bk-1979-0100.ch006

6 Single-Pass Leaching of Nuclear Melt Glass by Groundwater

Downloaded by UCSF LIB CKM RSCS MGMT on November 26, 2014 | http://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch006

DAVID G. COLES, HOMER C. WEED, DONALD D. JACKSON, and JAMES S. SCHWEIGER Nuclear Chemistry Division, L-233, Lawrence Livermore Laboratory, P.O. Box 808, Livermore, C A 94550

The work presented in this report was supported by the Department o f Energy (DOE) through the Nevada Operations O f f i c e (NVOO) as p a r t o f the R a d i o n u c l i d e M i g r a t i o n Program at the Nevada Test S i t e (NTS). The purpose of this program is to i n v e s tigate the p o t e n t i a l f o r underground m i g r a t i o n of r a d i o n u c l i d e s in groundwater from the sites of underground nuclear e x p l o s i o n s . Borg et al. have comprehensively reviewed t h i s subject and have i n c l u d e d an annotated b i b l i o g r a p h y ( 1 ) . Levy (2) discusses i n detail the specific aspects o f an underground nuclear e x p l o s i o n which are r e l e v a n t to r a d i o n u c l i d e contamination of the ground water. The technology r e q u i r e d to study the NTS groundwater problem is very s i m i l a r to that r e q u i r e d f o r studying the geo logic d i s p o s a l o f h i g h - l e v e l vitrified nuclear r e a c t o r wastes. The energy crisis o f the 1970's has emphasized the need f o r nuclear power and consequently there is c o n s i d e r a b l e i n t e r e s t i n developing an environmentally-acceptable means f o r d i s p o s i n g of the r e s u l t a n t nuclear wastes. d e M a r s i l y et al. (3) have d i s c u s s ed the feasibility f o r g e o l o g i c d i s p o s a l o f n u c l e a r wastes and have concluded that i o n exchange w i t h the rock surrounding the r e p o s i t o r y is the most important b a r r i e r between the waste and the b i o s p h e r e . The Study Group on Nuclear F u e l Cycles and Waste Management o f the American P h y s i c a l S o c i e t y (4) concluded that the management of n u c l e a r waste was well w i t h i n existing techno logy and that economic and political questions provided the g r e a t e s t problems f o r nuclear energy u s e . They a l s o s t a t e that hydrogeologic transport is the most likely route f o r contamina tion of the biosphere by a waste r e p o s i t o r y . Information on the interaction of r a d i o n u c l i d e s w i t h ground water in d e e p l y - b u r i e d , high-level, long-term "waste r e p o s i t o r i e s " is a v a i l a b l e at only a few l o c a t i o n s . One is the OKLO n a t u r a l r e a c t o r in Gabon which has f o r over 1.7 billion years r e t a i n e d some o f the r a d i o n u c l i d e s a l s o present in nuclear wastes ( 5 ) . Another is the Nevada T e s t Site, where r a d i o n u c l i d e s were first deposited underground on September 19, 1967 during the 1.7 k t 0-8412-0498-5/79/47-100-093$05.50/0 © 1979 American Chemical Society

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


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R a i n i e r e v e n t . The f i r s t o f 78 t e s t s b e n e a t h t h e r e g i o n a l w a t e r t a b l e was t h e 200 k t B i l b y e v e n t o n September 1 3 , 1 9 6 3 ; o t h e r s have b e e n d e t o n a t e d up t o May, 1976. OKLO a n d NTS a r e among t h e few a r e a s a v a i l a b l e f o r s t u d y , w h e r e r a d i o n u c l i d e c o m p o s i t i o n s have i n t e r a c t e d w i t h groundwater f o r s i g n i f i c a n t p e r i o d s o f t i m e . The c a v i t i e s f r o m t e s t s b e n e a t h t h e w a t e r t a b l e a r e a n a l o g o u s t o 78 s m a l l f l o o d e d w a s t e r e p o s i t o r i e s . The a n a l o g y b e t w e e n a NTS n u c l e a r c a v i t y and a waste r e p o s i t o r y i s p a r t i c u l a r l y c l o s e s i n c e the a c t i v i t y h a s t o b e l e a c h e d f r o m t h e p u d d l e g l a s s o r chimney d e b r i s b e f o r e e n t e r i n g t h e groundwater system and l e a c h i n g from the p u d d l e g l a s s i s s i m i l a r t o l e a c h i n g f r o m h i g h - l e v e l v i t r i f i e d r e a c t o r waste. N u c l e a r e x p l o s i o n phenomenology h a s b e e n r e v i e w e d by B o r g e t a l . ( 1 ) a n d t h e d i s t r i b u t i o n o f r a d i o n u c l i d e s w i t h i n the g l a s s and t h e r u b b l e chimney h a s b e e n d i s c u s s e d b y B o r g ( 6 ) and b y D u p u i s (7) . G e n e r a l l y , r e f r a c t o r y e l e m e n t s r e m a i n i n t h e m e l t g l a s s and v o l a t i l e e l e m e n t s condense o u t on t h e chimney rubble. A f i e l d t e s t a t NTS was r e c e n t l y u n d e r t a k e n i n o r d e r -to d e t e r m i n e t h e amount o f v a r i o u s r a d i o n u c l i d e s w h i c h h a d b e e n l e a c h e d f r o m t h e m e l t g l a s s a n d chimney r u b b l e p r o d u c e d d u r i n g the C a m b r i c e v e n t ( 8 ) . Samples w e r e o b t a i n e d a t d i f f e r e n t z o n e s w i t h i n t h e chimney and c a v i t y , a n d a n a l y z e d t o d e t e r m i n e t h e r a d i o n u c l i d e s o u r c e term a v a i l a b l e f o r p o s s i b l e groundwater t r a n s p o r t . However, l i t t l e q u a n t i t a t i v e d a t a o n t h e r a t e o f l e a c h i n g from t h e melt g l a s s r e s u l t e d . Recent q u a n t i t a t i v e l a b o r a t o r y d a t a o n t h e s t a t i c l e a c h i n g o f b o t h m e l t g l a s s and chimney r u b b l e h a v e b e e n o b t a i n e d , b y W o l f s b e r g ( 9 ) . He h a s b e e n a b l e t o do some l i m i t e d c o m p a r i s o n s b e t w e e n t h e s e l a b o r a t o r y measurements and t h e f i e l d measurements o b t a i n e d o n t h e C a m b r i c c a v i t y . I n t h e p a s t , most l e a c h i n g e x p e r i m e n t s h a v e b e e n done u s i n g a s t a t i c o r b a t c h method i n w h i c h s a m p l e s o f g l a s s w e r e immersed i n w a t e r o r w a t e r s o l u t i o n s f o r some s p e c i f i e d p e r i o d o f t i m e . The r e s u l t a n t aqueous phase was s e p a r a t e d f r o m t h e g l a s s and a n a l y z e d f o r t h e element o r r a d i o n u c l i d e o f i n t e r e s t . P a i g e (10) has d e s c r i b e d a l e a c h i n g e x p e r i m e n t i n w h i c h t h e l i q u i d i s r e c i r c u l a t e d over t h e g l a s s i n an e x t r a c t i o n - l i k e apparatus, Mendel (11) has p r e p a r e d a r e v i e w o f l e a c h i n g t e s t methodology, and t h e IAEA h a s s u g g e s t e d a s t a t i c ( b a t c h ) l e a c h i n g p r o c e d u r e as a s t a n d a r d p r o c e d u r e ( 1 2 ) . O t h e r s h a v e s t u d i e d g l a s s d u r a b i l i t y (13,14) a n d p e r f o r m e d a f i e l d t e s t on v i t r i f i e d r e a c t o r w a s t e s ( 1 5 ) . Some v e r y l i m i t e d l e a c h i n g t e s t s on n u c l e a r e x p l o s i o n m e l t g l a s s were r e v i e w e d b y B o r g e t a l . ( 1 ) . Our l e a c h i n g c o n c e p t d i f f e r s s i g n i f i c a n t l y f r o m t h o s e d i s c u s s e d p r e v i o u s l y b e c a u s e i t u s e s a dynamic f l o w - t h r o u g h s y s t e m i n w h i c h t h e s o l i d sample i s c o n t i n u a l l y e x p o s e d t o f r e s h l e a c h ing s o l u t i o n . Such a dynamic s y s t e m i s d e s i g n e d t o s i m u l a t e a c t u a l underground c o n d i t i o n s . I t i s a n o n e q u i l i b r i u m system i n w h i c h t h e d i s p l a c e m e n t from e q u i l i b r i u m can be v a r i e d by changing the f l o w r a t e . The s a m p l e s l e a c h e d i n o u r s t u d y a r e c a r e f u l l y s e l e c t e d n u c l e a r e x p l o s i o n melt g l a s s e s , chosen t o r e p r e s e n t


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t y p i c a l NTS r a d i o n u c l i d e transport sources. They are w e l l c h a r a c t e r i z e d and are c l o s e analogues to v i t r i f i e d h i g h - l e v e l r e a c t o r wastes. This i n t e r i m r e p o r t covers the r e s u l t s of the f i r s t 120 days of l e a c h i n g out of a planned 420 days. A f i n a l report w i l l be issued a f t e r the experiment i s terminated January 3, 1979.


Experimental Equipment design: The d e t a i l s of the equipment are d e s c r i b ed by Weed and Jackson (16). S p e c i a l l y - d e s i g n e d sample c e l l s r e t a i n the glass by means of 0.1 ym pore s i z e Nuclepore f i l t e r s on the i n l e t and the o u t l e t sides (see Figure 1). A schematic drawing of the system i s shown i n Figure 2. Water i s metered through the c e l l s with two p e r i s t a l t i c pumps l o c a t e d immediately upstream from the c e l l s ; each pump can d r i v e up to 10 pumping c a s s e t t e s simultaneously. Connecting l i n e s c o n s i s t of small-bore p l a s t i c tubing and Luer p r e s s - f i t p l a s t i c connectors, and a l l the r e s t of the f l u i d handling system i s made of p l a s t i c . The water i s fed by g r a v i t y from an a t m o s p h e r i c a l l y - i s o l a t e d supply carboy and i s f i l t e r e d twice before e n t e r i n g the pumps. The 20 sample c e l l s are held at 25°C i n a constant temperature bath. Tubes e x i t i n g from the tops of the c e l l s enter d i r e c t l y i n t o p l a s t i c bottles. Two c e l l s (#7 and #17) are arranged so that the flow i s downward. Sample c h a r a c t e r i z a t i o n : Gram q u a n t i t i e s of NTS nuclear explosion melt g l a s s were obtained from core samples of the c o o l ed puddles i n the bottom of the explosion c a v i t y . Samples from three separate t e s t s were obtained f o r l e a c h i n g i n t h i s e x p e r i ment. A l l three t e s t s had been detonated from 1.5 to 0.5 years p r i o r to the commencement of leaching and were chosen f o r t h e i r high f i s s i o n t o f u s i o n r a t i o i n order to provide the highest l e v e l s and greatest v a r i e t y of r a d i o n u c l i d e s i n the g l a s s . For t h i s i n t e r i m r e p o r t , only l e a c h i n g data from the most recent (Melt Glass #3) t e s t are presented here s i n c e t h i s glass cont a i n s the highest a c t i v i t y concentration and hence gives optimum a n a l y t i c a l s e n s i t i v i t y f o r r a d i o a c t i v i t y i n the leachate. Samples from i n d i v i d u a l cores were hand separated at NTS from d r i l l i n g mud and rock chunks. Samples were s e l e c t e d f o r the experiment i f t h e i r s i z e exceeded 1 cm diameter and i f they were hard, v i t r e o u s , and r a d i o a c t i v e . Each core y i e l d e d from one to eight pieces of m a t e r i a l that met these c r i t e r i a . Samples were obtained from four cores from Test 1, ten cores from Test 2, and f i v e cores from Test 3. Upon r e c e i p t of the samples i n the l a b o r a t o r y , they were cleaned with d i s t i l l e d water i n an u l t r a s o n i c cleaner aided by a small brush, a i r d r i e d , and photographed f o r l a t e r reference. It was assumed that the surface l e a c h i n g of these chunks was small compared to the surface area a v a i l a b l e f o r l e a c h i n g a f t e r the samples were ground. A r e p r e s e n t a t i v e sample from each core was


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Figure 1.

Sample holder assembly, one-pass leaching system


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Fluid flow

Air inlets

Constant level supply

Radiochemical enclosure Initial filter

Supply manifold (20 channels)

Final filters (20 channels) 2 Metering pumps (10 channels ea) Constant temperature bath Sample holders (20 channels)

Leach solution receivers (20 channels)

Figure 2.

One-pass continuous leaching system—functional diagram


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then gamma-counted on a Ge(Li) gamma-ray spectrometer i n order to see i f any core was s i g n i f i c a n t l y d i f f e r e n t from the other cores. In p a r t i c u l a r , the r a t i o of a s e l e c t e d r e f r a c t o r y element (e.g. C e ) to a v o l a t i l e element (or an element with a v o l a t i l e precursor, e.g. C s ) was c a r e f u l l y observed. In a l l cases i t was determined that each core represented melt g l a s s . A l l samples were then ground to -100 mesh and the (-100 + 325) mesh f r a c t i o n was used f o r the l e a c h i n g experiment. Thus the sample r e p r e s e n t i n g each core contained p a r t i c l e s from 43 to 147 ym diameter. That p a r t i c u l a r s i z e range was chosen so that the smallest p a r t i c l e s were much l a r g e r than the l e a c h i n g c e l l f i l t e r pores (430 times l a r g e r i n t h i s case). A l s o , they could be handled without generating r a d i o a c t i v e dust but were s t i l l small enough to r e l e a s e a d e t e c t a b l e amount of r a d i o a c t i v i t y upon leaching. A f t e r g r i n d i n g , s i e v i n g , and mixing, a 0.5 g a l i q u o t from each core composite (10-30 g t o t a l ) was gamma-counted to document the a c t i v i t y of each core f o r l a t e r c o r r e l a t i o n with the p h y s i c a l nature of the o r i g i n a l core chunks and with the x-ray d i f f r a c t i o n data. Table I l i s t s the x-ray data: the cores range from 65 to 100 percent g l a s s content. From these data and the consistency of the c o r r e l a t i o n between gamma-counting data and x-ray data i t was determined to mix a l l core samples from Test 1 to produce a Test 1 composite sample. The Test 2 composite sample and the Test 3 composite sample were prepared i n a s i m i l a r manner. A f t e r i n t e n s i v e mixing, t r i p l i c a t e 0.5 g samples from each composite sample were analyzed by gamma spectroscopy and the mean of these t r i p l i c a t e analyses was used as the p r e l e a c h g l a s s r a d i o n u c l i d e content ( Q Q ) f o r each composite t e s t sample. Radionuclides observed i n the g l a s s and the leachate are l i s t e d i n Table I I . T r i p l i c a t e a l i q u o t s were taken f o r p a r t i c l e s i z e a n a l y s i s and two of those a l i q u o t s were mixed f o r BET surface area analy s i s : r e s u l t s are i n Table I I I . The nine samples were i n d i v i d u a l l y sieved f o r s i z e d i s t r i b u t i o n . A chi-squared t e s t was performed on each t r i p l i c a t e set i n order to check the apparent e f f i c i e n c y of composite mixing. For a l l three composite samples, there was a 90 percent p r o b a b i l i t y that each of the three r e p l i c a t e s from each composite sample came from the same popula tion. The A and C samples were combined and evaluated f o r s u r face area by n i t r o g e n a d s o r p t i o n (BET). The Β samples were then subjected to scanning e l e c t r o n microscopy (SEM) a n a l y s i s . Figures 3-5 are SEM photographs of each g l a s s composite at 2000f o l d m a g n i f i c a t i o n . These photographs show conchoidal f r a c t u r i n g i n d i c a t i v e of g l a s s , as w e l l as some very f i n e p a r t i c l e s attached to the l a r g e r p a r t i c l e s . Leaching experiment: The experimental scheme i s shown i n F i g u r e 6. The v a r i a b l e s i n t h i s experiment are the three g l a s s compositions from three d i f f e r e n t t e s t s and the two flow r a t e s . The g l a s s compositions have j u s t been discussed. The l i n e a r flow r a t e s are 36 and 6.7 cm/day, corresponding to volume flow r a t e s 14l+

1 3 7


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Figure 3.

SEM photograph of Ghss (1280X)

#1

Figure 4.

SEM photograph of Ghss 4P2 (1280X)

Figure 5. SEM photograph of Ghss #3 (1280X)


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NTS Well 5B water

High flow 185 ml/day 1 2 3 4 5 6 7 8910

6 cells with melt glass #1

ééé

èèé

1 2 3

111213

Figure 6.

Low flow 34 ml/day 111213 14151617 1819 20


éééé èééé

4 5 6 7 14151617


m 8 910

m

181920

Experimental matrix—melt glass leaching experiment: ( ), flow is downward through sample; ( ), for all others flow is upward.


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Table I .

Single-Pass

X-Ray D i f f r a c t i o n A n a l y s i s o f NTS Melt Glass Samples

Melt Glass No. Core No.

Glass

1-7 1-8 1-12 1- 18

75% 75 «100 %100


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2- 5 2-7 2-8 2-9 2-10 2-11 2-12 2-13 2-14 2- 15 3- 5 3-6 3-7 3-8 3-10

Quartz

gCristobalite

5% 5 Τ Τ

T* Τ -

Feldspar 10% 10 Τ Τ

75 75 85 85 75 75 65 75 75 75

5 5 5 5 5 5 10 5 5 5

Τ Τ Τ Τ Τ Τ

5 5 Τ Τ 5 5 5 5 5 5

85 85 85 85 %100

5 5 Τ 5 Τ

Τ Τ Τ Τ

Τ Τ Τ Τ Τ

Τ = trace Table I I .

Radionuclides Observed i n the Melt Glass and Leachate Samples

not observed i n any leachate samples )

observed only s p o r a d i c a l l y i n leachate samples



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of 185 and 34 ml/day. Temperature (25°C) and water composition were kept constant. S i x samples of each g l a s s were i n c l u d e d i n order to provide t r i p l i c a t i o n at each flow r a t e . A l s o , two c e l l s c o n t a i n i n g Melt Glass #2 (#7 and #17) were arranged w i t h reverse flow ( i n l e t at the top and o u t l e t through the bottom). T h i s was done to minimize s t i r r i n g of the g l a s s as might be expected with the upward flowing c e l l s . In r e t r o s p e c t , we t h i n k these two c e l l s should probably have been f i l l e d with ground o b s i d i a n and used f o r radiochemical b l a n k s . The water used f o r the l e a c h i n g was from the NTS water w e l l 5B, chosen because i t represents a t y p i c a l NTS groundwater. The composition of t h i s water was determined by spectrochemical a n a l y s i s and i s shown i n Table IV. A 2 0 8 - l i t e r polyethylenel i n e d drum was f i l l e d with t h i s water approximately every two months and shipped to Livermore. Two-month renewal i n t e r v a l s were chosen s i n c e the l e a c h i n g apparatus consumed water at about that r a t e . The pH of t h i s water was determined at Livermore one week a f t e r c o l l e c t i o n and found to be 8.13 + 0.01. Preweighed samples (about 0.5g) were loaded i n t o each c e l l by plugging the i n l e t p o r t , p u l l i n g a vacuum on the e x i t p o r t , and pouring the ground g l a s s through a funnel i n t o the s i d e access p o r t . T h i s procedure g r e a t l y reduced the chances f o r s p i l l a g e of the powder or spreading of dust. The c e l l was then f i l l e d w i t h w e l l 5B water and put o n - l i n e by connecting i t to a previously f i l l e d i n l e t l i n e . The sampling schedule was chosen i n order to accumulate e a r l y data r a p i d l y s i n c e we expected the l a r g e s t changes i n the leach r a t e at the beginning of the experiment. A l s o , when p l o t ted on l o g paper, t h i s sequence maintained approximately equal time s e p a r a t i o n between data p o i n t s . The procedure was to c o l l e c t and composite a l l water accumu l a t e d from the s t a r t through Day 1, then through Day 2, and so on. Composite samples were c o l l e c t e d on Days 1, 2, 3, 6, 11, 32, 38, 70, and 120. Thus the composite sample c o l l e c t e d on Day 6 a l s o included leachate from Days 4 and 5; that c o l l e c t e d on Day 70 i n c l u d e d leachate from Days 39 through 69; and so on. The sample f o r Day 230 has been c o l l e c t e d but r e s u l t s from i t are not included i n t h i s d i s c u s s i o n . The l a s t composite sample (Day 420) w i l l be c o l l e c t e d on 3 January 1979. C o l l e c t i o n at Day 32 was planned f o r Day 20 but a scheduling e r r o r moved i t back 12 days. Samples were c o l l e c t e d i n polyethylene b o t t l e s and at the end of a sampling p e r i o d or when the b o t t l e s were n e a r l y f u l l , they were t r a n s f e r r e d completely i n t o a o n e - l i t e r polyethylene b o t t l e which had been l i n e d w i t h a 5 χ 10 χ 30 cm polyethylene bag. The sample i n the bag was then evaporated at 85°C. I f more than one f i l l i n g was r e q u i r e d (as was expected f o r long sampling i n t e r v a l s ) the b a g - l i n e d b o t t l e could be r e f i l l e d s e v e r a l days a f t e r the evaporation was concluded. The f i n a l composite sample, a f t e r evaporation, c o n s i s t e d of a few grams of s a l t s i n the bottom of the polyethylene bag. The bag was r o l l e d so that


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Table I I I .

Single-Pass

Surface Area (m /gm) 2


MG# LA IB 1C 1A+1C

10%

Diameter (ym) 50%

90%

50 45 45

87 84 85

136 135 136

42 45 44

80 79 81

132 133 132

41 42 42

79 78 78

133 132 132

0.396

MG# 2A 2B 2C 2A+2C

0.465

MG# 3A 3B 3C 3A+3C

Al As Β Ca Cd Co Cu Fe

103

Melt Glass Surface Area and Size A n a l y s i s

Sample

Element

Leaching

0.457

Table IV.

Composition

(yg/ml)

Element

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