AIROX Dry Pyrochemical Processing of Oxide Fuels - ACS Publications

16 A I R O X D r y Pyrochemical Processing of Oxide Fuels

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A Proliferation-Resistant Reprocessing Method L . F . G R A N T H A M , R. G . C L A R K , R. C . H O Y T , and J. R. M I L L E R Rockwell International, Energy Systems Group, 8900 D e Soto Avenue, Canoga Park, CA 91304

During the e a r l y developmental phases of nuclear power, several low-decontamination methods of r e c y c l i n g spent f u e l were investigated. Continued development of these low-decontamination methods was abandoned i n favor o f high-decontamination methods o f r e c y c l i n g spent nuclear f u e l . However, recent concern over p o t e n t i a l d i v e r s i o n by n a t i o n a l or subnational groups of highly decontaminated f i s s i l e m a t e r i a l from power production has brought about a r é é v a l u a t i o n o f low-decontamination fuel c y c l e s . The AIROX process {1,2,3) i s a d r y , pyrochemical, low-decontamination method o f r e c y c l i n g spent f u e l . The AIROX process simultaneously declads and p u l v e r i z e s spent fuel by simple g a s - s o l i d r e a c t i o n s ; the p u l v e r i z e d f u e l i s r e e n r i c h e d , r e p e l ! e t i z e d , and r e c y c l e d to the r e a c t o r . A d i s c u s s i o n of p r o l i f e r a t i o n r e s i s t a n c e i s given f i r s t ; t h i s i s followed by a d e s c r i p t i o n o f the AIROX processing method. Fuel c y c l e s that could u t i l i z e t h i s method are then suggested; t h i s i s followed by decladding and p u l v e r i z a t i o n d a t a . This paper i s concluded with a d i s c u s s i o n o f the status o f AIROX reprocessing i n c l u d i n g advantages and disadvantages of t h i s reprocessing method. Proliferation

Resistance

Spent f u e l i s g e n e r a l l y regarded as p r o l i f e r a t i o n - r e s i s t a n t due to the high l e v e l o f r a d i o a c t i v i t y and the low concentration o f f i s s i l e material i n the f u e l removed from a r e a c t o r . High l e v e l s o f r a d i o a c t i v i t y promote p r o l i f e r a t i o n r e s i s t a n c e because s p e c i a l , e a s i l y monitored f a c i l i t i e s are required to process the f u e l and the high l e v e l o f r a d i o a c t i v i t y makes removal o f fuel without d e t e c t i o n extremely d i f f i c u l t . Spent fuel i s a l s o p r o l i f e r a t i o n - r e s i s t a n t because the concentration of f i s s i l e material i s below that r e q u i r e d to achieve a nuclear detonation.

0-8412-0527-2/80/47-117-219$05.00/0 ©

1980 A m e r i c a n C h e m i c a l Society

In Actinide Separations; Navratil, James D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

ACTINIDE


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SEPARATIONS

This discharged f u e l can be r e c y c l e d back to the LWR or the FBR a f t e r reprocessing and r e f a b r i c a t i o n . The reprocessing s t e p , i f conducted by conventional methods f o r r e c y c l e , would separate the f i s s i l e m a t e r i a l (plutonium) from the spent f u e l . P o t e n t i a l d i v e r s i o n of a p o r t i o n o f t h i s separated f i s s i l e m a t e r i a l to nuclear weapons by n a t i o n a l or t e r r o r i s t groups has caused the Federal Government to postpone a l l f u e l reprocessing and to r e evaluate p r o l i f e r a t i o n - r e s i s t a n t methods of r e c y c l i n g the spent f u e l . Several p r o l i f e r a t i o n - r e s i s t a n t reprocessing methods have been proposed, such as CIVEX(4,5), PYR0-CIVEX(6,7), and AIROX ( J . ) , the process t h a t w i l l be discussed i n t h i s paper. A l l of these r e p r o c e s s i n g methods r e c y c l e f u e l c o n t a i n i n g f i s s i o n products and avoid complete f i s s i l e - f e r t i l e separation to enhance p r o l i f e r a t i o n r e s i s t a n c e . Fuel c y c l e s u t i l i z i n g AIROX r e p r o c e s s i n g are p r o l i f e r a t i o n - r e s i s t a n t because (1) the f i s s i o n product content i s 3060% t h a t of spent f u e l , (2) no f i s s i l e - f e r t i l e m a t e r i a l separations are made, and (3) the process cannot be e a s i l y modified to e f f e c t f e r t i l e - f i s s i l e separation. AIROX Process The AIROX (Atomics I n t e r n a t i o n a l Reduction Oxidation) process i s being developed to reprocess spent uranium oxide-based f u e l . It i s a c y c l i c o x i d a t i o n - r e d u c t i o n process t h a t employs only gaseous and s o l i d m a t e r i a l s ; no l i q u i d s are used. Hence, t h i s process i s o f t e n r e f e r r e d to as a dry process which simultaneously declads and p u l v e r i z e s the f u e l . P u l v e r i z a t i o n permits r e l e a s e of v o l a t i l e f i s s i o n products and comminutes the f u e l f o r reenrichment and r e c y c l e . I t a l s o provides gaseous access to unreacted f u e l i n the c e n t e r of the p e l l e t . P u l v e r i z a t i o n takes place by o x i d a t i o n of the uranium d i o x i d e (UO2) w i t h a i r at e l e v a t e d temperatures ( - 4 0 0 C) which expands the f u e l volume; i f the o x i d a t i o n i s continued u n t i l U3O3 i s o b t a i n e d , a 30% volume expansion i s achieved. The volume increase ruptures the c l a d d i n g and p u l v e r i z e s the f u e l . Complete o x i d a t i o n of UO2 to IbOg i s not r e q u i r e d to o b t a i n s u f f i c i e n t volume expansion f o r p u l v e r i z a t i o n . The o x i d i z e d (JO? i s reduced back to UO2 by r e a c t i o n with d i l u t e hydrogen (10-20%) i n n i t r o g e n at about 600 C. A f t e r r e d u c t i o n , the f u e l can be r e o x i d i z e d to achieve f u r t h e r p u l v e r i z a t i o n i f d e s i r e d . U l t i m a t e l y , the f u e l i s reduced back to UO2 f o r enrichment and p e l l e t i z a t i o n p r i o r to r e c y c l e to the r e a c t o r . In the AIROX processing system, the hardware i s sheared o f f the ends of the f u e l assembly and i n d i v i d u a l f u e l pins are fed i n t o a continuous r o t a r y punch, which i s e s s e n t i a l l y a V - b e l t p u l l e y w i t h punch d i e s extended from the center of the groove. This provides small holes i n the f u e l pins spaced a t an optimum d i s t a n c e (2.5 to 4.0 cm apart) to permit gaseous r e a c t a n t (O2 and H2) access to the f u e l . The c l a d d i n g a f t e r AIROX processing i s shown i n Figure 1. A t y p i c a l - s i z e d i s t r i b u t i o n of f u e l p a r t i c l e s


In Actinide Separations; Navratil, James D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. Figure 1.

Cladding after AIROX processing


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a f t e r d i f f e r e n t processing c o n d i t i o n s i s shown i n Table I. After a s i n g l e o x i d a t i o n , the f u e l i s r e a d i l y p u l v e r i z e d to a s i z e s u i t able f o r r e p e l l e t i z a t i o n ( i . e . , 2-10 microns) by b a l l m i l l i n g f o r a few minutes. The f a c t that the f u e l does not p u l v e r i z e to micron s i z e during the f i r s t o x i d a t i o n i s advantageous. Submicron and m i c r o n - s i z e d p a r t i c u l a t e s of f u e l would tend to c l i n g to the r u p tured c l a d d i n g and to be dispersed by a i r currents i n the process vessels. P u l v e r i z a t i o n to about 10 mesh (2000 microns) i s a l l that i s r e q u i r e d to assure decladding and complete (>99.9%) f u e l cladding separation. Note a l s o i n Figure 1 that the cladding ruptures up to the end plugs; i t i s not necessary to shear the end plugs to assure complete removal of the f u e l from the f u e l p i n . Possible

Fuel

Cycles

AIROX processed spent f u e l must be enriched i n f i s s i l e con tent p r i o r to r e a c t o r r e c y c l e . LWR C y c l e . The source of t h i s enrichment material f o r an LWR can be e i t h e r v i r g i n uraniumU) or FBR blanket material con t a i n i n g the required amount of f i s s i l e m a t e r i a l . The f u e l c y c l e i s p r o l i f e r a t i o n r e s i s t a n t because of the high r a d i o a c t i v i t y of the product and the low concentration of f i s s i l e material i n a l l process steps. Between 50-85% of the spent LWR f u e l can be r e c y c l e d to the LWR t h a t produced the f u e l ; the remaining 15 to 50% o f the spent f u e l can be used to s t a r t u p a new l i g h t - w a t e r r e a c t o r , processed by a p r o l i f e r a t i o n - r e s i s t a n t method to recover the f i s s i l e m a t e r i a l or disposed of at a Federal r e p o s i t o r y . Only about 80% as much v i r g i n uranium would be required to r e f u e l the r e a c t o r using r e c y c l e d f u e l a f t e r AIROX processing as would be necessary to r e f u e l the r e a c t o r i f no spent f u e l were r e c y c l e d . The f r a c t i o n o f the f u e l that can be r e c y c l e d to the LWR that produced the spent f u e l can be varied from 50-85%, but as the f r a c t i o n of r e c y c l e d f u e l i n c r e a s e s , the concentration of U ^5 · the enrichment stream must a l s o be increased from approximately 5 to 20%. In order f o r t h i s enrichment stream to be p r o l i f e r a t i o n r e s i s t a n t , the maximum concentration of Ij235 · the enrichment stream should be l i m i t e d to -20%. The penalty f o r r e c y c l i n g f i s s i o n products back to an LWR i s that the concentration of the f i s s i l e material i n the f u e l must be s l i g h t l y higher (3.5 vs 3.2% f o r a pressurized-water r e a c t o r , PWR) to make up f o r the neutrons that w i l l be absorbed by the f i s s i o n products(8). The neutron cross s e c t i o n v a r i e s with each isotope o f each f i s s i o n product; t h e r e f o r e , the importance of removing each f i s s i o n product depends on the neutron spectrum and on the c o n c e n t r a t i o n and h a l f l i f e of each f i s s i o n product. The impor tance of removing the various f i s s i o n products f o r uranium and plutonium r e c y c l e i n the PWR neutron spectrum i s given i n Table II. The v o l a t i l e f i s s i o n products removed during AIROX processing decrease the p a r a s i t i c neutron absorption about 25%. 2

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Particle Size ( μ )

Partial Oxidation

TYPICAL SIZE DISTRIBUTION OF AIROX PROCESSED FUEL

TABLE I


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TABLE

II


FISSION PRODUCT ELEMENT RELATIVE REMOVAL IMPORTANCE F i s s i o n Product Neutron Absorption

Uranium Recycle Element Samarium Neodynium Cesium** Europium Rhodium Xenon** Molybdenum Technetium Palladium Promethium Ruthenium** Silver Praseodymium Zirconium Lanthanum Iodine** Krypton** Cerium Gadolinium A l l Others

(% of T o t a l ) * Plutonium Recycle

180 day c o o l i n g

10 year c o o l i n g

180 day c o o l i n g

21.7% 16.8 13.5 8.2 7.6 6.6 4.6 3.9 3.3 3.2 3.1 1.4 1.2 1.2 0.9 0.8 0.8 0.3 0.3 0.7

23.3% 17.3 13.2 8.1 7.8 6.7 4.8 4.0 3.4 0.3 3.2 1.4 1.2 1.2 0.9 0.9 0.8 0.2 0.7 0.7

18.2% 11.3 12.9 9.5 12.2 7.8 3.7 3.2 5.4 4.9 3.1 3.0 0.7 0.8 0.6 0.9 0.5 0.2 0.3 0.8

*Percent of neutrons absorbed by a s p e c i f i c f i s s i o n product element compared to the neutrons absorbed by a l l the f i s s i o n products i n a PWR neutron spectrum. **Elements removed during AIROX processing.



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AIROX Processing of Oxide Fuels

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FBR C y c l e . The AIROX p r o c e s s i s a l s o a p p l i c a b l e t o FBR f u e l recycle; The FBR d r i v e r f u e l must be e n r i c h e d i n f i s s i l e c o n t e n t b e f o r e r e c y c l e by t r a n s f e r r i n g f i s s i l e m a t e r i a l f r o m t h e s p e n t FBR blanket. S i n c e t h e c o n c e n t r a t i o n o f f i s s i l e m a t e r i a l i s t o o low and s i n c e t h e AIROX p r o c e s s does n o t s e p a r a t e f e r t i l e and f i s s i l e m a t e r i a l , t h e AIROX p r o c e s s must be used i n c o n j u n c t i o n w i t h a p r o c e s s w h i c h does s e p a r a t e f i s s i l e m a t e r i a l . To o b t a i n t h e e n r i c h m e n t m a t e r i a l , t h e s p e n t FBR d r i v e r f u e l and a p o r t i o n ( 2 0 - 3 0 % ) o f t h e s p e n t b l a n k e t f u e l must be p r o c e s s e d by t h e p r o l i f e r a t i o n - r e s i s t a n t CIVEX o r PYROCIVEX m e t h o d s . The r e m a i n d e r o f t h e s p e n t f u e l , 7 0 - 8 0 % o f t h e b l a n k e t can be r e c y c l e d a f t e r AIROX p r o c e s s i n g o n l y Q J . The p o r t i o n s o f t h e s p e n t b l a n k e t f u e l w i t h t h e h i g h e s t f i s s i l e c o n t e n t ( s u c h as t h e s p e n t i n t e r n a l and a x i a l b l a n k e t s ) as w e l l as a p o r t i o n o f t h e r e m a i n i n g e x t e r n a l b l a n k e t w o u l d be r e c y c l e d w i t h o u t f u r t h e r p r o c e s s i n g . The s i m p l i c i t y o f t h e AIROX p r o c e s s p r o v i d e s and e c o n o m i c i n c e n t i v e f o r development o f t h i s d u a l - r e p r o c e s s i n g system. The A t o m i c s I n t e r n a t i o n a l B u l l s e y e o r P a r f a i t C o r e FBR(9J f u e l c y c l e was a n a l y z e d t o d e t e r m i n e t h e maximum amount o f b l a n k e t f u e l w h i c h c o m p r i s e s 78% o f t h e s p e n t f u e l t h a t c o u l d be r e c y c l e d a f t e r AIROX p r o c e s s i n g o n l y . In t h i s a n a l y s i s , i t was assumed t h a t t h e enrichment stream from the Ci vex o r P y r o c i v e x r e p r o c e s s i n g p l a n t was a b o u t 48% f i s s i l e m a t e r i a l , 42% f e r t i l e m a t e r i a l , and 10 wt % f i s s i o n p r o d u c t s . In one f u e l management scheme, a b o u t 70 wt % o f t h e b l a n k e t f u e l c a n be r e c y c l e d a f t e r AIROX p r o c e s s i n g only. A b o u t 51% o f t h e s p e n t b l a n k e t f u e l i s r e f a b r i c a t e d i n t o LWR f u e l and a b o u t 21% i s used t o d i l u t e t h e f i s s i l e m a t e r i a l f r o m t h e C i v e x o r P y r o c i v e x r e p r o c e s s i n g p l a n t t o t h e r e q u i r e d FBR f i s s i l e content before i t i s r e f a b r i c a t e d into d r i v e r f u e l . Only 28% o f t h e s p e n t b l a n k e t f u e l r e q u i r e s p r o c e s s i n g by aqueous o r p y r o c h e m i c a l means. In a n o t h e r f u e l management scheme w h i c h r e c y c l e s - 25% o f t h e s p e n t b l a n k e t f u e l back t o t h e a x i a l b l a n k e t t o i n c r e a s e s u b s e q u e n t b l a n k e t f i s s i l e c o n t e n t , t h e amount o f t h e b l a n k e t f u e l w h i c h c a n be r e c y c l e d a f t e r AIROX p r o c e s s i n g o n l y i s o v e r 80% and l e s s t h a n 20% must be p r o c e s s e d by aqueous o f p y r o c h e m i c a l methods. About 21% o f t h e s p e n t d r i v e r f u e l i s r e f a b r i c a t e d i n t o d r i v e r f u e l w h i l e 40% o f t h e s p e n t b l a n k e t f u e l i s r e f a b r i c a t e d i n t o LWR f u e l . In t h e FBR f u e l c y c l e s , t h e f r a c t i o n ( i . e . , 21% i n t h e examp l e s above) o f t h e b l a n k e t f u e l r e c y c l e d f o r use i n r e f a b r i c a t i n g d r i v e r f u e l a f t e r AIROX p r o c e s s i n g o n l y depends on t h e c o n c e n t r a t i o n of f i s s i l e material i n the Civex or Pyrocivex product. As the f i s s i l e content of the Civex or Pyrocivex product i s decreased, t h e amount o f b l a n k e t f u e l r e c y c l e d f o r d r i v e r f u e l f a b r i c a t i o n must a l s o be d e c r e a s e d p r o p o r t i o n a t e l y and t h e amount o f s p e n t b l a n k e t f u e l p r o c e s s e d by t h e C i v e x o r P y r o c i v e x p r o c e s s must be increased.


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Several t e s t s were run to determine o x i d a t i o n and reduction r e a c t i o n rates of UO2 p e l l e t s at various processing c o n d i t i o n s . The r a t e of r e a c t i o n was followed by measuring the amount of gaseous r e a c t a n t consumed and confirmed by weighing and a n a l y z i n g the product at the end of a t e s t . The product from several of the t e s t s was sieved to determine the s i z e d i s t r i b u t i o n . The o x i d a t i o n of UO2 appears to proceed i n three d i s t i n c t s t e p s , i . e . , (1) i n d u c t i o n p e r i o d ; (2) r a p i d o x i d a t i o n ; and (3) moderate o x i d a t i o n . This i s shown i n Figure 2. During the i n d u c t i o n p e r i o d , e s s e n t i a l l y no oxygen i s removed from the surrounding gas; t h i s time probably corresponds to that r e q u i r e d f o r the f u e l to reach processing temperature, d i f f u s i o n of a i r i n t o the microcracks in the f u e l and the i n i t i a t i o n of surface oxidation. The r a p i d o x i d a t i o n corresponds to the formation of intermediate uranium oxide phases, such as U4O9 or UO2.6, ' c r y s t a l s t r u c t u r e changes from cubic to the l e s s dense tetragaonal phase. The oxygen-to-uranium r a t i o at the t r a n s i t i o n v a r i e s from 2.5 to 2.6 at o x i d a t i o n temperatures of 400-500 C. The upper boundary of the t h i r d o x i d a t i o n step i s the formation of U3O8 (0:U r a t i o of 2.67). Examination of Figure 2 i n d i c a t e s that the time required to perform the f i r s t two o x i d a t i o n steps i s markedly reduced as the o x i d a t i o n temperature approaches 480 C. Essentially, a l l of the f u e l has decomposed to f i n e p a r t i c l e s a f t e r the f i r s t o x i d a t i o n step where the UO2 i s o x i d i z e d only as f a r as the i n t e r mediate phase; complete o x i d a t i o n to U3O8 i s unnecessary. These r e s u l t s , along with previous results(2^_3,lO) i n d i c a t e that the decladding can be accomplished on the f i r s t o x i d a t i o n . a

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During the f i r s t o x i d a t i o n in AIROX p r o c e s s i n g , f a c t o r s such as temperature and oxygen p a r t i a l pressure c o n t r o l the amount of time r e q u i r e d to p u l v e r i z e the spent f u e l . With a i r as the o x i d i z i n g gas, a minimum r e a c t i o n time was found at 480 C. As the oxygen p a r t i a l pressure was increased at about 435 C, the minimum r e a c t i o n time occurs with about 50% oxygen. These data i n d i c a t e that i f one t r i e s to o x i d i z e too r a p i d l y by i n c r e a s i n g the temperature and oxygen content, the o x i d a t i o n rate can a c t u a l l y decrease. T h i s i s probably because r a p i d o x i d a t i o n expands the surface oxide so that microcracks i n the fuel are blocked and the oxygen must d i f f u s e through an oxide f i l m whereas i f the o x i d a t i o n i s conducted more s l o w l y , the oxygen can d i f f u s e through microcracks i n the f i l m . The degree of p u l v e r i z a t i o n does vary with the p u l v e r i z a t i o n c o n d i t i o n s as shown i n Tables I and III. Three general observations can be reached from examining the p u l v e r i z a t i o n data. F i r s t , over 70% of the product was l e s s than 297 microns in s i z e a f t e r the f i r s t p a r t i a l o x i d a t i o n ( a l l of the f u e l was l e s s than 2 m i l limeter in s i z e ) . Second, there i s l i t t l e product that i s l e s s than 297 microns and greater than 74 microns. In other words, once p u l v e r i z a t i o n of a fragment begins, the fragment p u l v e r i z e s


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AIROX Dry Pyrochemical Processing of Oxide Fuels - ACS Publications

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