Geochemical Behavior of Disposed Radioactive Waste - American

23 Characterization of Borosilicate Glass-Containing Savannah River Plant Radioactive Waste MCC-1 Tests and Durability in Geologic Repository Groundwaters NED E. BIBLER

Downloaded by UNIV LAVAL on December 15, 2015 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch023

Ε. I. du Pont de Nemours & Co., Savannah River Laboratory, Aiken, SC 29808

Results are presented from static leach tests on borosilicate glass containing high-level radioactive nuclear waste from the Savannah River Plant. Tests were performed in shielded facilities closely following MCC-1 procedures. Leachants were deionized water, MCC brine, or silicate water, all at 40°C. Normalized mass losses (g/m ) based on Cs, Sr, and Pu were calculated. Results of leach times of 3, 7, 14, 28, and 300 days are reported for deionized water. Results for 28 and 200 days are reported for silicate water and brine. Normalized mass losses and pH changes indicate that glass containing radioactive waste leaches similarly to glass containing nonradioactive, simulated waste. Release rates in the two simulated groundwaters were slightly less than in deionized water. Also, radiolysis of the leachant by alpha, beta, and gamma rays from the glass did not significantly affect the normalized mass losses or the pH changes due to leaching. Results of the long-term tests suggest that equilibrium concentrations of radionuclides will be achieved. Based on tests with different ratios of glass surface area to leachant volume, these concen trations are controlled more by solubility and surface layer effects than by the surface area of the glass. 2

137

90

238

An environmental a s s e s s m e n t ^ ) concluding that b o r o s i l i c a t e glass i s s u i t a b l e f o r the immobilization of Savannah River Plant (SRP) waste has r e c e n t l y been published by the Department of Energy. This assessment included r e s u l t s of leach t e s t s with glass c o n t a i n ing e i t h e r actual(2,3) or simulated(4) r a d i o a c t i v e waste i n leach ants that simulated groundwaters of p o s s i b l e geologic r e p o s i t o r i e s : s a l t , b a s a l t , t u f f , and g r a n i t e . In support of the environmental This is Part 3 in a series.

0097-6156/84/0246-0359S06.00/0 © 1984 American Chemical Society

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

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE


360

assessment, a d e t a i l e d report summarizing the extensive body of glass c h a r a c t e r i z a t i o n data was p r e p a r e d O ) . That report concent r a t e d on the performance of SRP b o r o s i l i c a t e glass i n generic r e p o s i t o r y environments. A major conclusion was that SRP waste glass could e f f e c t i v e l y l i m i t r a d i o n u c l i d e release as part of a waste package i n these generic r e p o s i t o r y environments. This paper presents f u r t h e r data on the leaching of SRP boros i l i c a t e glass containing a c t u a l r a d i o a c t i v e waste. Two previous papers d e s c r i b i n g t h i s glass are c u r r e n t l y being published. One deals with general methods of preparation and characterization(2). The second i s a study of d e v i t r i f i c a t i o n of the glass(j>). The r e s u l t s of that study i n d i c a t e that d e v i t r i f i c a t i o n (up to 19 v o l %) does not s i g n i f i c a n t l y a f f e c t the leach r a t e . The present paper presents the r e s u l t s of MCC-1 leach tests(7) and of leach tests i n which the leachant was p e r i o d i c a l l y changed. Leaching was at 40°C i n deionized water or i n the MCC-1 brine or s i l i c a t e s o l u t i o n s . This temperature approximates that expected at the onset of leaching i n a s a l t or g r a n i t e r e p o s i t o r y ( 5 ) . The brine simulates groundwater i n a s a l t r e p o s i t o r y . The s i l i c a t e s o l u t i o n simulates groundwater i n a hard rock r e p o s i t o r y . Normalized mass losses were c a l c u l a t e d based on the amounts of C s , 90sr, or P u a c t i v i t y leached from the g l a s s . Because o f the intense r a d i o a c t i v i t y of the glass (surface dose rate o f approximately 10 r a d / h r ) , a l l the t e s t s had to be performed remotely in shielded f a c i l i t i e s . Results i n d i c a t e that t h i s r a d i o a c t i v i t y did not a f f e c t the leaching process. A l s o , i n the long-term t e s t s (approximately 200 days), r a d i o a c t i v i t y measurements i n d i c a t e that the r a d i o n u c l i d e s are approaching e q u i l i b r i u m concentrations. /

2 3 8

4

Glass Preparation

and

Composition

The glass was prepared by feeding an aqueous s l u r r y of g l a s s forming f r i t (SRP F r i t 131) and waste (approximately 70 wt % f r i t on a dry b a s i s ) to a joule-heated melter i n the s h i e l d e d c e l l s . The waste was from SRP storage Tank 11. Major components of the waste were nonradioactive Fe, Mn, and A l r e s u l t i n g from chemical operations at SRP. These materials were h i g h l y contaminated with f i s s i o n products and transuranic r a d i o n u c l i d e s . The composition of the glass (Table I) was determined by elemental a n a l y s i s of s o l u t i o n s of d i s s o l v e d g l a s s . Determination o f the s p e c i f i c a c t i v i t y of the glass was performed on the d i s s o l v e d glass s o l u t i o n s f o l l o w i n g normal counting procedures. Sources o f r a d i o a c t i v i t y were beta from 90sr, gamma from 137cs and alpha from 238p S p e c i f i c a c t i v i t i e s (Table I) were very high. A l s o , the surface dose rate of the glass was estimated with an ion chamber to be 10 rad/hr. This was due p r i m a r i l y to 90g beta p a r t i c l e s . Q u a n t i t a t i v e metallography i n d i c a t e d that the glass contained

P u

u

u>

S r >

u

ν

Ί

· C. ,

where ( N L ) £ , i s the grams of glass leached per u n i t surface area i n t o t a l time t based on species i . V^ i s the volume of leachant, SA i s the surface area of the g l a s s , and Cj^g i s the concentration of species i i n the glass (mCi/gram g l a s s ) . C£ 1 i s the concentration of i i n the leachant (mCi/ml). This jt


o r

23.

BIBLER

Borosilicate

Glass-Containing

Waste

363

concentration was c a l c u l a t e d to include the amounts of r a d i o nuclides that had sorbed on the walls of the leach v e s s e l . In some of the leach t e s t s , the leachant was changed p e r i o d i c a l l y . The t o t a l normalized mass loss was then determined by summing the mass losses over a l l the previous time increments. These t e s t s were performed so that both short and long term data could be obtained with the same sample of g l a s s .


Leaching

Results

Tests i n Deionized Water. Results i n deionized water f o r MCC-l t e s t s up to 28 days and f o r t e s t s where the leachant was p e r i o d i c a l l y replaced up to 300 days appear i n Figure 1. For the l a t t e r type of t e s t s , the glass was p e r i o d i c a l l y t r a n s f e r r e d to a new v e s s e l containing f r e s h deionized water. The glass was not allowed to dry during these t r a n s f e r s . The e r r o r brackets i n Figure 1 i n d i c a t e the d e v i a t i o n of the r e s u l t s of d u p l i c a t e t e s t s . Only s i n g l e data points were a v a i l a b l e f o r the longer t e s t s measuring 2 3 8 p Data f o r the 3 day t e s t s f o r each r e s p e c t i v e r a d i o n u c l i d e are i n good agreement. For the 7, 14, and 28 day r e s u l t s , data f o r the MCC-l t e s t s are lower than the r e s u l t s f o r the t e s t s where the deionized water was changed at those respect i v e days. A p o s s i b l e explanation f o r t h i s i s that s a t u r a t i o n e f f e c t s i n the leachate are not as e f f e c t i v e i n the t e s t s i n which the leachate was changed. In a l l the t e s t s , most of the leaching occurs i n the f i r s t 28 days. The long-term t e s t s i n d i c a t e , e s p e c i a l l y f o r 137cs and 2 3 8 p that leaching has n e a r l y stopped. During t h i s time, leaching i s i n h i b i t e d by surface layers being formed on the glass and by leachate s a t u r a t i o n by various species from the g l a s s ( 4 , 8 ) . These r e s u l t s are c o n s i s t e n t with other long-term leach t e s t s of SRL 131 waste glass(9) and of P a c i f i c Northwest Laboratory 76-68 waste glass(8>) both of which contained nonradioactive simulated waste. On t h i s b a s i s , glasses containing a c t u a l and simulated waste leach s i m i l a r l y . Ue

Uj

28 Day Tests i n Deionized Water, S i l i c a t e Water, and B r i n e . Normalized mass losses f o r these t e s t s are shown i n Table I I . The r e s u l t s for deionized water and s i l i c a t e water are s i m i l a r with r e s u l t s at 40°C based on leaching Cs, Sr, and U from SRL 131 glass containing nonradioactive simulated waste(j)). Comparable data at 40°C f o r Cs and Sr i n brine are not a v a i l a b l e . For U i n b r i n e , a value of 0.07 g/m has been reported(£). Data i n Table I I i n d i c a t e that s i l i c a t e water i s less aggressive than deionized water i n agreement with r e s u l t s of Wicks(4) and Lokken(9). For b r i n e , the r e s u l t s i n Table I I i n d i c a t e that the l e a c h a b i l i t y of Sr i s much higher than i n the other leachants. This has been observed by Strachan(jS) and has been a t t r i b u t e d to the f a c t that the pH does not increase during leaching i n b r i n e as much as i t 2

yu


364

GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

Pu-238 RESULTS

*

•


100

·" 200

300

Sr-90 RESULTS

CO

Cs-137 RESULTS

100 200 Leach Time, days

300

Figure 1. Time dependence of normalized mass losses i n deionized water at 40°C based on Cs-137, Sr-90, and Pu-238 from SRP b o r o s i l i c a t e glass containing a c t u a l r a d i o a c t i v e waste. 0, MCC-1 t e s t s ; ·, t e s t s where leachant was replaced p e r i o d i c a l l y . E r r o r bars i n d i c a t e the p r e c i s i o n of d u p l i cate t e s t s .


23.

Borosilicate Glass-Containing Waste

BIBLER

365

does during leaching i n deionized water or s i l i c a t e water(8). Such a smaller increase i n pH f o r b r i n e was a l s o observed i n t h i s study (Table I I I ) . Another reason f o r the higher normalized mass loss f o r 9^Sr ( j possibly also ^ P u ) fc the low glass surface area to leachant volume r a t i o (SA/V) i n t h i s test compared to the tests with deionized and s i l i c a t e water. This e f f e c t of SA/V on normalized mass loss i s discussed l a t e r . The agreement of the r e s u l t s f o r normalized mass losses i n Table II i n the presence of s t a i n l e s s s t e e l baskets with r e s u l t s where s t e e l was absent,(9) suggest that the s t e e l i s not a f f e c t i n g the leaching process. Data i n d i c a t i n g t h i s have a l s o been obtained i n tests with nonradioactive g l a s s ( 4 ) . Thus, i t appears that the s t a i n l e s s s t e e l i s not a f f e c t ing the leaching chemistry. The pH changes i n these t e s t s (Table I I I ) are c o n s i s t e n t with H ions i n s o l u t i o n exchanging with N a ions i n the g l a s s . These changes are i n agreement with r e s u l t s with nonradioactive g l a s s ( 4 , 8 ) ; thus r a d i a t i o n from the glass c o n t a i n i n g a c t u a l waste does not a f f e c t the pH. 2


a n

Table I I I .

pH Changes f o r 28 Day MCC-l Leach Tests f o r Radioactive SRP 131/Tank 11 G l a s s *

Test 1 Initial Final

Leachant

0

PH Test 2 Initial Final


Deionized H2O

6.8

7.6

6.9

7.9

6.2

8.4

Brine***

6.5

6.6

6.5

**

6.5

**

7.5

8.4

7.5

6.3

7.5

9.3

Silicate

H 0*** 2

* Τ = 40°C. ** Data f o r f i n a l pH not a v a i l a b l e . *** Compositions are l i s t e d i n Table I I .



2

211

1.7 0.31 0.12 +0.03***

0.30 0.09

+0.01***

2.8 +0.7***

S

P u

0.12 +0.05***

0.18

0.83

1.4 +0.3***

Normalized Mass Loss, g/m 238 90 i

1.2

C £

2.2 +0.5***

137

* Τ = 40°C. ** Compositions are l i s t e d i n Table I I . *** Average value determined i n d u p l i c a t e t e s t s .

0.1 0.2

192

0.02 0.04

207 197

Brine**

S i l i c a t e H 0**

SA/V cm"*

Time, days

Long-Term Leach Results f o r Radioactive SRP 131/Tank 11 G l a s s *

Leachant

Table IV.



2

6.5E03

0.1 0.2

192

211

(1.3 +0.3)E04***

1.8E04

3.8E04

(2.8 +1.0)E02***

g 2.2E02

Η m

I

Η

PO

ο

δ

Χ

m

CO

> r

η

m

χ

η

Ο m Ο

00

On

3.8E02

(3.4 +0.5)E02***

Alpha

* T = 40°C, r e s u l t s f o r normalized mass losses are given i n Table IV. ** Compositions are l i s t e d i n Table I I . *** Average value determined i n d u p l i c a t e t e s t s .

S i l i c a t e H 0** (4.0 +0.DE03***

9.7E03

0.04

197

(2.9 +0.8)E04***

(9.6 +1.5)E03***

0.02

Brine**

217

Time, days

Leachate A c t i v i t y , dpm/ml 137Cs

A c t i v i t y f o r Long-Term Leach Tests of Radioactive SRP 131/Tank 11 G l a s s *

SA/V cm"*

Leachant

Leachant

Table V·



23.

BIBLER

Borosilicate Glass-Containing Waste

369

In regard to long-term geologic storage of b o r o s i l i c a t e nuclear waste g l a s s , these r e s u l t s imply that the surface area of the glass i s not the main f a c t o r that w i l l c o n t r o l the amount of r a d i o n u c l i d e s r e l e a s e d . The main f a c t o r s w i l l be the volume o f groundwater a v a i l a b l e , the s o l u b i l i t y of r a d i o n u c l i d e s i n that groundwater, and the formation of a g e l layer on the glass that i n h i b i t s d i f f u s i o n from the g l a s s . Surface area o f the glass w i l l be an important f a c t o r i n determining how f a s t these e q u i l i b r i u m concentrations w i l l be reached. Considering that the residence time of water i n an emplacement hole i n a r e p o s i t o r y w i l l probably be greater than 100 years,(5) s a t u r a t i o n of t h i s water and g e l layer formation w i l l e a s i l y be achieved. Recently, the use o f s o l u b i l i t i e s has been employed to demonstrate the e f f e c t i v e n e s s o f geologic storage f o r nuclear waste(12). pH Changes i n Long-Term T e s t s . The i n i t i a l and f i n a l pH values i n the long-term t e s t s are shown i n Table VI. Values f o r the longest test i n deionized water (day 82 to day 220 i n F i g u r e 1) are a l s o included. The values f o r deionized water and s i l i c a t e water are consistent with pH changes caused by leaching, i . e . the pH increased. The small changes observed f o r the t e s t s i n b r i n e are consistent with e a r l i e r d a t a ( 8 ) . During these long-term t e s t s , the leachant was continuously being i r r a d i a t e d by gamma rays and alpha and beta p a r t i c l e s from the g l a s s . The dose to the leachant was MO Mrad. The data i n Table VI c l e a r l y i n d i c a t e that t h i s dose i s not a f f e c t i n g the pH change during leaching by producing HNO3 from r a d i o l y s i s of the N , 0 , and H 0 present. Production of HNO3 and a lowering of the pH has been observed i n leach t e s t s where the e n t i r e system ( l e a c h a t e , g l a s s , and a i r ) was being simultaneously i r r a d i a t e d by an e x t e r n a l 60co γ-ray source(13-14). A l s o , other work has shown that r a d i o l y s i s of moist a i r produces HNO3O5), while the gamma r a d i o l y s i s of water and d i s s o l v e d a i r i n the absence of a vapor phase produces no HN0q(16). In the t e s t s described here with r a d i o a c t i v e g l a s s , the r a d i a t i o n dose to the vapor phase during the t e s t was small because of the low gamma f l u x coming from the g l a s s (~10 rads/hr based on ion chamber measure ments); thus, the amount of HNO3 formed i n t h i s manner would be small. A l s o , r a d i o l y s i s o f the leachant was p r i m a r i l y from f i s s i o n product beta p a r t i c l e s , which have the same e f f e c t as gamma r a y s ( 1 7 ) . Thus, l i t t l e HNO3 would be formed by t h i s process. Consequently, the HNO3 production was i n s u f f i c i e n t to e f f e c t the pH change due to l e a c h i n g . The above r e s u l t s and d i s c u s s i o n are i n complete accord with conclusions presented i n a recent review on t h i s subject by Burns, et a l . (18). 2

2

2

Conclusions The data presented i n t h i s paper support the f o l l o w i n g conclusions concerning the performance of SRP r a d i o a c t i v e nuclear waste glass i n deionized water and i n two simulated groundwaters.


370

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE


Table VI.

pH Changes f o r 200 Day Leach Tests of Radioactive SRP 131/Tank 11*


Leachant Deionized

H0 2

Brine** S i l i c a t e H 0** 2

PH Test 2 Initial Final

Test 3 I n i t i a l Final

6.7

8.0

6.8

8.5

-

-

6.5

5,9

6.5

6.3

6,5

5.7

7.5

8.4

7.5

8.2

7.5

8.5

* Τ = 40°C, SA/V = 0.02 ~ 0.2. ** Compositions are l i s t e d i n Table I I .


23. 1.

2. 3.

4.

BIBLER

Borosilicate

Glass-Containing

371

Waste

Leach behavior of b o r o s i l i c a t e g l a s s c o n t a i n i n g a c t u a l r a d i o a c t i v e SRP h i g h - l e v e l waste i s s i m i l a r t o g l a s s c o n t a i n i n g nonradioa c t i v e simulated waste. Release rates i n MCC b r i n e and s i l i c a t e water are lower than i n deionized water. R a d i o l y s i s of the leachants by r a d i a t i o n from the glass does not a f f e c t the leaching process nor does i t produce s u f f i c i e n t H N O 3 to cause a pH decrease. In long-term t e s t s ( 1 y r ) , the amount of P u , S r , and 137 released are a f f e c t e d more by the r a d i o n u c l i d e s o l u b i l i t y i n the leachant and layer formation on the g l a s s than by the surface area of the g l a s s . S t a i n l e s s s t e e l does not a f f e c t the leaching chemistry. 2 3 8

9 0

C s


5.

S i t e - S p e c i f i c Repository T e s t i n g Program at Savannah River Laboratory The above conclusions are a l l based on data f o r a generic r e p o s i tory system. Each of these conclusions w i l l now be tested under s i t e - s p e c i f i c c o n d i t i o n s . This i s one of the o b j e c t i v e s of the current program being developed at SRL. SRP b o r o s i l i c a t e waste glass (both r a d i o a c t i v e and simulated) w i l l be leached under cond i t i o n s expected i n three s i t e - s p e c i f i c r e p o s i t o r i e s . These are s a l t , t u f f , and b a s a l t . SRL i s c u r r e n t l y working with developers of these r e p o s i t o r i e s to estimate long-term storage c o n d i t i o n s so that r e a l i s t i c t e s t i n g parameters can be e s t a b l i s h e d . One unique feature of these t e s t s w i l l be that the a c t u a l r e p o s i t o r y rock w i l l be used f o r the primary leaching v e s s e l . Acknowledgment I wish to acknowledge D. Z. Nelson and F. A. Cheek, I I I f o r t h e i r t e c h n i c a l support and t h e i r e x c e l l e n t a b i l i t i e s on the masterslave manipulators while performing the leach t e s t s . The information contained i n t h i s a r t i c l e was developed during the course of work under Contract No. DE-AC09-76SR00001 with the U.S. Department of Energy. This paper was prepared i n connection with work done under Contract No. DE-AC09-76SR00001 with the U.S. Department of Energy. By a c ceptance of t h i s paper, the p u b l i s h e r and/or r e c i p i e n t acknowledges the U.S. Government's r i g h t to r e t a i n a nonexclusive, r o y a l t y - f r e e l i c e n s e i n and to any copyright covering t h i s paper, along with the r i g h t to reproduce and to authorize others to reproduce a l l or part of the copyrighted paper. Literature Cited 1.

Environmental Assessment. Waste Form S e l e c t i o n For SRP High-Level Waste, DOE/EA-0179, US Department of Energy, Washington, DC, 1982.


G E O C H E M I C A L BEHAVIOR O F RADIOACTIVE WASTE

372 2.

Bibler, Ν. E.; Smith, P. Kent, "Characterization of Boro silicate Glass Containing Savannah River Plant Radioactive Waste." To be published in Proceedings of the Materials Characterization Conference, August 16-18, 1982, Alfred, NY.

3.

Walker, D. D.; Wiley, J. R.; Dukes, M. D.; Leroy, J. H. Nuclear and Chemical Waste Management, 1982, 3, 91. Wicks, G. G.; Robnett, Β. M.; Rankin, W. D., "Scientific Basis for Nuclear Waste Management"; Elsevier Publishing Co.: New York, 1982, Vol. V, p. 15. Plodinec, M. J.; Wicks, G. G.; Bibler, Ν. Ε., "An Assess ment of Savannah River Borosilicate Glass in the Repository Environment"; DP-1629, Ε. I. du Pont de Nemours & Co., Savannah River Laboratory, Aiken, South Carolina, 1982. Bibler, Ν. E., "Characterization of Borosilicate Glass Containing Savannah River Plant Radioactive Waste. 2. Microstructure and Durability." To be published in the Proceedings of XIII International Congress on Glass, Hamburg, Germany, July 3-8, 1983. Nuclear Waste Materials Handbook Waste Form Test Methods, MCC-1P Static Test, DOE/T1C-11400, Pacific Northwest Laboratory, Richland, Washington, 1981. Strachan, D. Μ., in "Scientific Basis for Radioactive Waste Management," Lutze, W., Ed; Elsevier Science Publishing Co: New York, 1982; Vol. V., p. 182. Lokken, R. O.; Strachan, D. Μ., "Long-Term Leaching of SRL Glass and SYNROC"; PNL-SA-10976, Pacific Northwest Laboratory, Richland, Washington, 1983. Oversby, V. Μ., "Leach Testing of Waste Forms Interrelation ship of ISO and MCC Type Tests"; UCRL-87621, Lawrence Livermore Laboratory, Livermore, California, 1982. Pederson, L. R.; Buckwalter, C. Q.; McVay, G. L., "The Effects of Surface Area to Solution Volume on Waste Glass Leaching." PNL-SA-10306, Pacific Northwest Laboratory, Richland, Washington, 1982. Chambre, P. L.; Pigford, T. H.; Zavoshy, S., "SolubilityLimited Dissolution Rate in Groundwater." UCB-NE-4016, University of California, Berkeley, California, 1982. McVay, G. L.; Pederson, L. R. J. Amer. Cer. Soc. 1981, 64, 154. Barkatt, Α.; Barkatt, Α.; Sousanpour, W. Nature, 1982, 300,339. Jones, A. R. Radiation Research, 1959, 10, 655. Wright, J.; Linacre; J. K.; Marsh, W. R.; Bates, T. H. in Proc. Intern. Conf. Peaceful Uses Atomic Energy: UN-Geneva, 1955; Vol. 7, p. 560. Saver, M. C.; Hart, E. J.; Flynn, K. F.; Girdler, I. Ε., "A Measurement of the Hydrogen Yield in the Radiolysis of Water by Dissolved Fission Products." ANL-76-46, Argonne National Laboratory, Argonne, Illinois, 1976. Burn, W. G.; Hughes, A. E.; Marples, J. A. C.; Nelson, R. S.; Stoneham, A. M. J. Nucl. Materials 1982, 107 , 245.

4.


5.

6.

7. 8.

9.

10.

11.

12.

13. 14. 15. 16.

17.

18.

RECEIVEDOctober20,

1983


Geochemical Behavior of Disposed Radioactive Waste - American

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