Radioactive Waste in Geologic Storage - American Chemical Society

16 Consequences of Radiation from Sorbed Transplutonium 1

Elements on Clays Selected for Waste Isolation

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 21, 2018 | https://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch016

R. G. HAIRE and G. W. BEALL Oak Ridge National Laboratory, Oak Ridge, TN 37830

Recently there has been interest in the sorptive behavior of natural clays toward metal ions potentially present in radioactive wastes. Initial studies of the transplutonium elements have been carried out to define their sorption behavior with such materials (1). However, i t is also important to understand the stability of the clay-actinide product with regard to radiation damage and to be able to predict what changes in behavior may occur after exposure to radiation, so that accurate transport models may be constructed. In order to obtain some information on the effect of alpha radiation on the clay materials, two natural clays were selected for experiments with Es. This isotope was chosen to provide an intense source of alpha radiation (6 x 10 α min μg , 6.6-MeV α) in hopes that the time base of the experiments could be reduced. However, this isotope is available only twice a year and its short half-life limited the experiments to a short period of time. Reported here are the initial results of the experiments with Es-clay mixtures and the tentative implications these results have for the other transplutonium elements. 253

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The methods used to purify the Es have been reported (2). Portions of the purified Es chloride solution were evaporated to dryness in fused silica vessels and the residues taken up in 10 M HC1. These solutions were then used directly for the electron microscopy studies or mixed with sodium chloride-acetate buffer (pH=5) for the distribution coefficient experiments. For electron microscopy, small amounts of clay were metered by using aliquots of a colloidal solution of the clay. These solu tions were then mixed with the Es. Portions of the resulting -3

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Research sponsored by the Division of Nuclear Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corpora tion. 0-8412-0498-5/79/47-100-291$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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mixtures were p e r i o d i c a l l y taken, evaporated on e l e c t r o n microscope g r i d s and the m a t e r i a l examined i n a P h i l i p s EM 300 e l e c tron microscope. S i m i l a r clay suspensions without Es were used as c o n t r o l s . For the d i s t r i b u t i o n c o e f f i c i e n t s , an acetate b u f f e r adjusted to pH 5 was used and the sodium c h l o r i d e concentration of i t v a r i e d from 0.25 t o 4 M by d i s s o l v i n g sodium c h l o r i d e i n the b u f f e r . A l i q u o t s of the E s s o l u t i o n were mixed with the b u f f e r and a weighed quantity of the s o l i d clay was mixed with t h i s s o l ution. P e r i o d i c a l l y , these mixtures were c e n t r i f u g e d and an a l i q u o t of the c l a y - f r e e supernate taken f o r counting a n a l y s i s . Two n a t u r a l l y - o c c u r r i n g clays were s e l e c t e d f o r the experiments; one was l a b e l e d k a o l i n ( f o r the mineral k a o l i n i t e ) w h i l e the second was r e f e r r e d to as a t t a p u l g i t e (or p o l y g o r s h i t e ) . Both were obtained from the Source Clay M i n e r a l Repository (3) as standard clays r e p r e s e n t a t i v e of each c l a s s of clay and were used as r e ceived. Stable, c o l l o i d a l suspensions of each were prepared by u l t r a s o n i c a l l y d i s p e r s i n g weighed q u a n t i t i e s of each clay i n t r i p l e - d i s t i l l e d water.


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Two n a t u r a l l y o c c u r r i n g c l a y s , k a o l i n and a t t a p u l g i t e , were s e l e c t e d f o r the r a d i a t i o n damage study with E s on the b a s i s of t h e i r morphology. These clays together with a t h i r d c l a y , m o n t m o r i l l i n i t e , were also used f o r s o r p t i o n studies with the transplutonium elements ( 1 ) . From e l e c t r o n micrographs of the k a o l i n , and a t t a p u l g i t e m a t e r i a l s , i t i s apparent that the regul a r geometric forms of k a o l i n ( p l a t e l e t s ) and a t t a p u l g i t e (rods or f i b e r s ) are best s u i t e d f o r f o l l o w i n g any changes that may occur i n t h e i r morphology as a r e s u l t of r a d i a t i o n . From s o r p t i o n experiments with the rare earths and t r i v a l e n t a c t i n i d e s , i t was found that the d i s t r i b u t i o n c o e f f i c i e n t s f o r k a o l i n and a t t a p u l g i t e were q u i t e d i f f e r e n t , the l a t t e r having much h i g h e r values (jL) . Therefore, the r a d i a t i o n damage study with E s was made with clays having d i f f e r e n t morphologies and different sorptive capacities. In i n i t i a t i n g these experiments i t was recognized that only d i l u t e c o l l o i d a l suspensions of the clay could be employed to obtain reasonable Es/clay r a t i o s , as only small amounts of Es were a v a i l a b l e . During the course of the experiments, i t a l s o became evident that X-ray or e l e c t r o n d i f f r a c t i o n were of l i m i t e d value i n f o l l o w i n g the e f f e c t of r a d i a t i o n on the c l a y , i n cont r a s t to r a d i a t i o n damage studies of r a d i o a c t i v e compounds such as Pu0 . With the E s - c l a y mixtures, the s e n s i t i v i t y of these d i f f r a c t i o n measurements was lower s i n c e only the undestroyed clay m a t e r i a l was being monitored by d i f f r a c t i o n a n a l y s i s . There were a l s o experimental problems with the r a d i a t i o n from the Es f o r X-ray d i f f r a c t i o n a n a l y s i s such as f i l m blackening. Thus, 2 5 3

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t h i s r a d i a t i o n damage study was confined to f o l l o w i n g morpholog i c a l changes i n the clay s t r u c t u r e s . With s m a l l E s / c l a y l o a d i n g s , (1/1000-1/100 of the c l a y ' s c a p a c i t y , with the capacity being 10~ -10 meq./mg clay) i t was d i f f i c u l t to detect changes i n the clay a f t e r 1-2 weeks by e l e c t r o n microscopy. However, when the clays were loaded to t h e i r c a p a c i t y with Es, extensive d e s t r u c t i o n of the c l a y s t r u c t u r e s wereroted i n 2-4 days. In Figure 1 are micrographs of k a o l i n and a t t a p u l g i t e clays with and without exposure to Figures l b and Id show the e a r l y stages of the damage process i n the c l a y s . With a t t a p u l g i t e , the process proceeded as f o l l o w s : f i r s t small voids and rough edges appeared, followed by clumping of fragments from the d e s t r u c t i o n process around the remaining f i b e r s , and f i n a l l y complete fragmentation of the rods or f i b e r s occurred. For k a o l i n , the d e s t r u c t i o n process began by forming voids or holes i n the p l a t e l e t s , followed by fragmentation of the p l a t e l e t s i n t o smaller p i e c e s . I t should be noted that the p l a t e l e t s of k a o l i n were more s e n s i t i v e to the e l e c t r o n beam i n the e l e c t r o n microscope and i t was necessary to use lower beam currents with the k a o l i n samples. The r a t e of d e s t r u c t i o n of both clays was a f u n c t i o n of Es l o a d i n g , and i n accord w i t h i t s higher s o r p t i v e c a p a c i t y , the d e s t r u c t i o n appeared sooner with a t t a p u l g i t e samples. The fragmentation of both clays from r a d i a t i o n was s i m i l a r to the s e l f - i r r a d i a t i o n damage that has been seen i n the r o d - l i k e s t r u c t u r e s of Am(OH) and * C m ( O H ) ( 4 ) . Quantitative decomposition rates were not measured i n t h i s work, but i t was concluded that the d e s t r u c t i o n was more extensive than that expected i f i t was assumed that only one s i t e on the clay was destroyed per alpha event. Thus, many other s i t e s are destroyed along the path of the alpha p a r t i c l e . Even though the c l a y s t r u c t u r e s were destroyed by the Es r a d i a t i o n , the d i s t r i b u t i o n r a t i o s (Es i n clay/Es i n s o l u t i o n ) for E s - c l a y mixtures d i d not drop d r a s t i c a l l y w i t h time but i n f a c t increased s l i g h t l y . This behavior has been i n t e r p r e t e d i n the f o l l o w i n g ways: (1) that the Es remains a s s o c i a t e d w i t h the fragmented c l a y , which can s t i l l be c e n t r i f u g e d out of the s o l u t i o n ; and/or (2) that the remaining Es i t s e l f has become i n s o l u b l e (and n o n - c o l l o i d a l ) w i t h time and a l s o c e n t r i f u g e s out i n the s o l i d phase. The r a d i a t i o n f i e l d i s s u f f i c i e n t that i n time non-soluble Es species may be formed i n the pH range of the s o l u t i o n s employed here. Intense alpha r a d i a t i o n can l o c a l l y deplete the hydrogen i o n concentration, generate r a d i c a l s , e t c . that may promote the formation of an i n s o l u b l e hydroxide, oxyhalide, etc. species. I t i s apparent from the work that i n i t i a l l y the Es i s indeed sorbed on the clay m a t e r i a l s . Whether the Es i s r e l e a s e d by the damaged clay to the s o l u t i o n where i t again a s s o c i a t e s w i t h undamaged c l a y s i t e s , or whether i t forms an i n s o l u b l e product with the fragmented c l a y , cannot be determined from t h i s work. The important aspect i s that the Es d i d not form a s o l u b l e species i n the s o l u t i o n a f t e r d e s t r u c t i o n of the clay s i t e . 5


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Electron micrographs of attapulgite and kaolin before (la, 1c) and after (lb, Id) contact with Es 253


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The conclusions reached from t h i s p r e l i m i n a r y study i s that these c l a y s can serve as s o r b a t i v e m a t e r i a l f o r the t r i v a l e n t a c t i n i d e s and that even a f t e r damage to the c l a y s t r u c t u r e s , the a c t i n i d e w i l l not be r e l e a s e d as a s o l u b l e s p e c i e s .


References 1.

2. 3. 4.

Beall, G. W., Ketelle, Β. H . , Haire, R. G . , and O'Kelley, G. D . , "Sorption Behavior of Trivalent Actinides and Rare Earths on Clay Minerals," ACS Symposium Series on Radioactive Waste in Geologic Storage, R. F. Gould, E d . , Miami Beach, Florida, Sept. 11-15, 1978. Baybarz, R. D . , Knauer, J. B . , and Orr, P. B . , USAEC Rept. ORNL-4672 (1973). Source Clay Mineral Repository, University of Missouri, Rolla, Mo. Haire, R. G . , Lloyd, M. H., Milligan, W. O., and Beasley, M . L . , J. Inorg. Nucl. Chem., 39, 843 (1977).

RECEIVED January 16, 1979.


Radioactive Waste in Geologic Storage - American Chemical Society

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