Chemical and Physical Consequences of Î± and Î²- Decay in the Solid

21 Chemical and Physical Consequences of αand β Decay in the Solid State -

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J. P. YOUNG and R. G. HAIRE Oak Ridge National Laboratory, Transuranium Research Laboratory, Oak Ridge, TN 37830 J. R. PETERSON University of Tennessee, Department of Chemistry, Knoxville, TN 37996-1600 D. D . ENSOR Tennessee Technological University, Department of Chemistry, Cookeville, TN 38501 Interesting chemical and structural phenomena can occur when radioactive materials are stored in the solid state. Extensive studies have been made of both the chemical and physical status of progeny species that result from the α or β decay of acti nide ions in several different compounds. The samples have been both initially pure actinide compounds--halides, oxides, etc.--and actinides incorporated into other non-radioactive host mate rials, for example lanthanide halides. In general, the oxidation state of the actinide progeny is con trolled by the oxidation state of its parent (a result of heredity). The structure of the progeny compound seems to be controlled by its host (a result of environment). These conclusions are drawn from solid state absorption spectral studies, and where possible, from x-ray diffraction studies of multi-microgram sized samples. -

The geochemical behavior of disposed r a d i o a c t i v e wastes i s con t r o l l e d by a number of d i f f e r e n t p h y s i c a l and chemical phenomena. Some changes a r e caused by thermal processes which i n f l u e n c e chemistry o f both the stored m a t e r i a l and i t s host. These thermal e f f e c t s can a l s o i n f l u e n c e the p h y s i c a l c h a r a c t e r i s t i c s of the s t o r e d m a t e r i a l . Laboratory and f i e l d s t u d i e s are underway i n many s c i e n t i f i c and engineering f a c i l i t i e s t o understand these effects. The transmutation of one elemental species i n t o another by r a d i o a c t i v e decay i s a phenomenom that w i l l c e r t a i n l y i n f l u e n c e the behavior of stored r a d i o a c t i v e waste, but transmutation has r e c e i v e d l i t t l e a t t e n t i o n i n t h i s context. Although the cause can

0097-6156/84/0246-0335S06.00/0 © 1984 American Chemical Society

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


336

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

be considered a p h y s i c a l e f f e c t , i t can provoke both chemical and s t r u c t u r a l changes. The important question i s , i n what way w i l l t h i s transmutation a f f e c t the s t o r e d m a t e r i a l and i t s host? Too o f t e n transmutation i s considered simply as the p h y s i c a l change of one element to another. The r e a l process i s , however, the change of one chemical species i n t o another chemical s p e c i e s . What w i l l be the r e s u l t a n t o x i d a t i o n s t a t e ? How w i l l the daughter or progeny species achieve t h i s o x i d a t i o n state? What s t r u c t u r e w i l l the r e s u l t a n t species e x h i b i t ? The answers t o these questions w i l l be r e q u i r e d to demonstrate geochemical s t a b i l i t y of a l l p o s s i b l e storage forms and t o i d e n t i f y those forms most s u i t a b l e f o r storage. Note that these are questions to be answered f o r chemical species i n the s o l i d s t a t e . Numerous studies have been c a r r i e d out i n which the answers to questions such as these are based on the r e s u l t s of chemical analyses of s o l i d s d i s s o l v e d i n some solvent system p r i o r to a n a l y s i s . A survey of such studies has been published (1). The r e s u l t s of these s t u d i e s were proba bly i n f l u e n c e d by the d i s s o l u t i o n treatment and are probably not r e p r e s e n t a t i v e of e f f e c t s that a c t u a l l y occurred i n the s o l i d s t a t e . Our s t u d i e s to date do not answer a l l the questions posed above. A s t a r t has been made, however, with some very i n t e r e s t i n g i n v e s t i g a t i o n s that deal with the e f f e c t of h e r e d i t y and e n v i r o n ment on the chemical and p h y s i c a l consequences of α and f ~ decay i n the s o l i d s t a t e . A s u i t a b l e r a d i o a c t i v e decay s e r i e s i n which both α and $~" emission occur w i t h i n a reasonable time i s found i n the h e a v i e r a c t i n i d e s where:

2

5

3

-

rf*

5fer

2

4

9

«

The Cf-249 i s considered t o be s t a b l e over the time of our experiments. The growth and decay of the various isotopes i n t h i s s e r i e s , s t a r t i n g with pure Es-253, are shown i n Figure 1. Each member of t h i s s e r i e s has been prepared i n various compounds, mainly h a l i d e s and oxides, which have been c h a r a c t e r i z e d by absorption spectrophotometry and X-ray powder d i f f r a c t i o n where p o s s i b l e . The microchemical techniques f o r c a r r y i n g out these s t u d i e s have been p r e v i o u s l y reported ( 2 ) . The growth of the r e s p e c t i v e progeny species i n e i t h e r the berkelium or e i n s t e i n i u m compounds has a l s o been followed by the above techniques. Such s t u d i e s have been c a r r i e d out with i n i t i a l l y "pure" compounds, where the parent species was at as high a concentration as p o s s i ble. This was achieved by minimizing the time between i s o l a t i o n of the p a r t i c u l a r a c t i n i d e and i t s conversion t o a given compound f o r study. The conclusions d e r i v e d from the s t u d i e s of such bulk-phase compounds have been t e s t e d by changing the chemical composition a t various stages of time a f t e r the i n i t i a l isotope s e p a r a t i o n and by doping Es-253 i n t o various non-radioactive h o s t s , such as lanthanide or a l k a l i n e e a r t h h a l i d e s . The r e s u l t s of these s t u d i e s and our conclusions drawn from them w i l l be presented i n t h i s paper.


21.

YOUNG ET A L .

Results and

337

a and β~ Decay in the Solid State

Discussion

The various bulk-phase compounds that have been prepared s t u d i e d over periods of time are summarized i n Table I.


Table I.

Compound* Es 0 2

3

ESF3 ESCI3 EsBr3 ESI3

EsCl EsBr Eslo BkF BkCl

2

2

3

3

§ 3

and

A c t i n i d e Compounds Prepared f o r Time Studies

Min. Time of Study, Yrs.

Ref.

2 2 3 3 3 3 3 3 2 3

Unp 3 4 4 4 4,5 4,5 4 6 Unp

+

Compound* BkBr3

Bkl Cf 03 CfCl

Min. Time of Study, Yrs.

§

-

3

2

§

3

CfBr3 Cfl3

CfCl CfBr Cf I

3

2

2

2

CmBr3

-

-5

Ref. 7 Unp Unp Unp 8 9 10 8 9 Unp

*Isotopes used: Es-253, Bk-249, Cf-249, Cm-244 tUnpublished §2 or more s t r u c t u r e s

The references given i n Table I are those d e s c r i b i n g the prepara t i o n of a given compound; the reference may or may not contain i n f o r m a t i o n on the behavior of the compound with time. Note that the compounds have been synthesized i n d i f f e r e n t o x i d a t i o n s t a t e s and d i f f e r e n t c r y s t a l s t r u c t u r e s where p o s s i b l e . Not shown i n the t a b l e are e i n s t e i n i u m , berkelium, and c a l i f o r n i u m phosphates which have a l s o been prepared and are being s t u d i e d a t present (11). Bulk-Phase Compounds. Some of our r e s u l t s i n the studies of the bulk-phase compounds have been published (3-7). These s t u d i e s have shown that o x i d a t i o n s t a t e i s preserved f o r these a c t i n i d e s i n e i t h e r α or $~~ decay. T r i v a l e n t e i n s t e i n i u m w i l l transmute to t r i v a l e n t berkelium which transmutes to t r i v a l e n t c a l i f o r n i u m . I t has a l s o been observed that d i v a l e n t e i n s t e i n i u m y i e l d s d i v a l e n t californium. It i s i n t e r e s t i n g to note i n t h i s l a t t e r case that i t has not yet been p o s s i b l e to synthesize d i v a l e n t berkelium i n the bulk phase. Berkelium(II) has not been observed i n our aged e i n s t e i n i u m ( I I ) compounds e i t h e r , but i t would be l o g i c a l t o assume i t has been produced there. Our i n a b i l i t y to observe Bk(II) could be r e l a t e d to weak a b s o r p t i o n i n t e n s i t i e s and/or i n t e r f e r e n c e by absorption bands of e i n s t e i n i u m ( I I ) or



338

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

c a l i f o r n i u m ( I I ) . The r e s u l t s of our s t u d i e s of bulk-phase h a l i d e compounds of 2*^Bk have a l s o proven that c r y s t a l s t r u c t u r e i s preserved i n t h i s fT~ decay (_7). The spectrum of an aged o r t h o r hombic BkBr3 sample c o n t a i n i n g the οrthorhombic form of CfBr3 i s shown i n Figure 2. This form of CfBr3 has not been synthe s i z e d d i r e c t l y but was prepared by nature through the transmuta t i o n of orthorhombic BkBr3 over a 3-year p e r i o d . The absorption spectrum and X-ray d i f f r a c t i o n p a t t e r n of t h i s m a t e r i a l demon s t r a t e d that i t did indeed contain orthorhombic CfBr3 ( 7 ) . Heating t h i s compound t o 350°C converted i t t o the thermodynamically s t a b l e monoclinic form of CfBr3, the form that can be synthesized d i r e c t l y . With bulk-phase compounds our r e s u l t s suggest that l o c a l order i s a l s o preserved i n α decay. Conclusive proof of t h i s could not be obtained, however, since u s e f u l X-ray powder d i f f r a c t i o n data are d i f f i c u l t to o b t a i n from Es compounds (12). The b e l i e f that s t r u c t u r e i s maintained, a t l e a s t i n the l o c a l order or c o o r d i n a t i o n sense, i s a s u b j e c t i v e conclusion (4); i t should be remembered that long range order i n h i g h l y r a d i o a c t i v e compounds, such as 2 5 3 h a l i d e s , i s apparently destroyed by self-irradiâtion damage. L o c a l order i s not destroyed; i f i t were, i t would not be p o s s i b l e to obtain r e p r o d u c i b l e a b s o r p t i o n s p e c t r a from these compounds. Extensive data are not a v a i l a b l e f o r oxides or other types of oxyanion compounds, but nothing observed u s i n g such compounds, i n the s t u d i e s c a r r i e d out so f a r , i s i n disagreement with the conclusions described above. ES2O3 presents an i n t e r e s t i n g case i n that i t u l t i mately generates an u n i d e n t i f i a b l e form of CÎ20^; t h i s work continues (11). E s

M a t e r i a l Balance. It has been pointed out that i n our s t u d i e s o n l y t r i v a l e n t progeny grow i n t o a t r i v a l e n t parent compound, and only d i v a l e n t progeny grow i n t o a d i v a l e n t parent compound. No other species are seen. What of the q u a n t i t a t i v e nature of these observations? Is there a m a t e r i a l balance of the observable s p e c i e s ? With the i r r e g u l a r - s h a p e d samples that r e s u l t from the s y n t h e s i s , i t i s not p o s s i b l e to o b t a i n a b s o r p t i o n c o e f f i c i e n t s f o r even the reference compounds (2), l e t alone the compounds of mixed s p e c i e s . What was done, however, was to obtain the r e l a t i v e absorbance r a t i o s of s e v e r a l daughter Bk(III) and granddaughter C f ( I I I ) peaks to parent E s ( I I I ) peaks i n the s p e c t r a of e i n s t e i nium t r i h a l i d e s as a f u n c t i o n of time. These r a t i o s were compared to the r e l a t i v e absorbance r a t i o s of the same peaks i n the spectrum of a h a l i d e " f r e s h l y " synthesized from a sample of ES2O3 that had aged an i d e n t i c a l p e r i o d of time. I t was assumed i n t h i s study that a l l a c t i n i d e parent and progeny s p e c i e s , no matter what t h e i r form i n a sample of oxide, would be q u a n t i t a t i v e l y converted i n t o the r e s p e c t i v e t r i h a l i d e s and would t h e r e f o r e serve as s t a n dards to which the s t o r e d h a l i d e c o u l d be compared. Since only peak r a t i o s were compared, no absolute absorbance measurements needed t o be made, and v a r i a t i o n s i n the peak r a t i o s between the



YOUNG ET A L .

a and

β~ Decay

in the Solid

State

E L A P S E D T I M E , days

Figure 1· Growth and decay curves f o r progeny o r i g i n a t i n g from ^ ^ E s . Reproduced with permission from Ref. 2, copyright 1978, E l s e v i e r Sequoia, S.A. 2

500

WAVELENGTH (nm) 600 700 8 0 0 9 0 0

1100

llllllllllllllllllllllHlllllllllllllulllllllllllllllll

20

19 18 17 16 15 14 13 12 11 10 9 WAVE NUMBER (10 m-

Chemical and Physical Consequences of Î± and Î²- Decay in the Solid

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