8 Ceramic Forms for Nuclear Waste
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R. G. DOSCH Sandia Laboratories, Albuquerque, NM 87185
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Options for disposal of high level l i q u i d radioactive waste, a by-product of the nuclear fuel cycle, range from controlled surface storage i n remote areas to storage i n deep geologic formations and injection into outer space. Features or operations common to all these options are the production of a solid waste form, transportation, and disposal, with the ultimate goal of isolating the waste from the biosphere. Since some of the long -lived radionuclides are extremely toxic to man, isolation must be assured over long periods of time. Any solid waste form should serve to reduce waste volume and provide short term stabilization under conditions such as fracture, f i r e , or water immersion which could result from a transportation mishap. In the geologic storage scenarios currently receiving the most scrutiny, the most l i k e l y path to the biosphere has been identified as aqueous transport of nuclides v i a groundwater. Thus an acceptable waste form would also resist dissolution under ambient repository conditions, with the obvious benefit of assuring a sufficiently low nuclide release rate into an aquifer to preclude a significant threat to health and safety. At the present time, glass waste forms which have been under development for many years i n the United States and elsewhere, are considered the reference materials i n high l e v e l waste management (1,2). A vitrified waste form will almost certainly be employed i n the initial waste disposal programs. Their use i n deep geologic storage, however, involves some potential problems. For example, v i t r i f i e d wastes which are relatively insoluble i n water or brine under normal conditions, undergo modification and/or dissolution quite rapidly at temperatures of 250°C, a condition that is expected under the l i t h o s t a t i c pressures associated with the principal geologic repositories considered to date ( 3 , 4 ) . In addition, as metastable glasses inevitably devitrify, This work supported by the United States Department of Energy ( D O E ) , under Contract A T (29-1)-789. A U.S. D O E Facility. 1
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0-8412-0498-5/79/47-100-129$05.00/0 © 1979 American Chemical Society
Fried; Radioactive Waste in Geologic Storage ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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s i g n i f i c a n t changes i n p r o p e r t i e s such as l e a c h a b i l i t y can be expected. P a r a l l e l waste form development programs on a much s m a l l e r s c a l e have been and are being conducted i n a few l a b o r a t o r i e s . Most are addressing the development of ceramic waste forms, which are i n h e r e n t l y more s t a b l e i n a thermodynamic sense than g l a s s e s . Some o f these i n c l u d e : l ) Glass ceramics (5) - these m a t e r i a l s are prepared from g l a s s e s by u s i n g s p e c i a l heat treatments t o promote c r y s t a l l i z a t i o n ; 2) Supercalcine - the product o f a process whereby chemical c o n s t i t u e n t s are added t o l i q u i d waste w i t h the i n t e n t of forming s t a b l e c r y s t a l l i n e hosts f o r s p e c i f i c n u c l i d e s during c a l c i n a t i o n and subsequent c o n s o l i d a t i o n (6_); 3) Metal or g r a p h i t e encapsulated waste oxides (6_); and k) Atomic a l l y d i s p e r s e d waste oxides i n a ceramic t i t a n i u m d i o x i d e matrix ( 7 , 8 ) . An overview o f the process o f i n c o r p o r a t i n g r a d i o a c t i v e waste i n a t i t a n i u m d i o x i d e ceramic form w i l l be presented. The process goals were t o produce a s t a b l e , c r y s t a l l i n e waste form and a process e f f l u e n t which d i d not r e q u i r e remote h a n d l i n g , or p r e f e r a b l y c o u l d be t r e a t e d as a chemical waste. The n u c l i d e waste, up t o 25$ by weight, i s i n t r o d u c e d i n t o a t i t a n a t e mate r i a l by an i o n exchange process. The loaded t i t a n a t e s are con v e r t e d t o a t h e o r e t i c a l l y dense (U.5 g/cc) ceramic by pressure s i n t e r i n g . C h a r a c t e r i z a t i o n s t u d i e s o f the f i n a l ceramic form have been performed u s i n g t r a n s m i s s i o n e l e c t r o n microscopy, nond i s p e r s i v e x-ray a n a l y s i s , and other techniques i n order t o i d e n t i f y some o f the important p r o p e r t i e s o f these complex mate rials. The l o n g term l e a c h a b i l i t y o f the ceramic has been s t u d i e d and compared t o a g l a s s waste form. T h i s work i s summar i z e d along w i t h the i n i t i a l r e s u l t s o f some h i g h temperatureh i g h pressure l e a c h i n g s t u d i e s . The o v e r a l l development program i n c l u d e d the study o f other exchange m a t e r i a l s such as n i o b a t e s , z i r c o n a t e s , and t a n t a l a t e s , some o f which had s u p e r i o r i o n exchange and l e a c h i n g p r o p e r t i e s , but were i n i t i a l l y economically u n a t t r a c t i v e as compared t o the t i t a n a t e s . These a l t e r n a t e m a t e r i a l s w i l l be b r i e f l y d i s c u s s e d along w i t h a p p l i c a t i o n s t o n u c l i d e s t a b i l i z a t i o n i n other areas of n u c l e a r p r o c e s s i n g . Ion Exchange M a t e r i a l Preparation and
Chemistry
The t i t a n a t e m a t e r i a l used i n the waste s o l i d i f i c a t i o n process i s one o f a group o f m a t e r i a l s o f the g e n e r a l formula M [MxOyH ] where M i s an exchangeable c a t i o n o f charge -hi and M' may be T i , Nb, Z r , or Ta (9). In the i n i t i a l work w i t h these m a t e r i a l s M was a quaternary ammonium i o n , g e n e r a l l y the t e t r a methylammonium i o n , i n compounds w i t h a s t o i c h i o m e t r y o f the type (CH3)p[(Ti205H) and (CH3 )ΐμΝ(M^OgH) which were h i g h l y water s o l u b l e . These formulas are e m p i r i c a l , but i f one considers the compounds as simple s a l t s which d i s s o l v e t o form (CH^ and z
n
Fried; Radioactive Waste in Geologic Storage ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
8.
Ceramic
DOSCH
Forms for Nuclear
Waste
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(ΜχΟγ-Η )~ s p e c i e s , t h e f o r m u l a s a d e q u a t e l y d e s c r i b e t h e i r chemistry. The a n i o n i c s p e c i e s r e a c t q u a n t i t a t i v e l y w i t h m e t a l c a t i o n s i n s o l u t i o n t o f o r m p r e c i p i t a t e s w i t h c o m b i n i n g r a t i o s o f one (MiOyHz)" p e r u n i t p o s i t i v e charge o n t h e c a t i o n . These p r e c i p i t a t e s , s u c h a s Na(Ti2C>5H) o r Ca(Ti20511)2, a c t a s i o n exchange materials i nwhich the N a and C a ions are replaced by cations w i t h a g r e a t e r a f f i n i t y f o r t h e m a t e r i a l . The e a r l i e s t e x c h a n g e m a t e r i a l s were p r e p a r e d b y t h i s m e t h o d , b u t f o r t h e w a s t e s o l i d i f i c a t i o n w o r k , a more d i r e c t a n d e c o n o m i c a l m e t h o d was u s e d i n v o l v i n g r e a c t i o n o f t h e a p p r o p r i a t e m e t a l a l k o x i d e , T i (003117)14., Nb(002115)5, Ta(002115)5, o r Z r ( O C ^ H Q , ^ w i t h a n a l c o h o l s o l u t i o n o f a base c o n t a i n i n g t h e d e s i r e d exchangeable c a t i o n , e.g. a s o l u t i o n o f 10-20 w/o s o d i u m h y d r o x i d e i n m e t h a n o l . I n t h e f i r s t s t e p (Eq. l a ) , a n e x o t h e r m i c e s t e r i n t e r c h a n g e r e a c t i o n t a k e s p l a c e f o l l o w e d b y r e a c t i o n w i t h t h e hydroxide i o n t o form a product w h i c h i s s o l u b l e i n t h e a l c o h o l s o l u t i o n . The s e c o n d s t e p ( E q . l b ) i s a h y d r o l y s i s r e a c t i o n r e s u l t i n g i n a p r e c i p i t a t e which i s f i l t e r e d a n d d r i e d u n d e r vacuum a t a m b i e n t t e m p e r a t u r e . ζ
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+
+ +
NaOH ( m e t h a n o l ) + T i i O C ^ H y ) ^ -* I n t e r m e d i a t e Intermediate
(Eq. l a )
+ H 0 - N a ( T i 0 H ) s o l i d + C Hy0H 2
2
5
3
(Eq. l b )
The o v e r a l l y i e l d i s e s s e n t i a l l y 100$ b y a n y o f t h e p r e p a r a t i o n methods, b u t t h e p h y s i c a l c h a r a c t e r i s t i c s o f t h e i o n exchangers a r e dependent o n p r e p a r a t i o n c o n d i t i o n s . F o r e x a m p l e , s o d i u m t i t a n a t e p r e p a r e d b y E q s . l a a n d l b w i t h h y d r o l y s i s i n one l i t e r o f w a t e r p e r m o l e o f Τΐ(θθ3Ηγ)ΐ|. h a s a b u l k d e n s i t y o f 0 Λ 5 g/cm3 a n d a s p e c i f i c s u r f a c e a r e a o f 10-^0 m /g. The same m a t e r i a l p r e p a r e d b y E q s . l a a n d l b a n d h y d r o l y z e d i n a s o l u t i o n o f 100 m l o f w a t e r i n 1000 m l o f a c e t o n e f o r e a c h m o l e o f 11(003117)^ h a s a b u l k d e n s i t y o f 0.35 g/cm3 a n d a s p e c i f i c s u r f a c e a r e a o f 2 0 0 kOO m /g. I n a l l c a s e s , t h e m a t e r i a l s c o n s i s t o f a g g l o m e r a t e s o f 50-100 A p a r t i c l e s w i t h t h e d e g r e e o f a g g r e g a t i o n o f t h e p a r t i c l e s determining both t h ebulk density and surface area. A program t o c h a r a c t e r i z e t h e sodium t i t a n a t e m a t e r i a l has v e r i f i e d that t h e chemical and p h y s i c a l p r o p e r t i e s a r e repro d u c i b l e from b a t c h t o b a t c h f o r any o f t h e p r e p a r a t i o n methods. As e x p e c t e d , t h e m a t e r i a l s w i t h t h e h i g h e s t s u r f a c e a r e a s a r e most r e a c t i v e a n d have b e e n s t u d i e d most e x t e n s i v e l y f o r s o l i d i f i c a t i o n o f aqueous w a s t e s . A t t e m p t s t o d e t e r m i n e s t r u c t u r e o f t h e c o m p l e x a n i o n i n t h e e x c h a n g e m a t e r i a l were u n s u c c e s s f u l w i t h t h e exception o f t h e niobate f o r which a c r y s t a l l i n e form o f t h e t e t r a m e t h y l a m m o n i u m n i o b a t e s a l t was o b t a i n e d ( 1 0 , 1 1 , 1 2 ) . The p r i n c i p a l r e a c t i o n w i t h aqueous w a s t e s o l u t i o n s i s b e l i e v e d t o c a t i o n exchange a s i l l u s t r a t e d i n E q . 2 . 2
2
o
2NaTi 05H ( s o l i d ) + S r 2
+ +
-
Sr(Ti 0 H) 2
5
2
( s o l i d ) + 2Na
+
Fried; Radioactive Waste in Geologic Storage ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
( E q . 2)
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Another r e a c t i o n o f the t i t a n a t e which has an important e f f e c t i n waste s o l i d i f i c a t i o n i s the r e a c t i o n w i t h hydrogen i o n , as i n the h y d r o l y s i s r e a c t i o n shown i n Eq, 3 . NaTi 0 H (solid) + 1 ^ 0 2
Na
+
+ OH"
+ HTigOjH ( s o l i d )
(Eq. 3)
T h i s r e a c t i o n i s s i g n i f i c a n t f o r sodium t i t a n a t e (Keq ~ 1 0 " ^ m o l e s / l ) and sodium z i r c o n a t e , but i s n e g l i g i b l e f o r sodium niobate and some other " t i t a n a t e s " such as M g ( T i 0 5 H ) . Gela t i n o u s hydroxide p r e c i p i t a t e s which would appear l i k e l y based on Eq. 3 were not observed i n the r e a c t i o n o f sodium t i t a n a t e with aqueous waste, and s t o i c h i o m e t r i c l o a d i n g was achieved with p o l y v a l e n t c a t i o n s which form i n s o l u b l e hydroxides as w e l l as f o r those forming s o l u b l e hydroxides. Hydrogen i o n was found to be much l e s s exchangeable than Na; hence, the o v e r a l l c a p a c i t y o f (Na,H)Ti 0cjH f o r c a t i o n exchange i s n e a r l y p r o p o r t i o n a l to the Na content. However, the a f f i n i t y f o r various c a t i o n i c species v a r i e s w i d e l y with Na content. For example, i n t e s t s w i t h s t o i c h i o m e t r i c amounts o f s e v e r a l (Na,H)Ti 05H compounds batch e q u i l i b r a t e d with 0.015 M Cs s o l u t i o n s , d i s t r i b u t i o n c o e f f i c i e n t s (K ) were found t o vary from k f o r H T i 0 c H (hydrated T i 0 ) t o >1000 f o r ( N a g c H g c ) T i 0 H (K£ = [Cs] s o l i d / [ C s ] s o l u t i o n χ S o l u t i o n volume, ml/weignt s o l i d , g ) . The K f o r standard sodium t i t a n a t e , NaTi 0^H, was ~200. For purposes o f comparison, sodium niobate has a h i g h a f f i n i t y f o r Cs (K^>1000) and a high o v e r a l l c a p a c i t y , while sodium z i r c o n a t e has a very low a f f i n i t y f o r Cs (Κ^
