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Interaction of Groundwater and Basalt Fissure Surfaces and Its Effect on the Migration of Actinides G. F. VANDEGRIFT, D. L. BOWERS, T. J. GERDING, S. M. FRIED, C. K. WILBUR, and M . G. SEITZ Argonne National Laboratory, Argonne, IL 60439

Experiments are being performed at Argonne National Laboratory (ANL) (1) to identify interactions of radionuclides and repository components that effect nuclide migration and (2) to assess changes i n nuclide migration caused by modifications expected upon aging of the waste, b a c k f i l l , and rock. This paper describes the philosophy of these experiments, the experimental set up, and some early results of this effort. The experiments are conducted with radioactive borosilicate glass, bentonite and mechanically fissured basalt rock i n flowing water analogous to their configuration in a breach of a nuclear waste repository. Changes undergone by the groundwater as i t passed through the fissure include: (1) drop in pH from 10 to 8, (2) loss of suspended particulate, and (3) loss of dissolved/suspended U, Np, and Pu. These effects, also studied as functions of radiation dose and of laboratory "aging" of the repository components, are related to the predicted long-term performance of a nuclear waste repository.

T h i s paper i s a progress r e p o r t of an experimental study underway at ANL on the i n t e r a c t i o n of flowing simulated groundwater and the components that may be used i n a nuclear waste r e p o s i t o r y constructed i n b a s a l t . The components are placed i n the water stream analogously to the c o n f i g u r a t i o n that could occur from a breach of the r e p o s i t o r y ; hence, the experiments

0097-6156/ 84/ 0246-0229S06.00/ 0 © 1984 American Chemical Society

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

GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

230

are designated as "analog experiments"* In these experiments the movement of r a d i o n u c l i d e s by the flowing water i s determined by the chemical composition of the groundwater and i n t e r a c t i o n s with components at v a r i o u s p o s i t i o n s along the flow path* Radionuclide m i g r a t i o n from a breached nuclear-waste r e p o s i t o r y by groundwater flow depends on the l e a c h i n g of r a d i o n u c l i d e s from s o l i d waste and on the chemical r e a c t i o n s that occur as a r a d i o n u c l i d e moves away from the repository* T h e r e f o r e , m i g r a t i o n involves the i n t e r a c t i o n s of leached species and the groundwater components with (1) the waste form and c a n i s t e r , (2) the engineered b a r r i e r , and (3) the g e o l o g i c m a t e r i a l s surrounding the r e p o s i t o r y * Some of these i n t e r a c t i o n s would occur i n the r a d i a t i o n and thermal f i e l d s centered on the s o l i d i f i e d waste* Rather than t r y i n g to p r e d i c t what the important i n t e r a c t i o n s are and then to study them i n d i v i d u a l l y , we consider the combination of a l l p o t e n t i a l i n t e r a c t i o n s , u s i n g these analog experiments* A schematic of the apparatus f o r an analog experiment i s shown i n F i g u r e 1* In an experiment, groundwater i s pumped through the system so that i t passes through the f i r s t v e s s e l , which contains b a s a l t c h i p s , b e n t o n i t e , and the g l a s s waste form; then through the second v e s s e l , which contains more bentonite and b a s a l t c h i p s ; and then through a narrow b a s a l t f i s s u r e i n the t h i r d vessel* F i v e experiments have been i n i t i a t e d and a s i x t h e x p e r i ment i s planned; three of these experiments have been completed* Table I l i s t s these experiments, shows how they have d i f f e r e d ,

Table I*

Experiment Number 1 2 3

Analog Experiments Completed or C u r r e n t l y Underway C o n d i t i o n of Waste Form B a c k f i l l and Rock Unaltered Unaltered Unaltered

4

"1000 y r aged"

5 6

"2000 y r aged" "1000 y r aged" bentonite and rock/unaltered waste form

Apparatus and* Experimental Condition H a s t e l l o y , HC-276 Monel-400 H a s t e l l o y , HC-276, γ field H a s t e l l o y , HC-276 H a s t e l l o y , HC-276 H a s t e l l o y , HC-276

Status (3/22/83) Completed Completed Completed Near com pletion On l i n e Planned

'Apparatus constructed of the metals i n d i c a t e d ; γ f i e l d I n d i c a t e s experiments c a r r i e d out i n the presence of a Co-60 gamma source*


14.

VANDEGRIFT ET A L .

Actiniae Migration


232


and shows t h e i r current status* Experiments 4 and 5, l a b e l e d as "1000 y r aged" and "2000 y r aged", use r a d i o a c t i v e s o l i d , b e n t o n i t e , and b a s a l t rock that have been hydrothermally modified i n the l a b o r a t o r y to hydrate the surfaces i n a s i m i l a r manner and to a degree that would occur over extended periods of time i n the r e p o s i t o r y * "Unaltered" r e f e r s to m a t e r i a l s r e c e i v i n g no hydrothermal treatment before t h e i r use i n the experiments* Experimental M a t e r i a l s The simulated groundwater used i n these experiments was prepared to represent the compositon of groundwater from W e l l DC-6, Grande Ronde Formation, which samples water p r i m a r i l y from 200 m below the Umtanum u n i t on the U . S . DOE Hanford s i t e near R i c h l a n d , WA ( 1 ) · I t s composition i s shown i n Table II* The groundwater used i n these experiments was c o n t i n u a l l y sparged w i t h N2 gas before i t entered the apparatus to reduce the d i s s o l v e d oxygen l e v e l and to l i m i t CO2 p i c k u p . B a s a l t cores were cut from rocks s u p p l i e d by B a s a l t Waste I s o l a t i o n P r o j e c t (BWIP) personnel and ground to s i z e (6.83-cm diameter by 14.60-cm long) on a l a t h e w i t h d i s t i l l e d water coolant. These rocks were Pomona-flow b a s a l t from the Pasco b a s i n i n the v i c i n i t y of the Near Surface Test F a c i l i t y , U . S . DOE Hanford s i t e . C h a r a c t e r i z a t i o n of t h i s b a s a l t can be found elsewhere ( 2 ) .

Table I I .

Recipe f o r Simulated Groundwater Prepared f o r Analog Experiments 1

Compound NaCl Na S0

4

Ν ο

0

2

Β

- 1 0

H

2 4 7 2° NaF Na C0 NaHC0 Na2S10 *9 H2O K2SO4 CaS04 2 H2O MgS04*7 H 0 HCl (1M) 2

3

3

3

e

2

2

3

Quantity Added to 20L of H 0 (g) 2

3.51 3.19 0.246 1.64 1.20 1.18 11.46 0.085 0.111 0.0036 61 mL

*The c o n c e n t r a t i o n of major c o n s t i t u e n t presented i n Table I I I .

Concentration i n m Mol/L 2.99 1.12 0.0331 1.95 0.57 0.71 2.01 0.024 0.032 7.3 χ 10" 3.05 ions i n ppm are

2

Added from a concentrated solution—1 mL to 20 L .

3

Added to reduce the pH at 2 2 ° C of the groundwater to

9.9.


4

14.

Actinide Migration

VANDEGRIFT ET A L .

233

Because these core were not permeable to water even at high p r e s s u r e , they were mechanically s p l i t to provide a path f o r water flow. To s p l i t the c o r e , a sharp, hardened s t e e l edge was pressed along the length of the c o r e , u s i n g a h y d r a u l i c p r e s s , u n t i l the cores f r a c t u r e d . D e t a i l s of the o p e r a t i o n and the mounting of b a s a l t cores i n the core holder v e s s e l can be found elsewhere ( 3 ) . The bentonite b a c k f i l l m a t e r i a l i s sodium montmorillonite from Crook County, Wyoming. A 500 g sample of t h i s m a t e r i a l , SWy-1, was obtained from the Clay M i n e r a l S o c i e t y , Source Clay M i n e r a l s R e p o s i t o r y , Department of Geology, U n i v e r s i t y of M i s s o u r i , Columbus, MO. The waste form was prepared by adding to 40 g of PNL 76-68 f r i t c o n t a i n i n g 3.4 wt % U 0 : 0.65 l i C i of Cs, 100 uCi of S r , 45 y C i of B a , 1640 y C i of C e , 2000 vtCi of E u , 500 y C i of A m , 23 mg (1400 yxCl) of P u 0 , and 115 mg (80 y C i ) of Νρθ2· * β mixture was melted i n a 95% Pt/5% Au c r u c i b l e w i t h a procedure d e t a i l e d elsewhere ( 3 ) , and cut i n t o 24 wafers with an average weight of 0.4 g and average dimensions of 1.1-cm χ 0.90-cm χ 0.13-cm. These wasteform wafers were c h a r a c t e r i z e d i n an e a r l i e r p u b l i c a t i o n ( 4 ) . M a t e r i a l s of c o n t r u c t i o n of v e s s e l s , t u b i n g , and couplings f o r each experiment were e i t h e r H a s t e l l o y HC-276 o r Monel-400. 1 3 7

3

8 5

1 3 3

1 5 2

8

1 4 1

2 4 1

2 3 9

2

2 3 7

Τ η

Experimental Apparatus Analog experiments are run i n the apparatus shown i n F i g u r e 1 by flowing simulated groundwater through the apparatus at a r a t e of 0.5 mL/hr. Based on the geometric cross s e c t i o n a l areas of the f i r s t v e s s e l and of the b a s a l t f i s s u r e , t h i s flow r a t e corresponds to a l i n e a r v e l o c i t y of ^30 m/y past the waste form and ^500 m/y through the b a s a l t f i s s u r e . The groundwater i s i n i t i a l l y a t room temperature but i s brought to the temperature of the apparatus, 9 0 ° C , i n the length of tubing l e a d i n g to the f i r s t v e s s e l . There i s an i n i t i a l " e q u i l i b r a t i o n " of the groundwater and the g e o l o g i c components of the system before the waste form i s loaded i n t o the f i r s t v e s s e l . As i s evident from data to be presented l a t e r i n t h i s paper, an e q u i l i b r a t i o n p e r i o d of ^30 days i s s u f f i c i e n t f o r the groundwater to reach a s t e a d y - s t a t e c o n d i t i o n w i t h the r e p o s i t o r y components under the c o n d i t i o n s of the f i r s t three analog experiments. T h i s steady s t a t e c o n d i t i o n , which i s c h a r a c t e r i z e d by the e x i t i n g groundwater having a composition d i f f e r e n t from i t s e n t e r i n g composition, y e t , having a constant composition w i t h time, w i l l be discussed at g r e a t e r l e n g t h i n the r e s u l t s section. During the f i r s t experiment i t was found that to maintain flow through the b a s a l t f i s s u r e i t was necessary h o l d the f i s s u r e open by p l a c i n g two s t r i p s of 0.14-mm t h i c k gold r i b b o n


234

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

along the l e n g t h of the f i s s u r e * Before t h i s was done, the pump pressure to maintain the 0*5 mL/h flow rose i n s e v e r a l days from an i n i t i a l 11 p s i g to one g r e a t e r than 150 p s i g (the maximum transducer value)* The core had become clogged by bentonite that had been transported by the flowing e l u a t e from the f i r s t and second vessels* Even w i t h the gold s t r i p s i n p l a c e , the pump pressure rose s t e a d i l y to ^130 p s i g by the end of an experiment i n a l l experiments w i t h u n a l t e r e d r e p o s i t o r y components (Table I ; 1, 2, and 3 ) · T h i s same e f f e c t was not noted f o r analog experiments using h y d r o t h e r m a l l y - a l t e r e d bentonite; no i n c r e a s e i n pressure w i t h time has been noted f o r these experiments (Table I , 4 and 5 ) · The s p e c i a l p r o p e r t i e s of bentonite c l a y ( i . e . , s m a l l p a r t i c l e s i z e and s w e l l i n g c a p a b i l i t y ) made i t impossible to l o c a l i z e the c l a y i n the apparatus i n a loose state* To maintain the bentonite i n the system without o b s t r u c t i n g the flowing water, a s p e c i a l d i s p e r s i n g system f o r bentonite was devised* T h i s system comprised of a H a s t e l l o y C-276 tube (2" long by 1/4" I . D . ) , crimped at one end, and 1 g bentonite packed i n t o the tube to ^ 1 - i n c h from the top* When the tubes were contacted w i t h groundwater, the bentonite swelled out of the tube and bentonite p a r t i c l e s c o n s t a n t l y dropped i n t o the flowing groundwater* The t h i r d analog experiment was performed i n the presence of a c o b a l t - 6 0 gamma ray source* The average dose r a t e to experimental components was 1 χ 1 0 rad/h* 5

Experiment Shutdown At the end of an experiment, the apparatus was disassembled beginning w i t h the downstream end* Flow of water was continued throughout the shutdown phase* As a part of the apparatus was removed, i t was checked f o r bentonite and r a d i o n u c l i d e content* Large samples (150 mL) of eluate were taken at the second v e s s e l o u t l e t and then the f i r s t v e s s e l o u t l e t as the shutdown continued over a s e v e r a l week period* A n a l y t i c a l Methods Analog groundwater compositions were analyzed by emission spectroscopy (emission by i n d u c t i v e l y - c o u p l e d plasma, ICP) and by i o n chromatography* Uranium concentrations were measured by l a s e r f l u o r e s c e n t spectrometry and r a d i o i s o t o p e s by gamma- and alpha-counting analyses* Because of the high d i s s o l v e d s o l i d content (832 mg/L) of the eluant groundwater, i t was necessary to perform a chemical s e p a r a t i o n of P u and N p by hexone e x t r a c t i o n * These procedures are d e s c r i b e d elsewhere ( 5 ) . A c i d - b a s e / p H t i t r a t i o n s were performed on the simulated ground water eluant samples to e s t a b l i s h the carbonate plus bicarbonate concentrations* The i n i t i a l pH value of samples was used to 2 3 9

2 3 7


14.

VANDEGRIFT ET A L .

Actinide Migration

235

e s t a b l i s h the concentrations of [ Η β β Ι Ο ^ " ] , [Η^ΒΟβ"] and [HCO3"] at 2 5 ° C . T h i s procedures are a l s o described i n d e t a i l elsewhere (6). Hydro-, T h e r m o - A l t e r a t i o n of Components The f i s s u r e d b a s a l t cores and bentonite were a l t e r e d by p l a c i n g them i n an autoclave under simulated groundwater at 3 2 0 ° C f o r 30 and 60 days to simulate 1000 y and 2000 y a g i n g , r e s p e c t i v e l y * The waste-form wafers f o r both experiments were aged i n the same manner by t r e a t i n g them f o r 17 days i n saturated steam at 340°C* The d e t a i l s of these procedures and the r a t i o n a l e f o r t h e i r use have been published p r e v i o u s l y ( 7 ) . The e f f e c t s of saturated steam on b o r o s i l i c a t e g l a s s were discussed i n a recent p u b l i c a t i o n ( 8 ) . Results As the simulated groundwater passes through the three v e s s e l s of the analog expriment ( F i g u r e 1 ) , i t undergoes compositional changes which appear to a f f e c t i t s a b i l i t y to t r a n s p o r t a c t i n i d e ions through the system* The f o l l o w i n g two s e c t i o n s d e s c r i b e s e p a r a t e l y (1) the changes i n groundwater composition and (2) the a c t i n i d e m i g r a t i o n i n the rock core f i s s u r e * Following these r e s u l t s i s the d i s c u s s i o n s e c t i o n , which b r i e f l y attempts to t i e these two phenomena together* Changes i n Analog Groundwater Composition* The data i n F i g u r e 2 for the f i r s t three analog experiments (which used u n a l t e r e d waste form, b e n t o n i t e , and b a s a l t cores) show that the groundwater e x i t i n g the b a s a l t f i s s u r e reaches a s t e a d y - s t a t e composition for [Na ] and [ C a ] soon a f t e r the s t a r t of the experiment. (There are three symbols f o r the three separate experiments, experiments 1, 2, and 3 i n Table I , that are e i t h e r empty or solid. The term "cold" i n the f i g u r e legend r e f e r s to the e q u i l i b r a t i o n p e r i o d of the experiments, before the waste form was put i n t o v e s s e l 1. "Hot" r e f e r s to that p a r t of the experiment where the waste form was i n p l a c e . ) The steadys t a t e c o n c e n t r a t i o n of sodium i o n i n the groundwater eluant at the c o r e ' s e x i t i s ^2/3 of i t s o r i g i n a l groundwater concen tration. The c a l c i u m - i o n c o n c e n t r a t i o n i s higher by almost an order of magnitude at the s t e a d y - s t a t e c o n d i t i o n reached i n s i d e the core than at i t s o r i g i n a l c o n c e n t r a t i o n . An a u x i l i a r y experiment, performed at 9 0 ° C w i t h ground water and a column of crushed b a s a l t c h i p s and w i t h a b a s a l t surface area to groundwater volume r a t i o equivalent to that of the analog b a s a l t f i s s u r e (2 χ 1 0 c m " ) , has e l u c i d a t e d t h i s phenomenon. Data from t h i s experiment, which are presented i n F i g u r e 3, show that the sodium-ion c o n c e n t r a t i o n of the ground water does not decrease g r a d u a l l y from i t s i n i t i a l value of +

2 +

2

1



O ANALOG 1 COLD • ANALOG 2 COLD Ο ANALOG 3 COLD • ANALOG 1 HOT • ANALOG 2 HOT • ANALOG 3 HOT 2-

INITIAL CONCENTRATION

Ο

•

200

F i g u r e 2 . Attainment of s t e a d y - s t a t e c o n c e n t r a t i o n of sodium and c a l c i u m ions i n the f i r s t three analog experiments*


VANDEGRIFT ET A L .

Actinide Migration

F i g u r e 3. Attainment of s t e a d y - s t a t e of sodium and calcium i n an a u x i l i a r y

concentration experiment.


238


13.7 meq/L to i t s s t e a d y - s t a t e value of 13 meq/L, b u t , r a t h e r , i t i n i t i a l l y f a l l s to a much lower c o n c e n t r a t i o n , ^2 meq/L. G r a d u a l l y , over an - é q u i l i b r a t i o n - p e r i o d , the sodium-ion c o n c e n t r a t i o n r i s e s to i t s s t e a d y - s t a t e v a l u e . The analog experimental d a t a , F i g u r e 2, show a s t e a d y - s t a t e [Na ] of only 9.6 meq/L. T h i s d i f f e r e n c e may be a f u n c t i o n of a s l i g h t l y d i f f e r e n t surface area to volume r a t i o f o r the two experiments, o r , perhaps, to a bentonite p e r t u r b a t i o n of the b a s a l t / g r o u n d water i n t e r a c t i o n . The c a l c i u m - i o n c o n c e n t r a t i o n shows an o p p o s i t e , yet s i m i l a r , behavior. Calcium i o n ' s i n i t i a l groundwater c o n c e n t r a t i o n of 0.08 meq/L increases d r a m a t i c a l l y to 15 meq/L before g r a d u a l l y dropping t o a s t e a d y - s t a t e concent r a t i o n of 0.4 meq/L. Table I I I shows ICP, i o n chromatography, and uranium c o n c e n t r a t i o n data f o r o u t l e t groundwaters from the f i r s t analog experiment. The data f o r the second and t h i r d analog experiments show the same trends as those seen i n Table I I I . In g e n e r a l , a l k a l i n e - e a r t h elements show the same behavior as does c a l c i u m ; an i n i t i a l very high c o n c e n t r a t i o n g r a d u a l l y f a l l s to a s t e a d y - s t a t e c o n c e n t r a t i o n much higher than that of the i n i t i a l groundwater. Some p r e l i m i n a r y data f o r potassium i o n (not shown i n Table I I I ) shows that i t a l s o follows the trend of c a l c i u m i o n . The v a r i a t i o n i n anion concentrations i s small i n comparison to that of c a t i o n i c s p e c i e s . Table IV shows the major c a t i o n i c and a n i o n i c c o n s t i t uents i n the second analog experiment groundwater i n (1) i t s i n i t i a l c o n d i t i o n , (2) i t s c o n d i t i o n at s t e a d y - s t a t e on e x i t i n g the second v e s s e l , and (3) i t s c o n d i t i o n at s t e a d y - s t a t e on e x i t i n g the u n a l t e r e d b a s a l t f i s s u r e . Quite apparent from these data i s the s m a l l change i n groundwater composition caused by i n t e r a c t i o n of the groundwater w i t h the bentonite and waste form, when compared to that of the e f f e c t of groundwater i n t e r a c t i o n w i t h u n a l t e r e d b a s a l t . The major d i f f e r e n c e i n groundwater a f t e r i t passed through the two b e n t o n i t e c o n t a i n i n g v e s s e l s was a s l i g h t l y lower pH and sodium-ion concentration. (Lower concentrations of R^SiO^", H2BO3"", and CO3 "" a r e a l l a t t r i b u t a b l e to the lower pH v a l u e . ) Passage of the groundwater through the b a s a l t f i s s u r e g r e a t l y modified i t s c a t i o n i c content and i t s pH ( t h i r d column). A g a i n , there was no p e r c e p t i b l e change i n the t o t a l SO4 "", F ~ , C I " , s i l i c a t e , or borate c o n c e n t r a t i o n . The charge balance i s accomplished by the decrease i n pH (10 to 8) that occurs i n the rock f i s s u r e . T h i s decrease i n pH decreased the negative ions i n s o l u t i o n by n e u t r a l i z i n g much of the R^SiO^", H2BO3", and CO3 " i n the groundwater. The t o t a l (carbonate + bicarbonate) a l s o decreased by ^40% from passage through the rock c o r e . Thus f a r we have discussed only the i n t e r a c t i o n of analog groundwater w i t h u n a l t e r e d bentonite and b a s a l t . Table V , which contains data from the 1000 y analog experiment (Exp. 4, Table I ) , shows the q u i t e d i f f e r e n t , and q u i t e l i m i t e d , e f f e c t +

2

2

2



collected*

1.30 1.37 1.43 1.42 1.50 1.58 1.53 1.46 1.51 1.46 1.46 1.38 1.38 1.38 ND

Original Groundwater 1--10 24--31 31--38 1--3 6--9 13--17 27--29 37--45 52--59 66--73 78--85 98--105 105--114 114- -120

2

B 2

1

Major Constituent Concentrations of Groundwater Samples which E x i t e d the F i r s t Analog Experiment

Collection Time, days

Reported i n mg/L*

ND - no data

Conditioning Conditioning Conditioning Radioactive Radioactive Radioactive Radioactive Radioactive Radioactive Radioactive Radioactive Radioactive Radioactive Radioactive

Sampling Period

Table I I I *


1.82

1.58

4.84

4.51

2

2.19

2.43

2.29

nil

0.04

0.08

OH"

0.02

0.67

1.10

H3Si0

9.6

12.3

13.7

Na+

4

2 +

0.01

0.08

0.10

2

H B0

0.63

0.018

0.075

Ca

3

2 +

nil

0.82

0.96

CO,.

0.20

nil

nil

Mg

1.33

1.05

1.33

HCO3

10.4

12.3

13.8

9.6

11.8

12.4

£ Anions

ΣCations

Concentrations of OH", R^SiO^", H2BO3", CO3 , and HCO3" and pH are l i s t e d f o r s o l u t i o n s at 2 5 ° C . These would, of course, change d r a m a t i c a l l y at 9 0 ° C ; where R^SiO^ " i s a l s o an important a n i o n i c s p e c i e s .

1.84

SO.

7.8

Core O u t l e t 'VlOO Days

F"

9.6

2nd V e s s e l O u t l e t M.50 Days

4.65

Cl"

9.9

pH

1

Major Constituents of the Second Analog Experiment Groundwater S o l u t i o n i n meq/L (Unaltered B a s a l t )

I n i t i a l Groundwater

Solution

Table I V ·

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984. 2

0.04

2.7

1.9

5.8

0.80

0.79

1.56

H^SiO^

13.6

9.5

0.08

0.07

0.11

H2BO3

0.061

0.19

0.52

0.50

0.86

CO.."

1.4

0.3

2.00

2.03

0.82

HCO3

15.0

10.0

13.9

10.8

13.8

Σ Anions

13.8

0.060

13.7

0.05

Σ Cations

2

Ca +

+

Na

Concentrations of OH", 038104", H2BO3", CO3 , and HCO3" and pH are l i s t e d f o r s o l u t i o n s at 2 5 ° C . These would, of c o u r s e , change d r a m a t i c a l l y at 9 0 ° C ; where H2S104 " i s a l s o an important a n i o n i c s p e c i e s .

0.04

1.9

1.4

4.1

0.10

1.9

2.4

OH"

9.5

Core O u t l e t ^90 Days

SO "

9.5

2nd V e s s e l O u t l e t ^10 Days

F~

9.9

Groundwater

6.0

Cl"

pH

1

Major C o n s t i t u e n t s of the F o u r t h Analog Experiment Groundwater S o l u t i o n s i n meq/L ( A l t e r e d B a s a l t )

Solution

Table V .

242


on groundwater composition of contact by the h y d r o t h e r m a l l y altered basalt f i s s u r e . A comparison of the f i r s t and t h i r d data rows of Table V ( o r i g i n a l analog goundwater v s . that e x i t i n g the rock core ^90 days i n t o the experiment^ shows only l i m i t e d compositional changes i n the two s o l u t i o n s . The s t e a d y - s t a t e compositions of major groundwater c o n s t i t u e n t s i n the e x i t i n g groundwater are f a r c l o s e r to those of the o r i g i n a l groundwater than those noted f o r f r e s h b a s a l t experiments. The major d i f f e r e n c e s that d i d occur i n the f o u r t h analog experiment were s u b s t a n t i a l increases i n the c o n c e n t r a t i o n of potassium i o n , and the t o t a l of carbonate plus b i c a r b o n a t e . The decreases seen f o r R^SiO^" and H2BO3" are due to t h e i r p a r t i a l n e u t r a l i z a t i o n at the lower pH of the e x i t i n g groundwater. The data f o r the second v e s s e l o u t l e t i n Table V seem to show that groundwater composition i s more a f f e c t e d by aged bentonite than by i t s u n a l t e r e d form. The sodium i o n concen t r a t i o n dropped d r a m a t i c a l l y as calcium i o n and potassium i o n both r o s e . A n i o n i c species concentrations a l s o dropped. These d a t a , however, were c o l l e c t e d e a r l y i n the experiment ( i . e . , a f t e r t e n days) and are l i k e l y to be not r e p r e s e n t a t i v e of the s t e a d y - s t a t e c o n d i t i o n of the system. The s t e a d y - s t a t e composition of the second v e s s e l o u t l e t s o l u t i o n w i l l be measured, when the rock core i s taken o f f - l i n e at the end of the experiment. The d i f f e r e n c e s i n the sums of the a n i o n i c and the c a t i o n i c charges i n the data presented i n Tables IV and V are w i t h i n the experimental e r r o r l i m i t s of the a n a l y t i c a l methods (ICP, i o n chromatography, and a c i d / b a s e t i t r a t i o n and pH measurement) used to measure the concentrations of these s p e c i e s . A c t i n i d e M i g r a t i o n i n the Rock Core Table VI shows the concentrations of plutonium, neptunium, and uranium measured at the i n l e t and o u t l e t of the u n a l t e r e d and h y d r o t h e r m a l l y - a l t e r e d b a s a l t core f i s s u r e s i n the f i r s t f i v e analog experiments (see Table I ) . Under c o n d i t i o n s s i m u l a t i n g a r e p o s i t o r y that was u n a l t e r e d by groundwater i n t e r a c t i o n (Table I , Exp 1-3), both Np and P u , i n the concen t r a t i o n s developed i n these analog experiments from the l e a c h i n g of the waste form, were s u b s t a n t i a l l y retarded w i t h i n the 14.6-cm b a s a l t f i s s u r e . In f a c t , as can be seen from F i g u r e 4, almost a l l of " ' N p a c t i v i t y was sorbed on the f i r s t o n e - t h i r d of the rock f i s s u r e . The data i n F i g u r e 4 have an estimated e r r o r , based on counting s t a t i s t i c s above, of approximately 2 counts per 1000 seconds. Uranium r e t a r d a t i o n was determined to be not as complete. By f i l t e r i n g groundwater samples from the f i r s t and second v e s s e l o u t l e t s through 0.1 μm f i l t e r s , i t was determined that the neptunium was i n a s o l u b l e form, and that most of the plutonium was a s s o c i a t e d w i t h f i l t e r a b l e bentonite p a r t i c l e s


Actinide Migration

VANDEGRIFT ET AL.

ANALOG - I EXPERIMENT DURATION 120 DAYS

60

40

»

20

ο ο

k

ANALOG - 2 EXPERIMENT DURATION 150 DAYS

c a ο °

40

>

20

o
OOAr>r>r>n 12

16

D I S T A N C E A L O N G ROCK F A C E , cm

F i g u r e 4. fissures*

2 3

^ N p s o r p t i o n on u n a l t e r e d b a s a l t core



244

Table V I ·

S o r p t i o n of A c t i n i d e s by B a s a l t F i s s u r e s

Experiment 1

inlet outlet

0.4

Geochemical Behavior of Disposed Radioactive ... - ACS Publications

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