
PbO Na20
K20 t i lass ( ~ 6 . 2 9 g/cm3) Si02
PbO
B2°3
Concentrated zinc Zr bromide solution Br
H
0
Saturated t13 BOI, H
0
B
- 25 -
1.617
1.148
0.017
0.462
0.341
5.084
0.775
0.5588
1.3663
0.0639
0.5111
0.108
0.884
0.008
SPECIAL APPLICATIONS
MED I CAL APPL I CAT IONS
Medical applications are expected for californium-252 sources ranging from 0.5 vg to 50 vg. are stored in one location, shielding requirements (for storage) may range from 0.5 1.18 to 200 1-18. from a few centimeters to ,100 cm depending on the allowable dose rate at the container surface.
If a number of such sources
Shields, therefore, may range
Some examples which represent the dose rate in steel (Fe) followed by saturated boric acid solution or paraffin containing 1 wt % B4C are given in Figures 41 through 44. show the effect of surrounding the hydrogenous materials with more steel. This added component of dense material adds considerably to the shield by further attenuating the primary gamma rays, and also reducing the secondary gamma contribution. approximately 3 in. (7.62 cm) of steel on the outside of the shield will reduce both the primary gamma and the capture gamma dose rate by about a factor of 10. attenuation factor.)
Figures 45 and 46
In such shields,
(See Figure 5 for an approximate
Portability may be desirable in many medical applications, necessitating size and weight considerations. - ACTIVATION ANALYSIS
Californium-252 sources used for activation analysis are expected to range from 0.1 t o 100 mg. the thermal flux predicted at various thicknesses of some commonly used moderating materials.
Figures 47 and 48 indicate
Fast fluxes are also of interest because of the excitation produced in some elements in the inelas- tic scattering process.
- 26 -
SHIELDING EFFECTIVENESS OF COMMONLY USED CASKS
Most casks b u i l t f o r gamma s h i e l d i n g a r e inadequate f o r Some casks , using
water as a h e a t t r a n s f e r mechanism, may be used as 2 5 2 C f shipping casks. Concrete casks may a l s o be used; however, because t h e emphasis has i n t h e p a s t p r i m a r i l y been p l aced on t h e d e n s i t y , t h e hydrogen con ten t o f t h e conc re t e i s u s u a l l y unknown. i s t h e case , t h e b e s t way t o determine i t s s h i e l d i n g c a p a c i t y i s by measurement. A d e t a i l e d chemical a n a l y s i s of t h e conc re t e w i l l al low i t t o be c a l c u l a t e d with t h e ANISN code.
5 2 C f because they lack hydrogenous m a t e r i a l s .
I f t h i s
P a r a f f i n - f i l l e d drums used t o s h i p and s t o r e Po-Be and Pu-Be sources a r e u s u a l l y a v a i l a b l e , or may be f a b r i c a t e d a t low c o s t . A s shown i n Table IX, t h e s e drums a r e l i m i t e d t o l e s s than 1 mg of 2 5 2 C f .
TABLE I X
C a l c u l a ted E f f e c t i veness o f P a r a f f i n-Fi 11 ed Drums
Size , g a l
15
30
55
70
100
130
170
230
300
Drum Dia,
i n .
15%
18
2 242
27
30
33
36
40
44
Common Carrier
Shippin L i m i t , W t , 1-(g 2 % f l b
- 27 -
1 7
62
93
180
240
313
400
525
700
150
250
450
600
900
1,200
1,500
2,000
2,600
LIST OF FIGURES
Figure Page
1 Spher i ca l S h i e l d Geometry . . . . . . . . . . . . . 31
2 C y l i n d r i c a l Sh ie ld Geometry . . . . . . . . . . . . 31
4 "Benelex" - Fast Neutron, Thermal Neutron, Primary Gamma, and Capture Gamma Dose Rate t o T i s sue i n "Benelex" from a Po in t I s o t r o p i c F i s s i o n Source of 2 5 2 C f . . . . . . . . . . . . . . . . . . . . . . . 32
5 I r o n , Lead, and Normal Concrete - At tenua t ion of Primary Gamma Rays (and Capture Gamma Rays Resu l t ing from Most Hydrogenous Sh ie lds ) from a Po in t I s o t r o p i c F i s s i o n Source of 2 5 2 C f f o r I r o n , Lead, and Normal Concrete . . . . . . . . . . . . . . . . . . . . . . 33
Fast Neutron, Thermal Neutron, Primary Gamma, and Capture Gamma Dose Rate t o T i s sue from a Po in t I s o t r o p i c F i s s i o n Source of 252Cf
6 I n t 'Lucite" . . . . . . . . . . . . . . . . . . 34
7 I n P a r a f f i n . . . . . . . . . . . . . . . . . . . 35
44 16 I n Type LS-a Concrete . . . . . . . . . . . . . . 17 I n N.T.S. (Dry) S o i l . . . . . . . . . . . . . . 45
18 I n N.T.S. (50% Sa tu ra t ed ) S o i l . . . . . . . . . 46
- 28 -
Figure Page
19 In N.T.S. (100% S a t u r a t e d ) S o i l 47 0 . . . . . . . . .
20 1n.Medium of 10 vo l % Pb i n H 2 0 . . . . . . . . . 48
2 1 In Medium of 20 vo l % Pb i n H 2 0 . . . . . . . . . 49
2 2 In Medium of 10 vo l % Fe i n H 2 0 . . . . . . . . . 50
23 I n Medium of 20 v o l % Fe i n H 2 0 . . . . . . . . . 51
24 In Medium of 40 vo l % F e i n H 2 0 . . . . . . . . . 52
25 In Medium of 60 v o l % Fe i n H 2 0 . . . . . . . . . 53
26 Boron i n Water . E f f e c t of Adding Boron t o a Sphe r i ca l Water Sh ie ld on t h e Capture Gamma Dose Rate . . . . . 54
27 Boron i n P a r a f f i n . E f f e c t of Adding Boron t o a Sphe r i ca l P a r a f f i n S h i e l d on t h e Capture Gamma Dose Rate . . . . . . . . . . . . . . . . . . . . . . . . 55
F a s t Neutron. Thermal Neutron. Primary Gamma. and Capture Gamma Dose Rate from a Po in t I s o t r o p i c F i s s i o n Source of 2 5 2 ~ f
28 I n Medium of 0 .05 w t % B4C i n P a r a f f i n . . . . . 56
29 In Medium of 0 . 1 w t % B4C i n P a r a f f i n . . . . . . 57
30 In Medium of 0.5 w t % B4C i n Paraff in . . . . . . 58
31 In Medium of 1 .0 w t % B 4 C i n P a r a f f i n . . . . . . 59
3 2 In Medium of 2.0 w t % B 4 C i n Pa ra f f in . . . . . . 60
33 In Medium of 5.0 w t % B 4 C i n P a r a f f i n . . . . . . 61
34 Varying Amounts of B4C i n P a r a f f i n . T o t a l Dose Rate (Without Primary Gamma Dose Rate) i n Paraff in with Varying Amounts of B4C from a Po in t I s o t r o p i c F i s s i o n Source of 2 5 2 C f . . . . . . . . . . . . . . . . . . . 62
35 Lithium i n P a r a f f i n . E f f e c t of Adding Lithium t o Hydrogenous Sh ie lds on Capture Gamma Dose Rate . . . 6 3
. 29 .
Figure Page - Fast Neutron, Thermal Neutron, Primary Gamma, and Capture Gamma Dose Rate from a Point I s o t r o p i c F i s s i o n Source of 2 5 2 C f
36 I n a Laminated Glass and O i l Window Sh ie ld . . . . 64
37 I n a 50% ZnBr;! Window Sh ie ld . . . . . . . . . . . 65
38 I n a 65% ZnBr2 Window Sh ie ld . . . . . . . . . . . 66
39 I n a S a t u r a t e d ZnBr;! Window Sh ie ld . . . . . . . . 67
40 I n a S a t u r a t e d Boric Acid Window Sh ie ld . . . . . 68
4 1 I n a Sh ie ld with 1 - in . S t e e l and S a t u r a t e d Boric Acid So lu t ion . . . . . . . . . . . . . . . . . . 69
42 I n a S h i e l d with 2- in . S t e e l and Sa tu ra t ed Boric Acid S o l u t i o n . . . . . . . . . . . . . . . . . . 70
43 In a Sh ie ld wi th 2- in . S t e e l and 1.0 w t % BI+C i n P a r a f f i n . . . . . . . . . . . . . . . . . . . . . 71
44 In a Sh ie ld with 3- in . S t e e l and 1 .0 w t % B 4 C i n P a r a f f i n . . . . . . . . . . . . . . . . . . . . . 7 2
45 I n a Sh ie ld wi th 2- in . S t e e l , Sa tu ra t ed Boric Acid, and 2- in . S t e e l . . . . . . . . . . . . . . . . . 73
46 I n a Sh ie ld wi th 3 - in . S t e e l , 1 w t % B 4 C i n P a r a f f i n , and 3- in . S t e e l . . . . . . . . . . . . 74
47 Thermal Neutron Flux i n Be, BeO, D 2 0 , H 2 0 , and Graphi te as a Function of Distance from a Point I s o t r o p i c F i s s i o n Source of 252Cf . . . . . . . . . . 75
48 Thermal Neutron Flux i n Water, Polyethylene, and P a r a f f i n as a Function of Distance from a Point I s o t r o p i c F i s s i o n Source of 2 5 2 C f . . . . . . . . . . 76
49 Volume of a Sphere a s a Function of i t s Radius . . . . 77
50 Volume of a Cylinder as a Function of i t s Radius . . . 78
51 Weight of a Sphe r i ca l Sh ie ld as a Function of i t s Radius f o r M a t e r i a l s of D i f f e r e n t Density . . . . . . 79
52 Weight of a C y l i n d r i c a l Sh ie ld as a Function o f i t s Radius f o r M a t e r i a l s of D i f f e r e n t Density When h=2r . 80
- 30 -
Q
F I G . 1 SPHERICAL S H I E L D GEOMETRY
\ /
\ /
\ /
/ / \
I \
\ I \ /
*R
/ \
/ \ - / \
F I G . 2 C Y L I N D R I C A L S H I E L D GEOMETRY
F I G . 3 SLAB S H I E L D GEOMETRY
- 31 -
Radius, cm
F I G . 4 B E N E L E X - F A S T NEUTRON, THERMAL N E U T R O N , P R I M A R Y GAMMA, AND C A P T U R E GAMMA D O S E RATE T O T I S S U E IN " B E N E L E X " FROM A P O I N T I S O T R O P I C F I S S I O N S O U R C E OF 2 5 2 C f
- 32 -
. .. . - -. . . . . . . . _ _ . . .. .; . . . .
Radius, cm
F I G . 5 IRON, LEAD, AND NORMAL CONCRETE - ATTENUATION OF PRIMARY GAMMA RAYS (AND CAPTURE GAMMA RAYS R E S U L T I N G FROM MOST HYDROGENOUS S H I E L D S ) FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f FOR IRON, LEAD, AND NORMAL CONCRETE
- 33 -
Radius, cm
F I G . 6 L U C I T E - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, m T U R E GAMMA DOSE RATE TO T I S S U E I N " L U C I T E " FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- - - - - - - - - - - - - - - = Primary Gamma - - Fast Ne ut ro n - - -
- - - - Capture Gamma
Thermal Neutron - - - - -
A
- - - c - - - - -
- ~
E lemento I Compos1 tion
Element Density, g/c
H 0.140 C 0.812
0.952
- - - - - - - - - - - -
io & $0 8L ,A0 IhO I40
F I G . 7 GAMMA , FROM
aJ v) 0 n
Radius, cm
F I G . 8 POLYETHYLENE - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE TO T I S S U E I N POLYETHYLENE FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 36 -
Radius, cm
F I G . 9 WATER - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM O F WATER FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 37 -
/
Radius, cm A
F I G . 10 WATER-EXTENDED POLYESTER - FAST NEUTRON, THERMAL NEUTRON, w PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF WATER-EXTENDED POLYESTER (65%) FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 38 -
L L , E
E v
L v
cl
P * N L
I oc
io-'
IO-^
IO-^
IO-^
I o-6
io-'
I O-e
IO-^
lo-'o 0 20 40 60 80 I00 I20 140
Radius, cm F I G . 11 L I T H I U M - 6 STEARATE - F A S T NEUTRON, THERMAL NEUTRON,
PRIMARY GAMMA, AND CATTURE GAMMA DOSE RATE I N AN
I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f I N F I N I T E MEDIUM OF L I T H I U M - 6 STEARATE FROM A P O I N T
- 39 -
F I G . 12
10-1 4 ' I
Glvcerin Composition
Element Density, g/cm3
H 0.110 C 0.493 0 0.657
1.26
Q Radius, cm
GLYCERIN - FAST NEUTRON , THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N GLYCERIN FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 40 -
S .00192 K Ca .581 Fe .a726
-
2.33
0 20 40 60 80 100 120 140 Radius, cm
F I G . 13 T Y P E 01 CONCRETE - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N TYPE 01 CONCRETE FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 41 -
Radius, cm
F I G . 14 T Y P E 0 2 A C O N C R E T E - F A S T NEUTRON, THERMAL NEUTRON, P R I M A R Y GAMMA, AND C A P T U R E GAMMA DOSE RATE IN T Y P E 02A C O N C R E T E FROM A P O I N T I S O T R O P I C F I S S I O N S O U R C E OF 2 5 2 C f
8
K 0.004 Ca 0.582 Fe 0.026
2.368
0 F I G . 15 T Y P F 03 CONCRFTF - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N TYPE 03 CONCRETE FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 43 -
Radius, cm
F I G . 16 T Y P E LS-a CONCRETE - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N TYPE L S - a CONCRETE FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 44 -
Q
I-
N E V
V W v)
-
I
1 I 1
W 0 [L
c
Atomic Composition
Element IO'' (atoms/cm3)
H 8.553 0 22.68
AI 2.014 Si 9.533 1
I I I I 0 20 40 60 80 100 I20 I40
Radius, cm F I G . 17 N.T.S. S O I L (DRY) - F A S T NEUTRON, THERMAL NEUTRON,
PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF N.T.S. DRY S O I L FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 45 -
Atomic Composition
Element lo2' (atoms /cm3)
H 9.82 0 23.3 A I 1.83 Si 8.68 i
I I I I I I I 80 100 I20 140 20 40 60
Radius, cm
FIG. 18 N.T.S. SOIL (50% SATURATED) - FAST NEUTRON, THERMAL
IN AN INFINITE MEDIUM OF N.T.S. SOIL (50% SATURATED) NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE
FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
F I G . 19 N.T.S. SOIL (100% SATURATED) - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA. AND CAPTURE GAMMA DOSE RATE _ _ - IN AN INFINITE MEDIUM OF N.T.S. SOIL (100% SATURATED) FROM A P O I N T I S O T R O P I C SOURCE OF 2 5 2 C f
- 47 -
Radius, cm
F I G . 20 10% L E A D I N WATER - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF 10 VOL % Pb in H 2 0 FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 48 -
A
Radius, cm F I G . 21 20% LEAD I N WATER - F A S T NEUTRON, THERMAL NEUTRON,
I N F I N I T E MEDIUM OF 20 VOL % P b I N H 2 0 FROM A P O I N T PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN
I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 49 -
F I G . 22 10% I R O N I N WATER - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF 10 VOL % Fe I N H 2 0 FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
Radius, cm F I G . 23 20% I R O N I N WATER - F A S T NEUTRON, THERMAL NEUTRON,
RIMARY GAMMA. AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF 20 VOL % Fe I N H20 FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 5 1 -
'O" r--- lo-' I\
Fast Neutron
Thermal Neutron
c - -
L - v
0 -
- c -
a3 0 [r: c
- c -
Radius, cm F I G . 24 40% I R O N I N WATER - FAST NEUTRON, THERMAL NEUTRON,
PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF 40 VOL % Fe i n H 2 0 FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 5 2 -
. - . - __ .- . . .. . . .. -
Q
Radius, cm F I G . 25 60% I R O N I N WATER - FAST NEUTRON, THERMAL NEUTRON,
PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF 60 VOL % Fe I N H 2 0 FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 5 3 -
0.8
0.6 0) u) 0 n
E E
CY
CY c3
f? 0.4 3 Q 0 0
Q) > CY a, m
c
.- c
-
0.2
C
I I
-
Shield Thickness, cm -
3 I .o 2 .o 3.0 4.0
Boron -10 Concentration, mg/cm3
F I G . 26 BORON IN WATER - E F F E C T O F A D D I N G BORON TO A S P H E R I C A L WATER S H I E L D ON T H E C A P T U R E GAMMA D O S E RATE
0.6
a, 0 E t
a, fn 0 n
E 0.4 E 0 c3
e 3 Q 0 0 a, > 0 a,
t
.- t
- 0.2
Natural Boron, mq/cm3
0 20 40 60 80 too Radius, cm
F I G . 27 BORON I N P A R A F F I N - EFFECT OF ADDING BORON TO A SPHERICAL P A R A F F I N S H I E L D ON THE CAPTURE GAMMA DOSE RATE
- 55 -
IO0
lo-'
E I 1 1 I 1 I h
Radius, cm 28 0.5 WT % B4C I N P A R A F F I N - FAST NEUTRON, THERMAL
I N AN I N F I N I T E MEDIUM OF 0.05 WT % BbC I N P A R A F F I N NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE
FROM AN P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 56 -
IO-^
IO-^
IO-^
I o-6
to-'
I o-8
IO-^
"-'OO 20 40 60 80 I00 120 140
Radius, cm F I G . 29 0.1 WT % B4C I N P A R A F F I N - F A S T NEUTRON, THERMAL
NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF 0.1 WT % B4C I N P A R A F F I N FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 57 -
Radius. cm ~
F I G . 30 0.5 WT % B4C I N P A R A F F I N - F A S T NEUTRON, THERMAL
I N AN I N F I N I T E MEDIUM OF 0.5 WT % B4C I N P A R A F F I N NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE
FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 58 -
Q
(u
E u u Q) In
c
c-5
8 -
I
2 c 3 Q) c \
r - L
I
E
E v
c
L v
n N L
P * Q)
0 c
LT Q) In 0 n
F I G . 31
to-'
lo-2
IO-^
IO-^
IO-^
IO+
to-'
I O-e
IO-^
10-'O
Radius, cm 1.0 WT % B4C I N P A R A F F I N - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N AN I N F I N I T E MEDIUM OF 1.0 WT % BI+C I N P A R A F F I N FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 59 -
IO0
lo-'
8 Radius, cm
F I G . 32 2.0 WT % B4C I N P A R A F F I N - FAST NEUTRON, THERMAL
I N AN I N F I N I T E MEDIUM OF 2.0 WT % B4C I N P A R A F F I N NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE
FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 60 -
Radius, cm F I G . 33 5.0 WT % B Q C I N P A R A F F I N - F A S T NEUTRON, THERMAL
I N AN I N F I N I T E MEDIUM OF 5.0 WT % B4C I N P A R A F F I N NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE
FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
@
- 61 -
Radius, cm
F I G . 34 VARYING AMOUNTS OF B4C I N P A R A F F I N - TOTAL DOSE RATE (WITHOUT PRIMARY GAMMA DOSE RATE) I N P A R A F F I N W I T H VARYING AMOUNTS OF B4C FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f 8
- 6 2 -
. .. . - .
0.6
0.4
0.2
0 2 0 40 60 80 100
Radius, cm
F I G . 35 L I T H I U M I N P A R A F F I N - EFFECT OF ADDING L I T H I U M TO HYDROGENOUS S H I E L D S ON CAPTURE GAMMA DOSE RATE
- 63 -
Radius, cm
F I G . 36 GLASS AND O I L WINDOW S H I E L D - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N A LAMINATED GLASS AND OIL WINDOW S H I E L D FROM A P O I N T ' ISOTROPIC F I S S I O N OF 2 5 2 C f
Q
- 64 -
Radius, cm
50% Z n B r 2 WINDOW S H I E L D - FAST NEUTRON, THERMAL NEUTRON PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N A 50% Z n B r 2 WINDOW FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
F I G . 37
- 65 -
r
d
F I G . 38 65% Z n B r n WINDOW S H I E L D - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N A 65% Z n B r , WINDOW FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
Q
3 a, c \ - L
I Jz
E
i! v
h
v L
D
k d
N L
a, 0 [L
c
F I G
lo-' - - - - -
: - - - - - - -
IO-^ =- - - - - - - -
IO-^ =- - - - - - - -
IO-^ - - - - - - -
- - - - - - -
IO-^ =- - - - - - - -
I I 1 I 0 20 40 60 80 100 120 140
Radius, cm
I . 39 SATURATED Z n B r 2 WINDOW S H I E L D - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I A SATURATED Z n B r , WINDOW FROM A P O I N T I S O T R O P I C F I S S I SOURCE OF 2 5 2 C f
N ON
- 67 -
F I G . 40
IO0
lo-'
lo-*
IO-^
t 1 1 I 1 I I
Q
IO-^
10-9
t Radius, cm
B O R I C A C I D WINDOW S H I E L D S - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, CAPTURE GAMMA, AND TOTAL DOSE RATE I N A SATURATED BORIC A C I D WINDOW FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 68 -
IO0
lo-'
IO-^
i l0-'OO 20 40 60 80 100 120 140
Radius, cm SATURATED BORIC ACID AND 1-IN. STEEL - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE IN A SHIELD CONTAINING 1 IN. OF STEEL AND A SATURATED BORIC ACID SOLUTION FROM A POINT ISOTROPIC FISSION SOURCE OF 252CF
- 69 -
0, cn 0 n
I I I I I 1 I 20 40 60 80 100 120 140 0
Radius, cm
F I G . 42 SATURATED B O R I C A C I D AND 2 - I N . STEEL - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N A S H I E L D C O N T A I N I N G 2 I N . OF STEEL AND A SATURATED BORIC A C I D SOLUTION FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 70 -
Radius, cm 1.0 WT % B + C I N P A R A F F I N AND 2 - I N . STEEL - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N A S H I E L D C O N T A I N I N G 2 I N . OF S T E E L
F I S S I O N SGURCE OF 2 5 2 C f AND 1.0 WT % B+C I N P A R A F F I N FROM A P O I N T I S O T R O P I C
A F I G . 43
- 71 -
Radius, cm
F I G . 44 1.0 WT % B4C I N P A R A F F I N AND 3 - I N . S T E E L - FAST NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N A S H I E L D C O N T A I N I N G 3 I N . OF
I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f S T E E L AND 1.0 WT % B4C I N P A R A F F I N FROM A P O I N T
- 72 -
I . , .- .. . - ... . . - - - -
n
N E 0
V W In
c
- - I
I
2 + 3 W c - \ L
I Jz
E
E v
.. h
v L
n
P d-
N L
W 0 c
e W In 0 n
F I G . 45
l o o r------ to-'
IO-^
IO-^
0 . ' . * 0 J *
lo-'oO 20 40 60 80 100 120 140 Radius, cm
2 - I N . STEEL, SATURATED B O R I C A C I D , AND 2 - I N . STEEL - F A S T NEUTRON, THERMAL NEUTRON, PRIMARY GAMMA, AND CAPTURE GAMMA DOSE RATE I N A S H I E L D C O N T A I N I N G 2 I N . OF STEEL, SATURATED B O R I C A C I D SOLUTION, AND 2 I N . OF STEEL FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE O F 2 5 2 C f
- 73 -
N E u
0 W In
c
A - 0
I
2 4-
3 Q) c \
L c I
-
E 2 E W
c A
w L
n N L
k d- Q) c
C F Q, v) 0 n
F I G . 46
IO+
I 1 0 - 7 ~ o-8
Thermal Neutron
'I I w t o/o B4C in Paraffin
lo"oo 20 40 60 80 100 120 140 Radius. cm
3 - I N . STEEL, 1 WT % B 4 C I N P A R A F F I N , AND 3 - I N . STEEL - FAST NEUTRON. THERMAL NEUTRON, PRIMARY GAMMA, AND
1; CAPTURE-GAMMA DOSE RATE IN A SHIELD CONTAINING 3 IN. OF STEEL, 1 WT % B 4 C I N P A R A F F I N , AND 3 I N . OF STEEL FROM A P O I N T I S O T R O P I C F I S S I O N SOURCE OF 2 5 2 C f
- 74 -
Q
I- I
N E
E L Q f Y
I-
n
Radius, cm
FIG. 47 THERMAL NEUTRON FLUX IN Be, BeO, D20, H 2 0 , AND GRAPHITE A S A FUNCTION OF DISTANCE FROM A POINT ISOTROPIC FISSION SOURCE OF 252Cf
- 75 -
N E u n
I 0 4, m I C
-
2 c 3 4, c \
0 4, v) I
(u I
- I
E
e 0
C I
c '3 4, C
0
Ql
E
f L
U
c n
Radius, cm FIG. 48 THERMAL NEUTRON FLUX I N WATER, POLYETHYLENE, AND
PARAFFIN A S A FUNCTION OF DISTANCE FROM A POINT ISOTROPIC FISSION SOURCE OF 252Cf
- 76 -
Q
A
I o4
io3
I o2
IO‘
I I I 1 0 20 40 60 80 100 120 140
Radius, cm
FIG. 49 VOLUME OF A SPHERE AS A FUNCTION OF ITS RADIUS
- 77 -
I o4
I o3
IO2
I I I I I 20 40 60 80 100 120 140 IO'
0 Radius, cm
FIG. 50 VOLUME OF A CYLINDER AS A FUNCTION OF ITS RADIUS
FIG. 51 WEIGHT OF A SPHERICAL SHIELD AS A FUNCTION OF ITS RADIUS FOR MATERIALS OF DIFFERENT DENSITY
- 79 -
FIG. 52 WEIGHT OF A CYLINDRICAL SHIELD AS A FUNCTION OF ITS RADIUS FOR MATERIALS OF DIFFERENT DENSITY WHEN h=2r
I o6
Radius, cm
. . . . -
- 80 -
. -.I_ -. . .
ACKNOWLEDGMENTS
The au tho r s would l i k e t o thank t h e Radiat ion Sh ie ld ing Information Center f o r providing t h e computer code packages t h a t made t h i s work p o s s i b l e .
1.
2.
3.
4.
5.
5A.
6.
7.
8 .
9.
10 *
REFERENCES
G . T. Seaborg. "Californium-252: Radioisotope with a Future. ' ' Californium-252. CONF-681032, USAEC Divis ion of Technical Information, p . 1 -3 (1969).
D. H . Stoddard. Radiation Properties of Californium-252. USAEC Report DP-986, E . I . du Pont de Nemours and Co., Savannah River Laboratory, Aiken, S. C . (1965).
E . K . Hyde. The Nuclear Properties o f the Heavy Elements. Vol. 111, p. 240, P r e n t i c e Hal l , Englewood Cl i f f s , N . J . (1964).
U. S. Code of Federal Regulations, T i t l e 10, Chapter 1, Par t 20, "Standards f o r P r o t e c t i o n Against Radiat ion."
U. S. Code of Federal Regulations, T i t l e 49, Chapter I , Part 173, "Shippers. I '
L . B. Shappert . Cask Designers Guide: A Guide f o r the Design, Fabrication, and Operation of Shipping Casks fo r NucZear Applications. Nat ioa l Laboratory, Oak Ridge, Tenn. (1970).
H . E . Hootman. Estimation of 52Cf Shielding Requirements. USAEC Keport DP-1232, Savannah River Laboratory, E . I . du POnt De Nemours G Co., Aiken, S. C . (1970).
USAEC Report OWL-NSIC-68, Oak Ridge
W. W. Engle, Jr . ANISN Users Manual: A One Dimensional Discrete Ordinates Transport Code w i t h Anisotropic Scat ter ing. USAEC Report K-1693, Oak Ridge Gaseous Di f fus ion P l a n t , Oak Ridge, Tenn. (1967).
J . R . Knight and F . R . Mynatt. MUG, A Program f o r Generating Multigroup Photon Cross Sections. Ridge Nat ional Laboratory, Oak Ridge, Tenn. (1970).
USAEC Report CTC-17, Oak
W . E . Ford and D . H. Wallace. POPOP4: A Code f o r Converting Gamma Ray Spectra t o Gma-Ray Production Cross Sections. USAEC Report CTC-12, Oak Ridge Nat ional Laboratory, Oak Ridge, Tenn. (1969).
The National Bureau of Standards Handbook 6 3 . P r i n t i n g O f f i c e , Washington, D . C . (1957).
U . S. Government
- 81 -
11. H. C . C la iborne and D . K . Trubey. "Gamma-Ray Dose Rates i n a S lab Phantom." Trans. Am. Nuel. Soc. 12, 383 (1969).
12. E . P . B l i za rd . "Shield Opt imiza t ion with Respect t o Weight." Reactor Handbook, E . P . B l i za rd , Ed., 2nd e d . , Vol. 111, Part B , p . 185, I n t e r s c i e n c e P u b l i s h e r s , N e w York (1962).
13. W . J . Stapp and M . Tetenbaum. ! 'Radiation Damage.'' Reactor Handbook, C . R . Lipton, J r . , Ed. , 2nd e d . , Vol. I , p . 1102, I n t e r s c i e n c e P u b l i s h e r s , New York (1960).
14. Reports by R E I C . Radia t ion Effects Information Cen te r , Ba t te l le Memorial I n s t i t u t e , Columbus, Ohio.
15. G . D . O l ive r , J r . and E . Bai ley Moore. "The Neutron Sh ie ld ing Q u a l i t i e s of Water-Extended-Polyesters." Health Physics 19, 578 (1970).
16. K . T . Faler. Castable Neutron ShieZd. U . S . Pa ten t 3,471,414 (October 7 , 1969).
17. T . E . Northup. High-Density Concrete for Gama and Neutron Attenuation. USAEC Report ORNL-3704, Oak Ridge Nat iona l Laboratory, Oak Ridge, Tenn. (1965).
18. F . J . Al l en , A . F u t t e r e r , and W . Wright. Neutron Ref lec t ion and FZux Versus Depth f o r Nevada Tes t SoiZ. BRL-1190, Ba l l i s t i c Research Labora to r i e s , Aberdeen Proving Grounds, Md. (1963).
USAEC Report
19. W. B. Doe. Zinc Bromide Solut ion f o r Use i n Shielding Windows. USAEC Report ANL-4879, Argonne Nat iona l Laboratory, Chicago, I l l . (1952).
20. T . Rockwell. Construction of Cheap Shield: A Survey. USAEC Report AECD-3352, Oak Ridge Nat iona l Laboratory, Oak Ridge, Tenn. (1950).
