ARTICLES

DEVELOPMENT POWER

OF

FUEL

ELEMENTS

FOR

FAST

REACTORS

I . S. G o l o v n i n , Yu. K. a n d T . S. M e n ' s h i k o v a

Bibilashvili,

UDC 621.039.54

The m a i n stages in the development of s o d i u m - c o o l e d f a s t power r e a c t o r s in the USSR a r e : the s u c cessful operation of the B R - 5 r e a c t o r , r e c o n s t r u c t e d now into the BR-10 r e a c t o r ; the development and s t a r t - u p of the BOR-60 r e a c t o r , r e a c h i n g nominal power in 1971; the completion of the construction of the BN-350 e x p e r i m e n t a l industrial r e a c t o r ; the c o n s t r u c t i o n of the BN-600 r e a c t o r [1-4]. The e x p e r i m e n t a l data accumulated in the c o u r s e of this p r o g r a m p e r m i t the development and construction of l a r g e - s c a l e industrial installations with f a s t r e a c t o r s . Scientists in m a n y countries have e s t i m a t e d that the optimum e l e c t r i c power p e r unit lies in t h e 1000-2000 MW r a n g e . P r a c t i c a l l y all s o d i u m - c o o l e d f a s t r e a c t o r s u b a s s e m b l i e s w e r e difficult to c o m p l e t e b e c a u s e of technical innovations and l a c k of adequate e x p e r i e n c e in r e l a t e d fields of technoiogy. This kind of installation r e q u i r e s h e a v y - d u t y sodium pumps and heat e x c h a n g e r s , s t e a m g e n e r a t o r s , strong l a r g e - s c a l e r e a c t o r v e s s e l s , t u r b o g e n e r a t o r s , etc. The development of fuel e l e m e n t s for f a s t r e a c t o r s r e q u i r e s s e r i o u s e f f o r t s . Studies of the nuclear p h y s i c s , c h e m i c a l - m e t a l l u r g i c a l , and technological c h a r a c t e r i s t i c s of a n u m b e r of fuel m a t e r i a l s and p o s sible s t r u c t u r a l m a t e r i a l s have sufficed to d e t e r m i n e the direction of development of fuel e l e m e n t s f o r the c o r e and b r e e d i n g blanket of s o d i u m - c o o l e d f a s t r e a c t o r s for the next 10-15 y e a r s . Austenitic s t a i n l e s s steel is the m o s t suitable m a t e r i a l f o r f u e l - e l e m e n t cladding and will be the basic s t r u c t u r a l m a t e r i a l during the next decade. F a s t r e a c t o r s use u r a n i u m oxide and u r a n i u m - p l u t o n i u m fuel b e c a u s e of its good c o m p a t i b i l i t y with s t r u c t u r a l m a t e r i a l s and the sodium coolant, its good radiation r e s i s t a n c e , and the s i m p l i c i t y of its p r o d u c tion technology. It is a p p r o p r i a t e to note that the development of oxide fuel e l e m e n t s f o r f a s t r e a c t o r s s t a r t e d in the Soviet Union on the b a s i s of the e x p e r i m e n t a l w o r k on the B R - 5 r e a c t o r , and has been taken as b a s i c by all European countries including F r a n c e , England, and Italy, and at the p r e s e n t t i m e the USA a l s o . The use of oxide fuel avoids a n u m b e r of difficulties connected with the production of r e l i a b l y operating fuel e l e m e n t s , and a c c e l e r a t e s the accumulation of f a s t r e a c t o r operating e x p e r i e n c e and data on which the design of l a r g e - s c a l e power s y s t e m s can be based. This f a c i l i t a t e s a p o s s i b l e subsequent shift to c a r b i d e , rdtride, or c a r b o n i t r i d e fuel, and finally to the m o s t alluring - m e t a l l i c fuel - if f a v o r a b l e scientific solutions a r e found. The development of oxide fuel e l e m e n t designs p e r m i t t i n g a burnup of up to 10% of the h e a v y a t o m s f o r l i n e a r specific loadings up to 600 W / c m and cladding operating t e m p e r a t u r e of the o r d e r of 700~ is i t self a difficult p r o b l e m . The l a c k of e x p e r i m e n t a l a r r a n g e m e n t s p e r m i t t i n g the production of actual o p e r a t ing conditions of the fuel e l e m e n t s (irradiation by an i n t e g r a t e d flux of m o r e than 1023 f a s t n e u t r o n s / c m 2, dynamics of burnup, etc. ) led to a s o m e w h a t belated d i s c o v e r y of such phenomena as the i o d i n e - c e s i u m interaction of the c o r e with the cladding, and the e m b r i t t l e m e n t and swelling of steel under high radiation d o s e s . T h e s e phenomena have still not been adequately investigated quantitatively and so f a r t h e r e is no p o s s i b i l i t y of c o m p l e t e l y c o r r e c t i n g e a r l i e r designs. H o w e v e r , t h e r e existence has not stopped the developmerit of oxide-fueled s o d i u m - c o o l e d r e a c t o r s s t a r t e d e a r l i e r . Studies have enabled us to understand the p r o c e s s e s o c c u r r i n g in c o r e s of oxide fuel e l e m e n t s at high burnups and huge t e m p e r a t u r e g r a d i e n t s , including the m e c h a n i c a l interaction of the c o r e and cladding, and to produce a dynamic model of t h e s e p r o c e s s e s s e r v i n g as a b a s i s f o r f u e l - e l e m e n t calculations.

Translated from Atomnaya submitted September 14, 1972.

]~nergiya, Vol. 34, No. 3, pp. 147-153, March,

1973.

Original article

9 1975 Consultants Bureau, -a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

196

!

5

Our basic ideas consist of the following [5-7]: 1. After a burnup of m o r e than 3% of the heavy atoms oxide fuel elements with loadings of 500-600 W / c m a r e p r a c t i c a l l y completely (more than 80-90%) f r e e of gaseous fission products. Thus swelling is a m i n i m u m as c o m p a r e d with other f o r m s of fuel. The volume of uranium dioxide and mixed uranium and plutonium oxides i n c r e a s e s 1% on the average f o r 1% burnup, while f o r loadings of ~ 200250 W / c m Nutonium dioxide swells 1.5% for 1% burnup. 2. Oxide fuel softens at t e m p e r a t u r e s above 900~ its N a s t i c i t y i n c r e a s i n g s h a r p l y with t e m p e r a t u r e . Radiation intensifies this p r o c e s s . In the compact oxide c o r e of a thin fuel rod operating at high linear loadings only the outer 0.15-0.2 m m l a y e r r e m a i n s rigid and exerts the main mechanical action on the cladding.

Fig. 1. C o r e fuel element design for l o a d ing of BN-350 r e a c t o r : 1) lower cap; 2) sleeve; 3) ttpper e~p; 4) cladding; 5) w i r e .

3. The high t e m p e r a t u r e gradients which a r i s e during a change in r e a c t o r power cause radial c r a c k s in a compact c o r e . F o r loadings above ~350 W / c m t h e s e c r a c k s a r e "healed" during s t e a d y - s t a t e operation by an evaporation - c o n d e n s a t i o n m e c h a n i s m with m a s s t r a n s f e r into the colder part of the c o r e . As a result of radial m a s s t r a n s f e r the initial gap between the cladding and c o r e is r a t h e r rapidly eliminated under operating conditions until the m e c h a n i s m of " f r a g m e n t " swelling is brought into play. In the cold state the gap is determined by the difference in t h e r m a l expansions of the m a t e r i a l s .

4. During operation an oxide c o r e undergoes s t r u c t u r a l changes leading to the f o r m a t i o n of s e v e r a l c h a r a c t e r i s t i c zones: an outer zone with the original s t r u c t u r e , an equiaxed grain zone, and a c o l u m n a r grain zone. The zone boundaries c o r r e s p o n d to the radial t e m p e r a t u r e distribution determining the radial variation of mechanical p r o p e r t i e s of the c o r e m a t e r i a l . The s t r u c t u r a l changes in the c o r e o c c u r as a r e s u l t of the f o r m a t i o n and m i g r a t i o n of m o s t l y l a r g e p o r e s inward into the h i g h - t e m p e r a t u r e zone, f o r m i n g a central hole or i n c r e a s i n g the size of the existing hole during the initial period of i r r a d i a t i o n if the fuel element was c o n s t r u c t e d with a central void. The accumulation of solid f r a g m e n t s has a r e l a t i v e l y small effect on s t r u c t u r a l changes up to 10% burnup. 5. The outer rigid l a y e r of an oxide c o r e m u s t have a uniformly distributed initial p o r o s i t y to c o m pensate f o r the swelling of this l a y e r during the accumulation of fission f r a g m e n t s . The m e c h a n i s m of this p r o c e s s can be explained by the production and diffusion of vacancies in the m i c r o s c o p i c regions of the t h e r mal spikes produced in the slowing down of fission f r a g m e n t s . The m i n i m u m value of the initial p o r o s i t y is d e t e r m i n e d by the r e q u i r e d burnup. It is a s s u m e d that the i n c r e a s e in volume due to the accumulation of solid f i s s i o n f r a g m e n t s does not exceed 0.4% f o r 1% burnup. 6. The mean effective fuel density in a c r o s s section of a fuel element, computed by taking account of the internal p o r o s i t y of the pellets, the central hole in the c o r e , and the gaps, m u s t be limited to a value, depending on the construction, which prevents melting of the inner portions of the c o r e with subsequent axial m a s s t r a n s f e r . 7. The power density in a fuel element is limited to a value which does not c a u s e melting of the central p a r t of the c o r e during the operating period. In this case the contraction of the c e n t r a l hole in the c o r e t o ward the end of the operating period as swelling o c c u r s under the r e s t r i c t i v e action of the cladding is taken into account, as is the lowering of the melting point of the dioxide with poisoning by fission p r o d u c t s . 8. The swelling of s t e e l in a neutron field significantly changes the pattern of s t r e s s and s t r a i n in the fuel element cladding. E s t i m a t e s based on a design model which a s s u m e s that the r a t e of swelling of the c o r e is independent of the extent of its m e c h a n i c a l interaction with the cladding shows that the swelling of steel has a favorable effect on the efficiency of the central fuel elements of an a s s e m b l y . The jackets of t h e s e fuel elements "escape ~ f r o m the c o r e , as it w e r e , and the mechanical loadings d e c r e a s e . Because

197

7

A-A Central ,/0,4 ~404

A

A-A -

Boundary

-

r 8+4r

r [

"6 3

F

n

Fig. 2, Core fuel element ing of the BN-350 r e a c t o r : 3) can; 4) lower end shield 6) sleeve; 7) porous plug; wire (tape); 10) cladding.

design f o r the second I o a d 1) l o w e r cap; 2) gas space; briquet; 5) c o r e briquet; 8) upper c~p; 9) spacing

of the nonuniformity of the t e m p e r a t u r e aroun d the p e r i m e t e r of the cladding of a fuel element on the p e r i p h e r y of an a s s e m b l y f u r t h e r s t r e s s e s a r i s e as a r e s u l t of the nonuniform swelling of ste~l. The magnitude of these s t r e s s e s depends on the t e m p e r a t u r e of the " r o s e t t e . , T h e r e f o r e it is desirable to take m e a s u r e s to d e c r e a s e the nonuniformity of the t e m p e r a t u r e around the p e r i m e t e r of the peripheral fuel elements. In Soviet r e a c t o r s d i s p l a c e r s [6] a r e introduced into the peripheral cells of an a s s e m b l y to accomplish this [6]. The ideas presented above can be c a r r i e d over completely to the operation of a fuel element with a vibrocompaeted core of powdered dioxide fuel. The only difference is in the initial period of irradiation during which the powder is s i n t e r e d into a compact rod with a central hole. F o r such fuel elements the initial operation of the r e a c t o r at power must follow a special p r o g r a m to e n s u r e the solidification of the hollow c o r e without melting. The safety f a c t o r of the cladding was calculated by taking account of the t h e r m a l and mechanical s t r e s ses f r o m the gas p r e s s u r e and the swelling of the c o r e . The l o n g - t e r m strength and l o n g - t e r m plasticity w e r e also taken into account, as was the relaxation of s t r e s s e s [6]. The design of fuel elements f o r the c o r e of the BN-350 r e a c t o r , intended f o r the f i r s t loading, was developed before the ealculational and design methods f o r oxide fuel elements were in final f o r m . However, the ability to operate up to 5% burnup was verified by direct experiment in a sodium loop of the MIR-2 r e a c t o r and in the BR-5. The fuel element design is shown in Fig. 1. It consists of a stainless steel tube 6.1 m m in d i a m e t e r with a wall thickness of 0.55 m m filled with sleeves of sintered uranium dioxide f o r m i n g a c o r e 1060 m m long. The a v e r a g e effective density in a c r o s s section of the fuel element is 8 g / c m 3. The nominal initial diametral gap between the cladding and the c o r e is 0.3 m m . The ends of the cladding a r e closed by argon arc welds. T h e r e is p r a c t i c a l l y no gas c o l l e c t o r . The empty volume in

198

T A B L E 1. C o m p a r a t i v e C h a r a c t e r i s t i c s of Fuel Element Designs 3N-350 fuel element BN-600fuel Characteristics

basic

Diarneter, mm Length of active portion, mm Maximum cladding temperature, ~

680

II variant [element 6,9 230+830 =t060 700

6,9 90+660=750 7t0

Maximum fuel tem- i800 perature, ~

2500

2500

450

530

53o

5

t0

10

Maximum heat loading, W/crn

I

Maximum burnup,%1

/

t00 Gas pressure at end [ 140 of operating per- ] iod, atm [ t,8 Maximum$ tangen-] 0,25 tim strain of clad- I ding,% 1 Safety factor of / 1,4 t,37 (t,05 $ cladding (with re- L spect to tension)at 1 end of operating ] period

40 t,6

1,55(t,06 $ )

*Temperatures indicated for hot spots. 1"Calculations performed without taking account of the effect of reactor radiation on material properties. STime to rupture decreased by a factor of 100 in comparison witla the properties of steel without taking account of the effect of reactor radiation on them.

the fuel e l e m e n t is m a d e up of the c e n t r a l hole of the c o r e , the gap b e t w e e n the c l a d d i n g and the c o r e , and the s m a l l s p a c e in the u p p e r p a r t of the e l e m e n t (20-25 m m ) w h i c h c o m p e n s a t e s f o r the t h e r m a l e x p a n s i o n , t a k i n g a c c o u n t of the a l l o w a n c e f o r the height of the c o r e . T h e s p a c i n g of the fuel e l e m e n t s in an a s s e m b l y is m a i n t a i n e d by a h e l i c a l l y wound w i r e . The d e s i g n p r e s s u r e of g a s e s in the fuel e l e m e n t at the end of the o p e r a t i n g p e r i o d is 140 a i m , but the s a f e t y f a c t o r of the c l a d d i n g r e m a i n s above ons f o r a m a x i m u m c l a d d i n g t e m p e r a t u r e of about 680~C at the b e g i n n i n g of the o p e r a t i n g p e r i o d and 650~ at the end. Hexagonal a s s e m b l i e s a r e f o r m e d of 169 fuel e l e m e n t s . In the s e c o n d l o a d i n g of the BN-350 r e a c t o r it is p r o p o s e d to use the m o r e r e f i n e d c o r e fuel e l e m e n t d e s i g n shown in Fig. 2. T h e d i a m e t e r of the fuel e l e m e n t h e r e is i n c r e a s e d to 6.9 r a m , and 127 of them can be placed in a h e x a g o n a l a s s e m b l y of the s a m e s i z e , m a i n t a i n i n g the l o a d i n g of the f i s s i o n a b l e isotope. A gas s p a c e is p r o v i d e d in the l o w e r c o l d e r p a r t of the fuel e l e m e n t , and the l o w e r end r e f l e c t o r is c o m b i n e d with the fuel c o r e in a s i n g l e j a c k e t . T h i s p e r m i t s a d e c r e a s e in the p r e s s u r e of f i s s i o n p r o d u c t g a s e s i n s i d e the c l a d d i n g to a m a x i m u m of 100 a t m f o r 10% b u r n u p of the heavy n u c l e i . A c e r t a i n i n c r e a s e in the a v e r a g e t e m p e r a t u r e of the c o r e d e c r e a s e s its m e c h a n i c a l a c t i o n on the c l a d d i n g d u r i n g s w e l l i n g . Since this was c o n f i r m e d on e x p e r i m e n t a l s a m p l e s i r r a d i a t e d in the SM-2 r e a c t o r , the fuel e l e m e n t d e s i g n d e v e l o p e d t u r n e d out to be o p e r a b l e to a b u r n u p of 100,000 MW d a y s / t o n of UO 2.

It should be noted that the l o w e r 230 m m of the fuel c o r e d i r e c t l y a d j o i n i n g the end r e f l e c t o r (Fig. 2) is m a d e of solid r a t h e r than hollow b r i q u e t s . The i n c r e a s e in e f f e c t i v e fuel d e n s i t y in the c r o s s s e c t i o n of the fuel e l e m e n t a c h i e v e d in this way l e a d s to an i n c r e a s e in the s u r f a c e t e m p e r a t u r e of the b r i q u e t s and to a d e c r e a s e in the m e c h a n i c a l a c t i o n of the c o r e on the c l a d d i n g in this p a r t . F i g u r e 3 shows the l o n g i t u d i n a l d i s t r i b u t i o n of the t a n g e n t i a l s t r a i n of the fuel e l e m e n t c l a d d i n g for the s e c o n d l o a d i n g . T h e i n c r e a s e in e f f e c t i v e fuel d e n s i t y in the l o w e r p a r t of the fuel e l e m e n t f r o m 75 to 86% of t h e o r e t i c a l d e c r e a s e s the s t r a i n of the c l a d d i n g toward the end of the o p e r a t i n g p e r i o d f r o m 2.3 to 1.8%, which s i g n i f i c a n t l y i n c r e a s e s its o p e r a t i n g r e s e r v e [7, 8]. An a l t e r n a t i v e fuel e l e m e n t d e s i g n p r o v i d e d f o r the i n c o r p o r a t i o n of both the l o w e r and u p p e r end r e f l e c t o r s into a s i n g l e j a c k e t . T h i s d e s i g n was not s u c c e s s f u l in the BN-350 r e a c t o r , h o w e v e r , b e c a u s e of a s i g n i f i c a n t i n c r e a s e in the h y d r a u l i c r e s i s t a n c e of the a s s e m b l y . T h i s idea has b e e n e m p l o y e d in the d e s i g n of a BN-600 fuel e l e m e n t now b e i n g d e v e l o p e d . One v e r s i o n of this fuel e l e m e n t is shown in F i g . 4. T h e fuel e l e m e n t was d e s i g n e d for a 10% b u r n u p of h e a v y n u c l e i , and can u s e a fuel c o r e of both u r a n i u m d i o x i d e and a (UPu)O 2 m i x t u r e . The j a c k e t has a l a r g e gas s p a c e (800 m m ) and a " h e a t i n g " r e g i o n of the fuel c o r e (90 m m ) f o r d e c r e a s i n g the t a n g e n t i a l s t r a i n of the c l a d d i n g . S a m p l e s of fuel e l e m e n t s c l o s e to the d e s i g n d e s c r i b e d a r e b e i n g t e s t e d in the BOR-60 r e a c t o r at the p r e s e n t t i m e . T a b l e 1 l i s t s the c o m p a r a t i v e c h a r a c t e r i s t i c s of the fuel e l e m e n t d e s i g n s d e s c r i b e d . T h e q u a l i t y of m a n u f a c t u r e of the fuel c o r e has an a p p r e c i a b l e effect on the e f f i c i e n c y and o p e r a t i n g c h a r a c t e r i s t i c s of fuel e l e m e n t s . P e l l e t i z a t i o n is an a c c e p t e d p r o d u c t i o n p r o c e s s in fuel c o r e m a n u f a c t u r e in the USSR and o t h e r c o u n t r i e s . R a t h e r e f f i c i e n t a u t o m a t i c p r e s s e s e n s u r i n g low p r o d u c t i o n l o s s e s have b e e n d e v e l o p e d [5, 6, 9]. Although the c h a r g i n g m a t e r i a l i n t e n d e d f o r p r o c e s s i n g by an a u t o m a t i c p r e s s r e q u i r e s m o r e c a r e f u l p r e p a r a t i o n to e n s u r e c o n s t a n c y of the buik d e n s i t y and the d u p l i c a t i o n of s i z e s and p r o p e r t i e s of the i n d i v i d u a l p e l l e t s , the a m o u n t of p l a s t i c i z e r a c c e p t a b l e in it is s i g n i f i c a n t l y l e s s than in

199

Z,5

2,o

I I I

Fig. 3. Tangential s t r a i n of cladding along the length of a c o r e fuel element of the BN-350 r e a c t o r in which the effective density along a length of 230 m m f r o m the lower end of the active p a r t of the fuel element is: ) 75% of t h e o r e t i c a l ; ~ . ) 86% of t h e o r e t i c a l .

o,5

0

250

500

750

r

Length of active part, mm powders used in other methods of forming oxide c o r e s . This e n s u r e s higher quality of the product after sintering: uniform density, c o r r e c t g e o m e t r i c shape, s m a l l e r deviations of dimensions from nominal. The latter p e r m i t s the omission of the grinding p r o c e s s except to c o r r e c t pellets rejected, for example, as the result of w e a r of the p r e s s i n g device. A p o i n t i n f a v o r of the pelletization p r o c e s s is the possibility of using automatic p r e s s e s with a lubricated p r e s s i n g device to f o r m "damp" pellets of powder practically without the addition of a plasticizer. This f u r t h e r improves the a c c u r a c y of the pellet m a n u f a c t u r e and their quality, and p e r m i t s an i n c r e a s e in the fuel charge in the fuel elements. The optimum technology uses a starting powder with a m i n i m u m amount of m a t e r i a l s which a r e eliminated in the sintering p r o c e s s . Some r e m a r k s on the purity of the starting m a t e r i a l are in o r d e r . In choosing the condition of the uranium dioxide the developer and user generally start f r o m the possibilities of the supplier but t r y to use the purest product. In principle the supplier can produce a product of any d e g r e e of purity, and a high degree of purity may turn out to be economically advantageous to him. An increased contamination of the initial uranium dioxide, p a r t i c u l a r l y by highly volatile admixtures, can affect the quality of the pellets produced, the capacity of the fuel with respect to heavy atoms, and the efficiency of the fuel elements. Not aI1 i m p u r i ties worsen the working capacity of fuel element c o r e s , however, and the p r e s e n c e of impurities below a certain level has a negligible effect o n the fuel element p r o p e r t i e s . T h e r e has been little r e s e a r c h on the dependence of the technological p r o p e r t i e s and radiation stability of fuel elements on the purity of the s t a r t ing m a t e r i a l , yet it is one way of reducing the cost of the fuel cycle. Another aspect of the problem of quality and economy of the production of c e r a m i c c o r e s is the choice of p l a s t i c i z e r . In our p r a c t i c e the most widely used binders are aqueous solutions of h i g h - m o l e c u l a r alcohols. P l a s t i c i z e r s of this type permit the use of simple technological equipment. However, they are not optimum In at least two r e s p e c t s : first, they r e q u i r e the selection of r a t h e r n a r r o w p r e s s u r e limits in f o r m i n g pellets; second, they prolong the sintering p r o c e s s because of the difficulty of eliminating moisture. Anhydrous p l a s t i c i z e r s are m o r e suitable: h i g h - m o l e c u l a r fatty acids, their salts (stearates and behanates), p a r t i c u l a r l y if there is a problem of obtaining a given uniform initial porosity in the sintered m a t e r i a l [10]. The binding p r o p e r t i e s of these substances are manifested even for small additions to the charge, and can improve the quality of the production. The sintering p r o c e s s is the most important technological operation for obtaining products of compact uranium dioxide. It can be p e r f o r m e d in a vacuum or in various a t m o s p h e r e s . The most common is s i n t e r ing in a hydrogeneous atmosphere. This ensures adequate stability of pellet size and p r o p e r t i e s (density, s t o i c h i o m e t r i c composition) and guarantees a low carbon content in the product.

200

I A-A

Central

g4+~O~

c A-A

Boundary

Fig. 4. Alternative design of a fuel element f o r the c o r e of the BN-600 r e a c t o r : 1) lower cap; 2) gas s p a c e ; 3) can; 4) lower and shield briquet; 5) c o r e briquet; 6) sleeve; 7) upper end shield briquet; 8) porous plug; 9) upper cap; 10) spacing wire (tape); 11) cladding. Uranium dioxide will be used as core fuel in the f i r s t loadings of the BN-350 and BN-600 fast power r e a c t o r s . After r e a c t o r s of this type have been m a s t e r e d the fuel will be mixed oxides (UO2 + 15-20% PuO2). It is proposed to make fuel elements of this type by p r e s s i n g pellets. The technological p r o c e s s of making fuel elements with c o r e s of mixed-oxide fuel involves certain special features. The f i r s t feature has to do with the method of making the original powder. This powder can be obtained either by mechanical mixing of powdered uranium and plutonium oxides, or by coprecipitating them f r o m solutions [11]. Choosing one or the other method r e q u i r e s taking account of the n e c e s s i t y of a uniform distribution of plutonium in the fuel core. A solid solution of plutonium dioxide in uranium dioxide f o r m e d by sintering pellets p r e s s e d f r o m mechanically mixed powders may have significant nonuniformities in the distribution of components in small volumes, which leads to a l a r g e Doppler coefficient of reactivity of the s y s t e m . F u r t h e r technological operations to equalize the distribution of concentrations in such fuel m a y i n c r e a s e t h e cost of the p r o c e s s . The coprecipitation p r o c e s s andthe subs equent firing lead to the f o r m a tion of c r y s t a l s of a solid solution of oxides, and the distribution of the fissionable component in such powd e r s is v e r y uniform. It should be kept in mind that the use of coprecipitated m i x t u r e s does not r e q u i r e the complete separation of uranium and plutonium in the r e p r o c e s s i n g of spent fuel elements, and this may reduce the cost of the external fuel cycle. Of c o u r s e in the initial phase of production the choice of the method of manufacture of mixed-oxide fuel may be determined by the c u r r e n t possibilities of the manufacturing plants in the country, but for a stabilized p r o c e s s of muitiple r e p r o c e s s i n g of fuel the method of c h e m i c a l coprecipitation appears to be preferable.

201

A second f e a t u r e of the p r o c e s s of manufacturing pellets of m i x e d - o x i d e fuel is the difference in affinities of uranium and plutonium for oxygen and hydrogen. This l e a d s to a significant effect of the a t m o s p h e r e onthe sintering p r o c e s s . In sintering in a reducing a t m o s p h e r e a two-phase s t r u c t u r e of solid solutions is f o r m e d in the c o r e w i t h the phase concentrations depending on the sintering r e g i m e . A s i n g l e - p h a s e s t r u c t u r e can be obtained by sintering in an oxidizing a t m o s p h e r e , but the n e c e s s i t y of such a p r o c e s s to i m p r o v e the radiation stability of the fuel m u s t be confirmed e x p e r i m e n t a l l y . A third f e a t u r e of the production of mixed oxides is the high toxicity of the product. The p r e s e n t l y existing a r e a s for the m a n u f a c t u r e of fuel e l e m e n t s with c o r e s containing plutonium dioxide [10] a r e equipped with glove boxes and provide f o r c a r r y i n g out the p r o c e s s manually. However, f o r l a r g e - s c a l e industrial production of plutonium fuel the safety r e q u i r e m e n ! s m a y demand significant c o r r e c t i o n s and n e c e s s i t a t e p a r t i a l o r complete r e m o t e control, p a r t i c u l a r l y for multiple r e p r o c e s s i n g of fuel. Remote control o r d i n a r ily involves an i n c r e a s e in production costs. F r o m our point of view a r e a s o n a b l e mechanization and automation of technological p r o c e s s e s will be advantageous in the l a r g e s c a l e production of plutonium fuel and f o r sufficiently developed equipment will p e r m i t the elimination of hand w o r k except for b r i e f manual o p e r a tions to c o r r e c t faults or r e p l a c e equipment. This d e c r e a s e s the r e q u i r e m e n t s f o r high reliability, d u r a bility, and dependability of operation which can be demanded of a r e m o t e control s y s t e m r e p r o c e s s i n g m a t e r i a l s with a biologically dangerous radiation level. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

202

CITED

A . I . Leipunskii et a l . , (USSR) SMEA Symposium on the State and P r o s p e c t s of Construction of F~st R e a c t o r P o w e r Plants [in Russian], Vol. 1, Obninsk (1967), p. 249. A . I . Leipunskii et al., i b i d . , p . 123. A . I . Leipunskii et al., Atomnaya ]~nergiya, 30, No. 2, 165 (1971). A . I . Leipunskii et al., Atomnaya ]~nergiya, 2--5, No. 5 , 3 8 0 (1968). I . S . Golovnin et al., P a p e r at the F r a n c o - S o v i e t Symposium on the Development of Fuel Elements f o r the BOR-60 R e a c t o r [in Russian], K a d a r a s h (1970). A . I . Leipunskiiet a l . , P a p e r 4 9 / P / 4 6 0 at the Fourth Geneva Conference [in Russian] (1971). I . S . Golovnin et al., Atomnaya Energiya, 3_..00,No. 2, 216 (1971). R. Klipot and A. Smolders, P o w d e r Metallurgy, 12, 24, 305 (1969). M. Batler et al., P a p e r M 88/33 at a Symposium on the Use of Plutonium as a R e a c t o r Fuel [in R u s sian], B r u s s e l s (1967). E . A . Evans et al, P a p e r P/236 (USA) at the T h i r d Geneva Conference (1964). C. Sory et al., J. Nuclear Mat., 35, 267 (1970).