Radioactive Waste in Geologic Storage - ACS Publications - American

3 Hydrologic Considerations Related to Management of Radioactive Waste

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch003

GEORGE D. DEBUCHANANNE and WARREN W. WOOD

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U.S. Geological Survey, Reston, VA 22092

The objective of geologic isolation of radioactive wastes is to preclude their reaching the biosphere until after they have decayed to the extent that they no longer constitute a health hazard. Concern over radioactive wastes from military, industrial and research uses has elicited many lines of commentary and deep concern from many individuals. In California, the concern about waste disposal was the focal point in establishing a moratorium on the construction of new reactors u n t i l a satisfactory waste disposal technology could be demonstrated. There are several pathways by which buried wastes can enter the biosphere; these include emission of gases from decaying organic material buried with the waste, erosion of overlying material resulting in waste exposure, waste movement to the land surface through tectonic activity, intrusion by man, and leaching and seepage into ground-water systems. It is generally agreed that the long-range problem of intrusion by man is the most l i k e l y mechanism for contact with the biosphere, as institutional cont r o l , even on the most restrictive burial sites, is likely to be less than 100 years. The earth science community generally perceives that transport by ground water is the most important natural mechanism for bringing buried wastes to the biosphere. The hydrologic evaluation associated with the transport of radionuclides from a repository can be centered on three areas of uncertainty: (1) characterizing the existing hydrologic system at a given s i t e , (2) predicting events that w i l l alter the hydrologic properties with time, and (3) evaluating the interaction of the wastes with the burial environment. Most ground-water hydrologists would prefer to speak in terms of waste isolation, emplacing waste in a repository, or waste dispersion, rather than disposal. Disposal, of course, not only implies that no attempt w i l l be made to retrieve the material but i t also implies future neglect. Isolation as, for example, Current address: Department of Geosciences, Texas Tech University, Lubbock, Texas 79409. 1

This chapter not subject to U.S. copyright. Published 1979 American Chemical Society

Fried; Radioactive Waste in Geologic Storage ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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i n a r e p o s i t o r y , o r p l a n n e d d i s p e r s i o n , on t h e o t h e r h a n d , c a r r y w i t h i t t h e i m p l i c a t i o n t h a t t h e w a s t e s a t a s i t e w o u l d be m o n i t o r e d and managed. A l l s e d i m e n t a r y r o c k s e q u e n c e s p e r m i t some movement o f w a t e r t h r o u g h them, and, o f c o u r s e , m o v i n g w a t e r w i l l l e a c h t h e b u r i e d waste r e s u l t i n g i n t r a n s p o r t w i t h the moving water. R e a l i s t i c a n s w e r s t o p r o b l e m s o f w a s t e i s o l a t i o n must c o n s i d e r a p h i l o s o p h y of c o n t r o l l e d d i s p e r s i o n over v e r y l o n g time p e r i o d s . Volume o f W a s t e The DOE ( D e p a r t m e n t o f E n e r g y ) w h i c h i s t h e l e a d a g e n c y i n m a t t e r s o f n u c l e a r e n e r g y , has s p e c i f i e d t h r e e c a t e g o r i e s o f waste: (1) h i g h - l e v e l , (2) t r a n s u r a n i u m c o n t a m i n a t e d , and (3) other than h i g h - l e v e l waste, g e n e r a l l y c a l l e d l o w - l e v e l . Highl e v e l waste r e s u l t s from r e p r o c e s s i n g of spent n u c l e a r r e a c t o r fuel. The t r a n s u r a n i c - c o n t a m i n a t e d w a s t e (TRU) c o n t a i n s c o n c e n t r a t i o n s g r e a t e r t h a n 10 n a n o c u r i e s p e r gram ( n C i / g ) o f t h e manmade e l e m e n t s w i t h a t o m i c numbers g r e a t e r t h a n 92. The r e m a i n d e r o f t h e w a s t e , w i t h t h e e x c e p t i o n o f m i n e and m i l l t a i l i n g s and spent f u e l rods i s c o n s i d e r e d low l e v e l . F u e l r o d s have o n l y r e c e n t l y b e e n c o n s i d e r e d a w a s t e f o r m as a r e s u l t o f t h e A d m i n i s t r a t i o n ' s d e c i s i o n t o e l i m i n a t e r e p r o c e s s i n g of spent n u c l e a r f u e l and a r e c l a s s i f i e d as h i g h - l e v e l w a s t e . Low-Level Waste. L o w - l e v e l w a s t e s a r e f u r t h e r d i v i d e d i n t o c a t e g o r i e s o f s p e c i a l n u c l e a r m a t e r i a l , s o u r c e m a t e r i a l , and b y p r o d u c t m a t e r i a l , d e p e n d i n g on t h e i s o t o p e s c o n t a i n e d . Special n u c l e a r m a t e r i a l r e f e r s t o u r a n i u m 2 3 3 , p l u t o n i u m 239, and u r a n i u m c o n t a i n i n g more t h a n t h e n a t u r a l a b u n d a n c e o f u r a n i u m 235. Source m a t e r i a l r e f e r s t o m a t e r i a l s c o n t a i n i n g 0.05 p e r c e n t o r more o f t h o r i u m o r u r a n i u m i n any p h y s i c a l o r c h e m i c a l f o r m e x c e p t t h a t covered under s p e c i a l n u c l e a r m a t e r i a l . By-product m a t e r i a l s c o n s i s t o f a l l o t h e r r a d i o a c t i v e m a t e r i a l s i n c l u d i n g f i s s i o n and a c t i v a t i o n products. B u r i a l o f t r a n s u r a n i u m and l o w - l e v e l w a s t e i n s h a l l o w p i t s and t r e n c h e s h a s o c c u r r e d s i n c e t h e e a r l y d a y s o f t h e M a n h a t t a n Project. I n t h e U n i t e d S t a t e s , 14 b u r i a l s i t e s h a v e b e e n o p e r a t e d b y t h e D e p a r t m e n t o f E n e r g y , 12 o f w h i c h a r e p r e s e n t l y a c t i v e ; and s i x s t a t e - o w n e d s i t e s h a v e b e e n o p e r a t e d by l i c e n s e d commercial f i r m s . Three of the commercial s i t e s are p r e s e n t l y active (figure 1). T h r o u g h 1977, a p p r o x i m a t e l y 1,306,000 c u b i c m e t e r s (1) of m i l i t a r y w a s t e m a t e r i a l had b e e n b u r i e d i n a p p r o x i m a t e l y 280 h e c t a r e s a t t h e DOE s i t e s . The r a t e o f a c c u m u l a t i o n a t t h e s e s i t e s has r e m a i n e d r e l a t i v e l y c o n s t a n t and i s now a p p r o x i m a t e l y 33,000 c u b i c m e t e r s p e r y e a r . T h r o u g h 1977, a p p r o x i m a t e l y 500,000 c u b i c m e t e r s o f w a s t e had b e e n b u r i e d i n 300 h e c t a r e s a t t h e s i x c o m m e r c i a l s i t e s ( 2 ) . The p r e s e n t a n n u a l r a t e o f b u r i a l i s a p p r o x i m a t e l y 60,000 c u b i c m e t e r s (3) and i s i n c r e a s i n g r a p i d l y



Figure 1.

Locations of burial grounds of solid low-level radioactive waste


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(figure 2). Between 1946-1970, a p p r o x i m a t e l y 28,000 f i f t y f i v e - g a l l o n drums o f m i l i t a r y and c o m m e r c i a l l o w - l e v e l w a s t e w e r e d i s p o s e d of i n t h e A t l a n t i c Ocean and a p p r o x i m a t e l y 47,000 i n t h e P a c i f i c (4). T h i s p r a c t i c e was d i s c o n t i n u e d i n 1970 and s i n c e t h e n a l l l o w - l e v e l w a s t e has b e e n b u r i e d i n s h a l l o w t r e n c h e s . Holcomb (2) r e p o r t s t h a t t h r o u g h 1976, c o m m e r c i a l b u r i a l g r o u n d s had d i s p o s e d o f 951,468 k i l o g r a m s (kg) o f s o u r c e m a t e r i a l , approximately 4 m i l l i o n c u r i e s of by-product m a t e r i a l , and 1,667 kg o f s p e c i a l n u c l e a r m a t e r i a l as w e l l as 113 kg o f TRU w a s t e ( 3 ) . The DOE s i t e s , a c c o r d i n g t o D u g u i d ( 1 ) , c o n t a i n 7,292,124 kg o f s o u r c e m a t e r i a l and 11 m i l l i o n c u r i e s o f b y p r o d u c t a s w e l l as more t h a n 700 k g o f TRU w a s t e . An a d d i t i o n a l 6 m i l l i o n l i t e r s o f a w a s t e - c e m e n t g r o u t s l u r r y has b e e n i n j e c t e d i n t o h y d r o f r a c t u r e d s h a l e a t t h e Oak R i d g e d i s p o s a l s i t e i n Tennessee. H i g h - L e v e l W a s t e . T h e r e a r e a p p r o x i m a t e l y 285 m i l l i o n l i t e r s c o n t a i n i n g a p p r o x i m a t e l y 590 χ 10^ c u r i e s (5) o f m i l i t a r y and 2.3 m i l l i o n l i t e r s o f c o m m e r c i a l h i g h - l e v e l l i q u i d w a s t e (6) p r e s e n t l y s t o r e d i n t a n k s . A l t h o u g h t h e volume of h i g h - l e v e l m i l i t a r y w a s t e i s much g r e a t e r , t h e c u r i e c o n t e n t o f s t r o n t i u m 90 of b o t h s o u r c e s i s a p p r o x i m a t e l y t h e same i f s t o r e d s p e n t f u e l rods from commercial r e a c t o r s are i n c l u d e d i n t h e i n v e n t o r y . The t o t a l c u r i e c o n t e n t w i l l be t h e same f o r b o t h s o u r c e s by 1985 a t t h e p r e s e n t r a t e o f u s e ( 5 ) . To d a t e , t h e r e h a s b e e n no d i s p o s a l o f any h i g h - l e v e l w a s t e . Waste D i s p o s a l P r o b l e m R a d i o a c t i v e w a s t e i s o l a t i o n i n a g e o l o g i c medium i s an example o f a problem t h a t w i l l extend beyond t h e f o r e s e e a b l e d u r a t i o n o f e x i s t i n g human i n s t i t u t i o n s . The p o t e n t i a l e f f e c t s a r e e x t r e m e l y i m p o r t a n t and p r e s e n t a s i g n i f i c a n t c h a l l e n g e t o i n s t i t u t i o n s d e s i g n e d f o r d e a l i n g w i t h p r o b l e m s o f l e s s e r com p l e x i t y and s h o r t e r d u r a t i o n . I n t h i s c a s e , n o t o n l y a r e t h e human i n s t i t u t i o n s s t r a i n e d , b u t t h e s c i e n c e o f h y d r o l o g y p r e s e n t l y l a c k s t e c h n i q u e s and m e t h o d o l o g y t o d e f i n e a d e q u a t e l y t h e f l o w i n s e v e r a l common t y p e s o f h y d r o l o g i e s y s t e m s . There are t h r e e areas of h y d r o l o g i e u n c e r t a i n t y i n v o l v e d i n the g e o l o g i c i s o l a t i o n of r a d i o a c t i v e waste. The f i r s t c o n c e r n s the c h a r a c t e r i z a t i o n of e x i s t i n g h y d r o l o g i e systems; t h a t i s , our i n a b i l i t y to q u a n t i t a t i v e l y d e f i n e the e x i s t i n g c o n d i t i o n s con t r o l l i n g t h e movement o f r a d i o n u c l i d e s . The s e c o n d i s t h e u n c e r t a i n t y o f p r e d i c t i n g d i s r u p t i v e e v e n t s such as earthquakes and c l i m a t i c c h a n g e s w h i c h a l t e r t h e p r o p e r t i e s and h y d r o l o g i e c o n d i t i o n s of t h e system. The t h i r d a r e a o f u n c e r t a i n t y i s how t h e w a s t e i t s e l f and t h e method o f emplacement a f f e c t t h e h y d r o l o g y of a system. D i s r u p t i v e events are g e n e r a l l y not considered important i n



DEBUCHANANNE AND WOOD

Figure 2.

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Yearly total volume of commercial low-level waste buried within the conterminous United States



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the e v a l u a t i o n of l o w - l e v e l d i s p o s a l s i t e s because of the s m a l l amount o f l o n g - l i v e d r a d i o n u c l i d e s p r e s e n t . However, t h e o t h e r a r e a s o f u n c e r t a i n t y w o u l d a f f e c t modes o f g e o l o g i c i s o l a t i o n o f h i g h - l e v e l w a s t e . The e f f e c t o f w a s t e emplacement on t h e h y d r o l o g i c p r o p e r t i e s of t h e s i t e have g e n e r a l l y been g i v e n o n l y c u r s o r y e v a l u a t i o n i n p a s t s t u d i e s of l o w - l e v e l waste s i t e s . T h i s has p r o v e d t o be s h o r t s i g h t e d , and s e l e c t i o n c r i t e r i a f o r new s i t e s w i l l a l m o s t c e r t a i n l y c o n t a i n s p e c i f i c g u i d e l i n e s on s u c h t h i n g s as cap o r c o v e r d e s i g n and t y p e o f w a s t e f o r m p e r m i t t e d t o be b u r i e d . These t h r e e broad a r e a s of u n c e r t a i n t y form t h e b a s i s of p r e s e n t and p r o p o s e d r e s e a r c h on r a d i o a c t i v e w a s t e and a r e t h e s u b j e c t of t h i s paper. Consider f i r s t the problem i n d e f i n i n g e x i s t i n g systems i n terms of t h e h y d r o g e o l o g i c p r o p e r t i e s i n r e l a t i o n t o t r a n s p o r t by g r o u n d w a t e r . T r a d i t i o n a l l y , g r o u n d w a t e r h y d r o l o g i s t s h a v e b e e n I n v o l v e d w i t h f i n d i n g and e v a l u a t i n g s u p p l y s y s t e m s f o r i r r i g a t i o n , i n d u s t r i a l , m u n i c i p a l , and domest i c users. Waste i s o l a t i o n s t u d i e s have been j o k i n g l y r e f e r r e d t o as " a n i t h y d r o l o g y " b e c a u s e h y d r o l o g i s t s a r e b e i n g f o r c e d t o s t u d y f l o w systems e x h i b i t i n g v e r y low, r a t h e r t h a n h i g h , groundw a t e r f l u x e s . T h i s change i n p h i l o s o p h y has p r e s e n t e d a m a j o r c h a l l e n g e to the study of ground-water h y d r o l o g y , i n v o l v i n g f o c u s i n g more a t t e n t i o n on t h e f u n d a m e n t a l s o f f l o w and geoc h e m i c a l r e a c t i o n s i n m a t e r i a l s o f low p e r m e a b i l i t y . T h e s e low p e r m e a b i l i t y s i t e s r e q u i r e new m e t h o d o l o g y and t e c h n q i u e s t o e v a l u a t e the systems. One o f t h e a p p r o a c h e s i n e v a l u a t i n g t h e p e r m e a b i l i t y o f an a q u i f e r s y s t e m i s t o p e r f o r m an a q u i f e r t e s t by pumping a w e l l and o b s e r v i n g t h e w a t e r - l e v e l d e c l i n e w i t h t i m e i n n e a r b y o b s e r v a t i o n w e l l s . I n systems of low p e r m e a b i l i t y , s u f f i c i e n t w a t e r f o r a pumping t e s t i s g e n e r a l l y p r i m a f a c i e e v i d e n c e t h a t i t i s u n a c c e p t a b l e as a d i s p o s a l s i t e . C o n s e q u e n t l y , a new s e t o f " p a r l o r t r i c k s " i s needed t o d e f i n e t h e p e r m e a b i l i t y of t h o s e types of media. S u i t a b l e techniques are not a v a i l a b l e to evaluate m a t e r i a l o f l o w p e r m e a b i l i t y i n an a r e a t h e s i z e o f e i t h e r a h i g h o r l o w level repository. O n l y a v e r y s m a l l a r e a o f a few t e n s o f c e n t i m e t e r s , s u c h as t h e a r e a i n a c o r e h o l e o r t h a t i m m e d i a t e l y a d j a c e n t t o t h e o b s e r v a t i o n w e l l , p r e s e n t l y c a n be evaluated. D r i l l i n g many h o l e s and a p p l y i n g t e c h n i q u e s s u i t a b l e f o r c h a r a c t e r i z i n g s m a l l a r e a s w o u l d c r e a t e numerous p o t e n t i a l w a s t e m i g r a t i o n p a t h w a y s . T h a t i s , t e s t i n g a s i t e a d e q u a t e l y by t h e s e l i m i t e d methods w o u l d d e s t r o y any i n t e g r i t y t h a t t h e s i t e p o s s e s s e d . A d d i t i o n a l l y , a new t e c h n i q u e i s n e e d e d t o i d e n t i f y s m a l l zones of h i g h p e r m e a b i l i t y w i t h i n a l a r g e volume of r o c k s w i t h low p e r m e a b i l i t y . T h e s e h i g h l y p e r m e a b l e z o n e s may be f r a c t u r e s or changes i n f a c i e s of t h e media, s o l u t i o n o p e n i n g s , o r t h e y may r e s u l t f r o m any o t h e r f a c t o r t h a t can l o c a l l y a l t e r t h e p e r m e a b i l i t y of a h y d r o l o g i e system. Another a r e a of u n c e r t a i n t y i n d e s c r i b i n g e x i s t i n g systems



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p e r t a i n s t o f l o w and t r a n s p o r t t h r o u g h f r a c t u r e s . B o t h s t o c h a s t i c and d e t e r m i n i s t i c approaches a r e p r e s e n t l y b e i n g pursued i n attempting t o model f l o w i n f r a c t u r e d media. O p i n i o n w i t h i n t h e ground-water p r o f e s s i o n i s d i v i d e d on w h i c h w i l l y i e l d t h e b e s t i n f o r m a t i o n a s n e i t h e r one h a s b e e n s u c c e s s f u l l y a p p l i e d i n the f i e l d . With t h ecurrent state of a b i l i t y t o define a given h y d r o l o g i e system, i t would appear t h a t t h e s t o c h a s t i c approach may g i v e t h e most u s e f u l r e s u l t s . However, i f new g e o p h y s i c a l o r o t h e r t e c h n i q u e s become a v a i l a b l e t o p e r m i t d e f i n i t i o n o f a f r a c t u r e system, t h e d e t e r m i n i s t i c m o d e l i n g approach might be more u s e f u l . For y e a r s , h y d r o l o g i s t s concerned w i t h water s u p p l y have used models developed f o r f l o w i n g r a n u l a r media t o p r e d i c t f l o w i n h i g h l y f r a c t u r e d m e d i a . However, a n y a p p a r e n t s u c c e s s i n t h i s a p p r o a c h was f o r t u i t o u s b e c a u s e ( a ) t h e pumping w e l l i n t e g r a t e d a l a r g e a r e a and was a b l e t o " s e e " many f r a c t u r e s a s i f t h e y w e r e p o r e s i n a g r a n u l a r m e d i a , a n d ( b ) b e c a u s e t h e permea b i l i t y c o n t r a s t b e t w e e n t h e t w o t y p e s o f f l o w was r e l a t i v e l y s m a l l i n t h e system. N e i t h e r o f these s i t u a t i o n s p r e v a i l s i n rocks o f low p e r m e a b i l i t y . A l t h o u g h t h e r e h a s b e e n some s u c c e s s i n p r e d i c t i n g w a t e r - l e v e l changes i n f r a c t u r e d media o f r e l a t i v e l y h i g h p e r m e a b i l i t y , t h e r e h a s b e e n l i t t l e o r no s u c c e s s i n p r e d i c t i n g t h e movement o f d i s s o l v e d m a t e r i a l s i n t h e s e m e d i a . The o i l i n d u s t r y h a s d e v e l o p e d and u s e d a d o u b l e p o r o s i t y m o d e l to d e s c r i b e f l o w i n c e r t a i n f r a c t u r e systems (7). L i t t l e d a t a on a p p l i c a t i o n t o a c t u a l f i e l d p r o b l e m s a r e a v a i l a b l e i n t h e l i t e r a t u r e s u g g e s t i n g t h a t t h i s model has had o n l y l i m i t e d s u c c e s s and has been used o n l y a f t e r t h e f a c t , n o t i n a p r e d i c t i v e mode. D e f i n i n g t h e movement o f w a t e r i n r o c k s o f l o w p e r m e a b i l i t y by c h e m i c a l m e t h o d s , s u c h a s t h e u s e o f t r a c e r s , a g e d a t i n g o f w a t e r , and i s o t o p e r a t i o s , h a s met w i t h l i m i t e d s u c c e s s f o r t h e same r e a s o n s a s h a v e t h e p h y s i c a l m e t h o d s . T h a t i s , t h e y a r e i n c a p a b l e o f d e s c r i b i n g t h e p e r m e a b i l i t y d i s t r i b u t i o n i n an adequately large area. The u n s a t u r a t e d z o n e , w h e r e most o f t h e l o w - l e v e l w a s t e i s b u r i e d , c o u l d p r o b a b l y be used f o r h i g h - l e v e l waste a f t e r c o o l i n g b u t i t does n o t l e n d i t s e l f t o t h e r e g i o n a l a p p r o a c h t o d e f i n i n g permeability. A d d i t i o n a l l y , t h e l a b o r a t o r y methods o f e v a l u a t i n g permeability of unsaturated m a t e r i a l a r e extremely d i f f i c u l t t o p e r f o r m c o n s i s t e n t l y . However, an a i r p e r m e a b i l i t y t e c h n i q u e t h a t may p r o v e u s e f u l i n e v a l u a t i n g t h e z o n e b e t w e e n t h e s u r f a c e and t h e w a t e r t a b l e was d e v e l o p e d b y Weeks ( 8 ) . The t e c h n i q u e uses n o r m a l b a r o m e t r i c f l u c t u a t i o n s measured a t d i f f e r e n t d e p t h s i n t h e f o r m a t i o n t o determine p e r m e a b i l i t y t o a i r which can be used t o determine p e r m e a b i l i t y t o water. C o n s i d e r now t h e e f f e c t s o f t h e w a s t e and emplacement o f t h e w a s t e on t h e h y d r o l o g y o f t h e s y s t e m . P r e s e n t t h i n k i n g s u g g e s t s t h a t a m i n e d u n d e r g r o u n d c a v i t y i n bedded s a l t w i l l b e t h e f i r s t choice f o r a r e p o s i t o r y f o r h i g h - l e v e l waste. The r a t i o n a l e f o r



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t h i s c h o i c e o f medium i s t h a t t h e p r e s e n c e o f a t h i c k l y bedded s a l t w h i c h has b e e n p r e s e n t f o r m i l l i o n s o f y e a r s w i t h o u t d i s s o l u t i o n must p r e c l u d e t h e e x i s t e n c e o f any s i g n i f i c a n t r e g i o n a l ground-water f l o w through the s a l t . Arguments a g a i n s t t h i s t h i n k i n g a r e weak i f b r e c c i a p i p e s , s o l u t i o n c a v i t i e s , b r i n e p o c k e t s or o t h e r e v i d e n c e of s o l u t i o n a r e absent from t h e formation. However, o t h e r a r e a s o f c o n c e r n t o t h e h y d r o l o g i s t are: (1) t h e h y d r o l o g i e e f f e c t s o f e x c a v a t i o n o f t h e s h a f t and c a v i t y on h y d r o l o g y ; f o r i n s t a n c e , w i l l f r a c t u r e s d e v e l o p and p e r m i t a b r e a c h o f t h e c o n f i n i n g beds t h a t have p r o t e c t e d t h e s a l t f o r g e o l o g i c p e r i o d s o f t i m e and c o n s e q u e n t l y p e r m i t f l o w of ground water? (2) W i l l t h e b a c k f i l l o f t h e m i n e d c a v i t y a f t e r emplacement o f t h e w a s t e be s u f f i c i e n t l y d e n s e t o p r e v e n t e x c e s s i v e s u b s i d e n c e and c r a c k i n g o f o v e r l y i n g b e d s and t h e r e f o r e permit a c c e s s of ground water? (3) W i l l t h e h e a t c a u s e d by t h e r a d i o a c t i v e d e c a y o f t h e w a s t e c a u s e t h e r m a l e x p a n s i o n and s u b s e quent c o n t r a c t i o n , r e s u l t i n g i n c o l l a p s e of t h e o v e r l y i n g format i o n and a c c e s s o f g r o u n d w a t e r t o t h e r e p o s i t o r y throug"h fractures? (4) W i l l t h e h e a t o f t h e w a s t e c a u s e i n t e r s t i t i a l o r g r a i n - b o u n d a r y w a t e r t o m i g r a t e t o w a r d t h e w a s t e and t h e r e b y s i g n i f i c a n t l y a l t e r the s t r u c t u r a l p r o p e r t i e s of the s a l t i n the immediate area of the waste? (5) How w i l l t h e r a d i o n u c l i d e s behave c h e m i c a l l y i n a b r i n e under r e d u c i n g c o n d i t i o n s at r e l a t i v e l y high temperatures? That i s , w i l l t h e i r s o r p t i v e c h a r a c t e r i s t i c s be s i g n i f i c a n t l y a l t e r e d , and i f s o , what t y p e o f p r e d i c t i v e c a p a b i l i t i e s do we p o s s e s s f o r t h e s e c o n d i t i o n s ? B e t t e r cap d e s i g n s ( t r e n c h c o v e r s ) a r e n e e d e d f o r l o w - l e v e l w a s t e s i t e s so t h a t t h e m a t e r i a l s a r e l e s s p e r m e a b l e . Present p r a c t i c e r e s u l t s i n as much as 5 0 - p e r c e n t v o i d s p a c e i n t h e t r e n c h , i n p a r t f r o m t h e d e c a y o f o r g a n i c m a t e r i a l and i n p a r t f r o m t h e p r o c e d u r e s f o r w a s t e emplacement. T h i s r e s u l t s i n c o m p a c t i o n o f t h e b u r i e d w a s t e and s u b s i d e n c e o f t h e cap. The c h e m i c a l c h a r a c t e r i z a t i o n o f l o w - l e v e l w a s t e i s more d i f f i c u l t than t h a t of h i g h - l e v e l waste because of the heterogeneous n a t u r e o f t h e w a s t e and a s s o c i a t e d m a t e r i a l s . Thousands of organicc h e m i c a l s i n c l u d i n g c h e l a t i n g a g e n t s may f o r m c o m p l e x e s w i t h n u c l i d e s r e n d e r i n g them n o n s o r b a b l e . Also, a viable microbiologic a l p o p u l a t i o n c o n t i n u a l l y a l t e r s t h e c o m p o s i t i o n and n a t u r e o f the waste. The c h e m i c a l b e h a v i o r o f w a s t e i n t h e u n s a t u r a t e d z o n e i s a l a r g e unknown. F o r e x a m p l e , d e f i n i t i o n o f t h e number o f s o r p t i o n s i t e s a v a i l a b l e to a s o l u t e i s d i f f i c u l t or impossible b e c a u s e unknown and v a r i a b l e amounts a r e o c c u p i e d by i n t e r s t i t i a l gas r a t h e r t h a n w a t e r . A r e t h e k i n e t i c s o f r e a c t i o n a f f e c t e d by less-than-complete saturation? These a r e not t r i v i a l q u e s t i o n s , as most o f t h e l o w - l e v e l d i s p o s a l s i t e s a r e c o n s t r u c t e d i n t h e u n s a t u r a t e d z o n e . W i n o g r a d (9) s u g g e s t e d t h a t c e r t a i n a r e a s u n d e r l a i n by t h i c k u n s a t u r a t e d z o n e s m i g h t make f e a s i b l e r e p o s i t o r i e s f o r h i g h - l e v e l wastes. Two a r e a s o f u n c e r t a i n t y , t h a t c o n c e r n i n g t h e c h a r a c t e r i z a -



3.


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t i o n o f e x i s t i n g h y d r o l o g i e systems and t h a t a s s o c i a t e d w i t h t h e h y d r o l o g i e e f f e c t s o f w a s t e emplacement a n d w a s t e - r o c k i n t e r a c t i o n s , lend themselves t o t h e s c i e n t i f i c approach o f t e s t i n g a h y p o t h e s i s by a s u i t a b l y d e s i g n e d e x p e r i m e n t . The f i n a l g e n e r a l c a t e g o r y o f u n c e r t a i n t y t o be d i s c u s s e d , t h a t o f d i s r u p t i v e e v e n t s , i s more d i f f i c u l t t o overcome b e c a u s e o f t h e g e o l o g i c t i m e i n v o l v e d a n d t h e l a c k o f b a s i c k n o w l e d g e a b o u t t h e fundament a l processes i n v o l v e d . That i s t o s a y , geology i s s t i l l b a s i c a l l y a science of d e s c r i p t i o n rather than p r e d i c t i o n . F o r e x a m p l e , we d o n ' t know t h e b a s i c mechanism c o n t r o l l i n g t h e development o f c o n t i n e n t a l g l a c i a t i o n and i t s r e c u r r e n c e interval. Again, t h i s i s n o t a t r i v i a l q u e s t i o n because t h e hydrology o f almost every area s e l e c t e d f o r a r e p o s i t o r y would c h a n g e s i g n i f i c a n t l y i f t h e t y p e o f g l a c i a t i o n t h a t ended 10,000 y e a r s ago were t o r e c u r . The d i s t r i b u t i o n o f h y d r a u l i c h e a d would be a l t e r e d s i g n i f i c a n t l y as would t h e t o t a l f l u x o f ground w a t e r m o v i n g t h r o u g h a g i v e n s y s t e m . The l e n g t h o f g r o u n d - w a t e r f l o w p a t h s w o u l d become s h o r t e r a n d t h e r a t e o f e r o s i o n o f s u r f i c i a l m a t e r i a l might be s i g n i f i c a n t l y i n c r e a s e d . T h e r e i s n e e d t o a v o i d l o c a t i n g a r e p o s i t o r y i n an a r e a where igneous i n t r u s i o n s would be l i k e l y t o o c c u r . This should n o t be d i f f i c u l t a s we h a v e m i l l i o n s o f y e a r s o f g e o l o g i c r e c o r d s and an o b s e r v e d s y s t e m a t i c p a t t e r n o f i n t r u s i o n . H o w e v e r , when we c o n s i d e r s e i s m i c e v e n t s , o u r h i s t o r y i s l i m i t e d t o a b o u t 200 years. That i s , i t would be extremely d i f f i c u l t t o s a y t h a t a g i v e n a r e a w i l l n o t have an e a r t h q u a k e o f , s a y , m a g n i t u d e 8 o r g r e a t e r ( R i c h t e r s c a l e ) i n t h e n e x t 250,000 y e a r s . Our c o n c e r n w i t h e a r t h q u a k e s i s n o t s o much t h a t t h e w a s t e w o u l d be b r o u g h t d i r e c t l y to t h esurface, but that f r a c t u r e s i nt h e formation c h o s e n f o r t h e r e p o s i t o r y w o u l d p e r m i t g r o u n d - w a t e r movement w h i c h c o u l d u l t i m a t e l y t r a n s p o r t t h e w a s t e t o t h e human e n v i r o n ment. To q u o t e a r e c e n t p u b l i c a t i o n o f t h e U.S. G e o l o g i c a l S u r v e y , "Past g e o l o g i c events such as f a u l t i n g , s e i s m i c d i s t u r b a n c e s , o r c l i m a t i c c h a n g e s h a v e n o t b e e n random, b u t a d e t e r m i n i s t i c e x p l a n a t i o n f o r t h e i r frequency, p l a c e o f occurrence, magnitude and r a t e o f c h a n g e a r e d i f f i c u l t t o e s t a b l i s h . Regardless of whether d e t e r m i n i s t i c o r p r o b a b i l i s t i c models a r e f a v o r e d t o e x p l a i n p a r t i c u l a r past g e o l o g i c events, t h e use o f t h e g e o l o g i c r e c o r d t o p r e d i c t f u t u r e events i s a f o r m i d a b l e t a s k . " (10) T h i s i s n o t t o s a y t h a t a l l i s doom and g l o o m . Although many o f t h e s e t a s k s a r e f o r m i d a b l e , t h e e a r t h - s c i e n c e community b e l i e v e s them t o b e t r a c t a b l e and t h a t a s u c c e s s f u l g e o l o g i c r e p o s i t o r y f o r r a d i o a c t i v e waste can be c o n s t r u c t e d . We o n l y plead t h a t our ignorance o f earth's processes be considered i n t h e d e v e l o p m e n t o f a r e p o s i t o r y a n d t h a t any r e p o s i t o r y c o n s t r u c t e d p r i o r t o t h e a c q u i s i t i o n o f t h e needed fundamental k n o w l e d g e c o n t a i n many i n d e p e n d e n t n a t u r a l and manmade b a r r i e r s to r a d i o n u c l i d e t r a n s p o r t t o compensate f o r o u r l a c k o f knowledge.


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Literature Cited 1.


2.

3.

4.

5. 6. 7. 8.

9. 10.

Duguid, J.O., "Assessment of DOE low-level r a d i o a c t i v e waste solid d i s p o s a l storage activity", Battelle Columbus Laboratory, BMI-1984, 1977. Holcomb, W . F . , Nuclear S a f e t y , 1978, 19, ( 1 ) , 50-59, "A summary of shallow land burial of r a d i o a c t i v e waste at commercial sites between 1962 and 1976, w i t h p r o j e c t i o n s " . N a t i o n a l Academy of S c i e n c e s , "The shallow land burial of l o w - l e v e l r a d i o a c t i v e l y contaminated solid waste", 2101 C o n s t i t u t i o n A v e . , N . W . , Washington, D . C . 20418, 150, 1976. Dyer, R.S., "Environmental surveys of two deep sea r a d i o a c t i v e waste d i s p o s a l sites u s i n g submersibles" i n Symposium, Management of R a d i o a c t i v e Waste from the Nuclear F u e l C y c l e , IAEA, 317-338, 1976. Krugmann, Η . , and von H i p p l e , F., S c i e n c e , 1977, 197, 883-885, " R a d i o a c t i v e waste: a comparison of U . S . military and civilian inventories". GAO Report to Congress, "Nuclear energy dilemma: d i s p o s i n g of hazardous r a d i o a c t i v e waste s a f e l y " , 73, 1977. Rossen, R.H., Soc. P e t r . Eng. J o u r . , June 1977, " S i m u l a t i o n of n a t u r a l l y f r a c t u r e d r e s e r v o i r s w i t h s e m i - i m p l i c i t source terms". Weeks, E.P., "Field determination of vertical p e r m e a b i l i t y to air in the unsaturated zone", U . S . G e o l . Survey P r o f . Paper 1051, 41, 1978. Winograd, Isaac J., EOS, 1974, 55, (10), 884-894, " R a d i o a c t i v e waste storage in the a r i d zone". Bredehoeft, J.D., England, A . W . , Stewart, D . B . , Trask, N.J. and Winograd, I.J., " G e o l o g i c a l d i s p o s a l of h i g h - l e v e l radioactive wastes--earth-science perspectives", U.S. G e o l . Survey Circ. 779, 1978.

RECEIVED January 16, 1979.


Radioactive Waste in Geologic Storage - ACS Publications - American

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