SHIELD DESIGN FOR SNAP REACTORS*
V. K e s h i s h i a n t A t o m i c s I n t e r n a t i o n a l
A Division of N o r t h A m e r i c a n Aviation, Inc.
Canoga P a r k , California
ABSTRACT
A s u r v e y has been m a d e of r e a c t o r shield r e q u i r e m e n t s in a satellite with n u c l e a r a u x i l i a r y p o w e r . The payloads r a n g e from rugged equipment through r a d i a t i o n s e n s i t i v e e l e c - t r o n i c g e a r . A n u m b e r of s y s t e m layouts from c o m p a c t to g r e a t l y extended, have been a n a l y z e d . Dependent upon r e a c t o r power, s y s t e m layout, and d e g r e e of payload p r o t e c t i o n r e - quired, the shield weight r e q u i r e m e n t s a r e found to extend from z e r o to s e v e r a l thousand p o u n d s .
The r a d i a t i o n originating f r o m a space n u c l e a r a u x i l i a r y power s y s t e m of the SNAP type m a y be r e d u c e d to a c c e p t a b l e payload p r o t e c t i o n l i m i t s by p r o p e r vehicle, component, and shielding a r r a n g e m e n t . Studies have shown that, depending on this a r r a n g e m e n t , shield weights can v a r y from z e r o to a few thousand pounds. Since few d e t a i l s of the vehicle or its m i s s i o n a r e known at the p r e s e n t t i m e , it is only p o s s i b l e to point out the high d e g r e e of i n t e r d e p e n d e n c e between the shield and the vehicle, and to show how widely the shield can v a r y , depending on the a r r a n g e m e n t .
T h e r e a r e t h r e e m a i n a p p r o a c h e s which m a y be taken in the design of the shield:
1) P l a c e all shielding a r o u n d the r e a c t o r . 2) P l a c e all shielding a r o u n d the payload.
3) Use a split shield, i. e . , u s e p a r t of shield a r o u n d the r e a c t o r and p a r t around the payload.
^ P r e s e n t e d at the Space P o w e r S y s t e m s C o n f e r e n c e , Santa Monica, California, S e p t e m b e r 27-30, I960.
Work done u n d e r AEC C o n t r a c t : AT(11 - 1 ) - G E N - 8
"("Compact P o w e r S y s t e m s D e p a r t m e n t
Since the SNAP s e r i e s of r e a c t o r s a r e v e r y s m a l l in size, it is advantageous to p l a c e all of the shield around the r e a c t o r in o r d e r to achieve a m i n i m u m s y s t e m weight.
It w a s a s s u m e d in this study that the r e a c t o r would be in a satellite in orbit, and that the r e a c t o r and the r e m a i n d e r of the s y s t e m would be in operation continuously for one y e a r . This r e q u i r e d d e t e r m i n a t i o n of the dose t o l e r a n c e s of s y s t e m components m o s t sensitive to r a d i a t i o n . A compilation of available data on radiation d a m a g e t h r e s h o l d s of e l e c t r o n i c components showed that t r a n s i s t o r s a r e the m o s t sensitive to radiation d a m a g e . This s u r v e y lead to the selection of
10l2 fast n e u t r o n s / c m ^ and 10? r o e n t g e n s of g a m m a radiation as r e a s o n a b l e lifetime e x p o s u r e s . It is quite p o s s i b l e that t h e s e values m a y be c o n s e r v a t i v e as t h r e s h o l d s of r a d i a t i o n d a m a g e , however, it is p o s s i b l e that noise g e n e r a t i o n r a t h e r than d a m a g e m a y be the limiting c o n s i d e r a t i o n . Since little information on this subject is available, it is believed that the above t h r e s h o l d s for radiation d a m a g e for t r a n s i s t o r s a r e r e a l i s t i c v a l u e s . Thus the r e a c t o r shield was designed so that t r a n s i s t o r s would have a useful life of one full y e a r .
In o r d e r to show the effect of vehicle layout on shield weight, a v a r i e t y of c a s e s w e r e c o n s i d e r e d . While shield weight m a y not be the m o s t u r g e n t p r o b l e m confronting the s y s t e m at the p r e s e n t t i m e , the u l t i m a t e feasibility, of at l e a s t s o m e m i s s i o n s , m a y depend on it. At this t i m e , a r e a c t o r and its a s s o c i a t e d components cannot be r e g a r d e d as an incidental p a r t of a r e a c t o r containing satellite, but m u s t be r e g a r d e d a s one of its c e n t r a l f e a t u r e s . The design of the e n t i r e vehicle m u s t be influenced and, p e r h a p s , dominated by both the a c c o m p l i s h m e n t of the m i s s i o n and the p r e s e n c e of the r e a c t o r . It a p p e a r s n e c e s s a r y , t h e r e f o r e , to m a i n t a i n an open mind on vehicle configurations.
In all the shielding calculations that w e r e p e r f o r m e d for the v a r i o u s configurations, the s a m e a s s u m p t i o n s w e r e used and a r e given below:
1) The r e a c t o r , including the r e f l e c t o r , is cylindrical in shape, D = H = 15.5 i n c h e s .
2) The r e a c t o r is in operation at full power (250 kw t h e r m a l ) for a p e r i o d of one y e a r .
3) The c o r e composition is s i m i l a r to a typical SNAP r e a c t o r .
4) The dose r a t e s at the payload w e r e set at the m o s t p e s s i m i s t i c levels which a p p e a r consistent with t r a n s i s t o r operation:
a. 3 x 10^ fast n e u t r o n s / c m ^ - s e c .
b . 10"^ r o e n t g e n s / h r for g a m m a radiation.
5^2
(These dose r a t e s c o r r e s p o n d to ΙΟ1^ fast n e u t r o n s / c m ^ and 10? r o e n t g e n s for g a m m a r a d i a t i o n for a p e r i o d of one y e a r continuous o p e r a t i o n . )
5) The shield is m a d e of lithium hydride, and in weight computations it is a s s u m e d to have a density of 0.90 g m / c m3.
6) In the calculation of the n e u t r o n s s c a t t e r e d from the r a d i a t o r , it is a s s u m e d that the a l u m i n u m r a d i a t o r has an effective t h i c k n e s s of 28 m i l s . 7) In the calculations it is a s s u m e d that the d i r e c t
dose and the s c a t t e r e d d o s e s a r e equal.
8) The r a d i a t o r h a s an a r e a of a p p r o x i m a t e l y 400 ft . In all the configurations, the g a m m a r a y dose at the p a y - load i n t e g r a t e d over a p e r i o d of one y e a r is l e s s than 10?
r o e n t g e n s . T h u s , all the shield d e s i g n s w e r e b a s e d on the fast neutron contribution. It should be noted that at v e r y high altitudes, the air density is e x t r e m e l y low and as a r e s u l t , the a i r s c a t t e r i n g contribution is negligible.
F i g u r e s 1 to 9 i l l u s t r a t e the v a r i o u s configurations with the r e s u l t i n g shield w e i g h t s .
F i g u r e 1
The r e a c t o r is located between the payload and the l a s t stage b o o s t e r , and all the shield is placed in front of the payload. The shield weight is calculated to be 11,000 pounds. In the r e m a i n d e r of this d i s c u s s i o n the shields a r e around r e a c t o r and shield weights a r e m u c h l e s s than
11,000 pounds.
F i g u r e 2
The r e a c t o r is again located between the payload, which is in the nose of the m i s s i l e , a n d l a s t stage b o o s t e r . The total shield weight is 3720 pounds i n s t e a d of 11,000 pounds, a s in the a r r a n g e m e n t of F i g u r e 1. The shield for the n e u t r o n s s c a t t e r i n g off the r a d i a t o r weighs 2840 pounds, w h e r e a s the d i r e c t shield weighs only 880 pounds. Calculations in- dicate that r a t i o of the r a d i a t o r s c a t t e r e d to u n s c a t t e r e d (or direct) n e u t r o n flux at the payload is a p p r o x i m a t e l y 0.72%. This value c o r r e s p o n d s to an a l u m i n u m r a d i a t o r t h i c k n e s s of 28 m i l s . Actually, the r a d i a t o r is only 14 m i l s thick, however, the r a d i a t o r steel pipes that contain the m e r c u r y vapor, r e p r e s e n t an effective additional 14 m i l s of a l u m i n u m for n e u t r o n s c a t t e r i n g c a l c u l a t i o n s .
The m o s t i m p o r t a n t i t e m in this figure is the l a r g e amount of shield that is r e q u i r e d for the r a d i a t o r - s c a t t e r e d n e u t r o n s .
F i g u r e 3
This a r r a n g e m e n t is v e r y s i m i l a r to F i g u r e 2, except that the r a d i a t o r is placed below the r e a c t o r . Note that the d i r e c t shield weight is not changed, w h e r e a s the s c a t t e r i n g
shield is 1463 pounds instead of the 2840 pounds in F i g u r e 2.
A disadvantage of this design is that the outer skin between the payload and the r a d i a t o r m u s t be ejected when the
s y s t e m is in orbit to p r e v e n t additional neutron s c a t t e r . Also, the s t r u c t u r e that holds the payload m u s t be in the solid angle of the d i r e c t shield, and this b e c o m e s a diffi- cult s t r u c t u r a l p r o b l e m .
F i g u r e 4
The r a d i a t o r folds outward to one side in four q u a d r a n t s , which further r e d u c e s shield weight. However, due to the r e q u i r e d folding action, r e l i a b i l i t y is d e c r e a s e d and it r e s u l t s in a difficult s t r u c t u r a l d e s i g n .
F i g u r e 5
R e a c t o r and r a d i a t o r a r e extended b a c k w a r d s 100 feet after orbit is achieved and shield weight is further r e - duced to 636 pounds. However, the extending m e c h a n i s m weight has not been included and s y s t e m r e l i a b i l i t y is d e c r e a s e d
F i g u r e 6
A single fin r a d i a t o r is used in this d e s i g n . The total shield weight is 552 pounds. The c y l i n d r i c a l skin around r e a c t o r and r a d i a t o r m u s t be ejected, when the s y s t e m is in orbit. In addition, s t r u c t u r a l r e i n f o r c e m e n t is n e c e s - s a r y to p r e v e n t oscillation or flexure and the consequent i n c r e a s e in s c a t t e r e d neutron flux at the payload.
F i g u r e 7
This design is s i m i l a r to the p r e v i o u s design, except that it has a 3 fin r a d i a t o r . The s a m e p r o b l e m s exist in this design, however it is s t r o n g e r s t r u c t u r a l l y .
F i g u r e 8
In this design the r e a c t o r and shield a r e moved back about 9 feet after achieving o r b i t . The d i r e c t shield weight d e - c r e a s e s to only 465 pounds, however, the r a d i a t o r s c a t t e r - ing shield weight i n c r e a s e s to 587 pounds due to the cylin- d r i c a l r a d i a t o r . It can be seen that if the r a d i a t o r w e r e positioned along the dotted l i n e s , the total shield weight could be r e d u c e d to that of the d i r e c t shield only. However, the l a s t stage b o o s t e r would have to be ejected, and an additional c y l i n d r i c a l skin would have to be placed around the new r a d i a t o r section during launch. On the other hand,
544
if the r e a c t o r is placed above the payload a s shown in F i g u r e 9, t h e s e p r o b l e m s would be o v e r c o m e .
F i g u r e 9
In this design the r e a c t o r is placed in the nose cone with the shield located d i r e c t l y below it. Note that no r a d i a t o r s c a t t e r i n g shield is n e c e s s a r y and that the total shield weight is only 450 pounds. The r a d i a t o r is in the f o r m of a f r u s t u m of a cone having the r e q u i r e d a r e a of 400 ft^.
This a r r a n g e m e n t is v e r y a t t r a c t i v e due to its s t r u c t u r a l simplicity and lack of s c a t t e r i n g surface which l e a d s to its high weight efficiency,
It should be r e a l i z e d that the shielding weights given in the 9 a r r a n g e m e n t s a r e only a p p r o x i m a t e , and that they a r e used only for design c o m p a r i s o n p u r p o s e s . If the l a s t design is used for SNAP Z and SNAP 8, the total shield weight would be a p p r o x i m a t e l y 300 and 450 pounds r e s p e c t i v e l y .
- 14 MILS AI RADIATOR
- CYLINDRICAL RADIATOR
DIRECT SHIELDING 8 8 0 lbs
SCATTER SHIELD 2840 lbs
Fig. 2.
CYLINDRICAL RADIATOR
DIRECT SHIELD 880lbs SCATTER SHIELDS
930 lbs SCATTER SHIELDS
533 lbs TOTAL SHIELD WEIGHT
2343 lbs >
n
I
-<
CO
Fig. 3.
RADIATOR FOLDED OUTWARD IN 4 QUADRANTS
TOTAL SHIELD WEIGHT 1043 lbs
Fig. 4.
EXTENSION SYSTEM
~WEIGHT~ 130 lbs
CYLINDRICAL RADIATOR
>
n T3
o
CO -<
CO —I
rn
CO
RADIATOR ~
VN
DIRECT SHIELD 392 lbs SCATTER SHIELD
160 lbs
TOTAL WEIGHT 5 5 2 lbs
Fig. 6.
DIRECT SHIELD 505 lbs SCATTER SHIELD
293 lbs
c/>
>
n
CO -<
en
TOTAL WEIGHT 798 lbs
Fig. 7.
VN
CYLINDRICAL RADIATOR-
NITIAL POSITION OF REACTOR ft SHIELD
DIRECT SHIELD 4 6 5 lbs
SCATTER SHIELD 587 lbs POSITION OF REACTOR 8
SHIELD WHEN IN ORBIT TOTAL SHIELD WEIGHT 1052 lbs
Fig. 8.
FRUSTUM OF CONE RADIATOR
CO
>
n
CO
TOTAL SHIELD WEIGHT 450 lbs
Fig. 9.
