Nuclide Migration in Fractured or Porous Rock - ACS Symposium

10 Nuclide Migration in Fractured or Porous Rock P. G. RICKERT, R. G. STRICKERT, and M. G. SEITZ 1

Downloaded by FUDAN UNIV on February 26, 2017 | http://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch010

Argonne National Laboratory, Argonne, IL 60439

R e p o s i t o r i e s built i n geologic bodies are being considered f o r permanent d i s p o s a l of nuclear wastes. In the context o f deep geologic r e p o s i t o r i e s , the rock formation itself may be considered as part o f the mechanism r e t a i n i n g the wastes w i t h i n the immediate neighborhood of the r e p o s i t o r y . The i n t e r a c t i o n s of nuclear wastes with geologic m a t e r i a l s are being studied i n l a b o r a t o r y experiments to evaluate geologic media as b a r r i e r s to n u c l i d e migrat i o n . M i g r a t i o n caused by flowing water is considered the most c r e d i b l e mechanism f o r moving wastes from a r e p o s i t o r y i n a w e l l chosen site. The extent and r a t e of migration o f wastes under the i n f l u e n c e o f groundwater, o f course, depend on s e v e r a l parameters; i n c l u d i n g the type and extent of the chemical r e a c t i o n of the r a d i o n u c l i d e s with the geologic media. In previous work (1,2,3) it was found that the k i n e t i c s o f s o r p t i o n was an important parameter a f f e c t i n g the migration of nuc l i d e s i n geologic media. For example, i n experiments designed to measure the k i n e t i c s o f r e a c t i o n f o r r a d i o n u c l i d e s i n s o l u t i o n with t a b l e t s o f rock, it was found that periods from s e v e r a l minutes t o s e v e r a l hours were r e q u i r e d f o r the r a d i o n u c l i d e s to reach steady s t a t e concentrations on the rock t a b l e t s and in the s o l u t i o n s . Figure 1 shows the r e a c t i o n curves found f o r the s o r p t i o n of plutonium and americium from solution by a tablet of granite. The r e a c t i o n r a t e s f o r the s o r p t i o n o f plutonium and americium from s o l u t i o n are not the same, and both r e q u i r e a number of hours to reach steady s t a t e c o n c e n t r a t i o n s . The observation that the s o r p t i o n process progresses as a f u n c t i o n of time i m p l i e s that under c o n d i t i o n s of s o l u t i o n flow through geologic media, there may not be instantaneous o r local e q u i l i b r i u m between a n u c l i d e bearing s o l u t i o n and geologic media, but r a t h e r , the n u c l i d e would be sorbed from s o l u t i o n through a length o r zone of the media. Following adsorption, the n u c l i d e would desorb a f t e r an amount of water passed over the media. The n u c l i d e would again be readsorbed downstream a d i s t a n c e dependent 1

Current address: Battelle Pacific Northwest Laboratories, Richland, Washington 99352.

0-8412-0498-5/79/47-100-167$06.00/0 © 1979 American Chemical Society

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

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upon the r a t e of adsorption and the flow r a t e of the s o l u t i o n . Desorption and readsorption would continue i n t h i s manner and the migration of the n u c l i d e would be defined i n terms of the r a t e of adsorption, the r a t e of desorption, and the e q u i l i b r i u m d i s t r i b u t i o n of the n u c l i d e between s o l u t i o n and geologic media. To quantify t h i s treatment of migration as i n f l u e n c e d by k i n e t i c s , a model has been developed i n which instantaneous or l o c a l e q u i l i b r i u m i s not assumed. The model i s c a l l e d the Argonne D i s p e r s i o n Code (ARDISC) (_4) . In the model, adsorption and de s o r p t i o n are treated independently and the r a t e s f o r adsorption and desorption are taken i n t o account. The model t r e a t s one d i mensional flow and assumes a constant v e l o c i t y of s o l u t i o n through a uniform homogeneous media. This paper describes an experimental study of the a p p l i c a b i l i t y of the ARDISC model to l a b o r a t o r y studies of n u c l i d e mi g r a t i o n i n geologic media. Basic D e s c r i p t i o n of Model The model assumes a one dimensional column d i v i d e d i n t o an a r b i t r a r y number (LEND) of u n i t s or zones. Each zone can sorb a given species such that at e q u i l i b r i u m the d i s t r i b u t i o n of the species between the surface of the zone and the volume of s o l u t i o n ( i n i t i a l l y at u n i t concentration of the species) i s

α/3 where a = amount of species sorbed on the and

(1)

surface

3 = 1 - α If a f t e r t h i s d i s t r i b u t i o n i s e s t a b l i s h e d , a second c y c l e occurs i n which the s o l u t i o n from the i n i t i a l zone i s t r a n s f e r r e d to a second zone with the same c h a r a c t e r i s t i c s as the f i r s t zone and a f r e s h s o l u t i o n c o n t a i n i n g none of the species i s placed i n the f i r s t zone, another e q u i l i b r a t i o n , t h i s time i n both zones w i l l occur. This process i s continued by the model f o r a d d i t i o n a l c y c l e s and zones. The d e s c r i p t i o n of the model up to t h i s point i s analogous to a d e s c r i p t i o n of a chromatographic model which incorporates the concept of the t h e o r e t i c a l p l a t e (5). I f , however, the movement from zone to zone i s f a s t enough that e q u i l i b r i u m does not occur, the f r a c t i o n sorbed from s o l u t i o n can be c a l c u l a t e d i f the r a t e of s o r p t i o n i s known (or can be estimated). A parameter, F, i s therefore defined p _ f r a c t i o n of species sorbed from s o i n , during one c y c l e time f r a c t i o n of species sorbed from s o i n , at e q u i l i b r i u m which can be r e l a t e d to the r a t e of adsorption

of the

species.


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The parameter, F, gives d i r e c t l y the amount adsorbed i n a zone i n which the species i n i t i a l l y i s present only i n the s o l u t i o n . For any species adsorbed on the surface o f a zone, the f r a c t i o n of the sorbed species which desorbs during the time of a c y c l e can be measured (or estimated). Another parameter, G, i s therefore defined


^ f r a c t i o n of species desorbed from surface during 1 c y c l e t i m e l y f r a c t i o n of species desorbed from surface at e q u i l i b r i u m to account f o r desorption under non-equilibrium c o n d i t i o n s . The parameter, G, gives d i r e c t l y the amount desorbed i n a zone where the species i s i n i t i a l l y present only on the s u r f a c e of a zone. Thus f o r each zone, during a given c y c l e , the adsorptiondesorption process i s separated i n t o two d i s t i n c t events with F or G d e s c r i b i n g the k i n e t i c s o f each event. Such an approach i s of course v a l i d only f o r f i r s t order r a t e r e a c t i o n s . In the l i m i t of low concentration, (such as that r e s u l t i n g from slow l e a c h i n g from a r e p o s i t o r y ) the r e a c t i o n s i t e s on the rock w i l l not approach s a t u r a t i o n and the number of r e a c t i o n s i t e s can be considered to remain constant during adsorption. Therefore, f o r a s i n g l e species i n s o l u t i o n at t r a c e r concentrations the reac t i o n should approximate a f i r s t order r e a c t i o n ( i . e . , where no complications such as concentration e f f e c t s , step-wise dehydra t i o n , d i s s o c i a t i o n , e t c . , are p r e s e n t ) . I f a c a l c u l a t e d c y c l e i s defined as a p a r t i a l e q u i l i b r a t i o n between the species on the surface of a zone and the species i n the s o l u t i o n of a zone, and i s followed by an instantaneous t r a n s f e r of the s o l u t i o n to the next zone, the f o l l o w i n g r e c u r s i v e formulae apply: R(£,Z) = GaR(£-l,Z) + (1-G)R(£-1,Z) + FaS(£-l,Z)

(4)

S(£,Z+1) = FgS(£-l,Z) + (l-F)Sa-l,Z) + G3R(^-1,Z)

(5)

or rearranging R(£,Z) = [Ga+l-G]R(£-l,Z) + FaS(il-l,Z)

(6)

S(£,Z+1) = [Fg+l-F]S(£-l,Z) + GgR(£-l,Z)

(7)

where R(£,Z) = amount of species sorbed on the surface i n the Zth zone a f t e r the £th c y c l e . S(£,Z+1) = amount of species i n s o l u t i o n i n the (Z+l)th zone a f t e r the £th c y c l e . ( the s o l u t i o n i n the Zth zone i s t r a n s f e r r e d to the (Z+l)th zone at the end of the £th c y c l e . ) Thus i f a, F, and G are known and i f I and Ζ are s p e c i f i e d , the d i s t r i b u t i o n o f the species both i n s o l u t i o n and on the media can be c a l c u l a t e d by the model. For equations 6 and 7, one must have


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an i n i t i a l amount of species

GEOLOGIC STORAGE

present.


Experimental I f the r a t e of adsorption, the r a t e of desorption, and the e q u i l i b r i u m p a r t i t i o n i n g of a n u c l i d e between a s o l i d medium and s o l u t i o n are known, then the r a t e of migration of a n u c l i d e through the medium can be p r e d i c t e d with the ARDISC model. The r a t e of a d s o r p t i o n and the r a t e of desorption are assumed to be dependent on the geometric r e l a t i o n s h i p of the rock and sol u t i o n ( i . e . , dependent upon the volume of s o l u t i o n and on the shape and area of the s o l i d medium i n contact with the s o l u t i o n ) . Because the dependence of s o r p t i o n k i n e t i c s on the geometric r e l a t i o n s h i p i s not known, the r a t e s f o r s o r p t i o n are determined by experiment f o r the p a r t i c u l a r geometry (surface area of rock to volume of s o l u t i o n ) f o r which the p r e d i c t i o n of n u c l i d e migration is desired. The a p p l i c a b i l i t y of the ARDISC model to l a b o r a t o r y migration experiments was studied i n two sets of experiments. In the f i r s t set of experiments, the r a t e of adsorption, the r a t e of desorpt i o n , and the e q u i l i b r i u m p a r t i t i o n i n g of americium I I I between s o l u t i o n and simulated f i s s u r e s were measured i n s t a t i c and batch experiments. The measured parameters were used i n the model to p r e d i c t the m i g r a t i o n of americium I I I through simulated f i s s u r e s . The p r e d i c t i o n s were compared with experimental r e s u l t s . In the second set of experiments, strontium was eluted through a column of g l a u c o n i t e at s i x s o l u t i o n flow r a t e s ranging from 14.4 to 721 centimeters per hour. To o b t a i n input data f o r the ARDISC model the e q u i l i b r i u m p a r t i t i o n i n g of strontium between s o l u t i o n and g l a u c o n i t e was determined from the r e s u l t i n g curves and the r a t e for adsorption and the r a t e f o r desorption were determined by curve f i t t i n g the r e s u l t s of one of the column i n f i l t r a t i o n experiments. The parameters that were determined i n t h i s way were used i n the model to p r e d i c t the migration of strontium through the column of g l a u c o n i t e f o r four of the s o l u t i o n flow r a t e s . The p r e d i c t i o n s were compared with experimental r e s u l t s . Fissures F i s s u r e s were f a b r i c a t e d by c u t t i n g slabs of rock from a core of gray hornblende s c h i s t . X-ray d i f f r a c t i o n a n a l y s i s of gray hornblende s c h i s t i d e n t i f i e d c h l o r i t e and an amphibole (hornblende) as the major mineral phases present and f e l d s p a r as a minor c o n s t i t u e n t . The s l a b s were cut i n t o r e c t a n g l e s approximately 2.54 cm wide by 5.08 cm long. Each piece was p o l i s h e d on one s i d e with s u c c e s s i v e l y f i n e r s i z e d abrasive with the f i n a l a b r a s i v e s i z e being 600 mesh. To form a f i s s u r e , two rectangular pieces were put together. Wax gaskets were placed along the long edges of the rectangular pieces to form a 0.027 cm gap between


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the two p i e c e s . The surface area to volume r a t i o produced was 74 cm to 1 mL and the surface area to volume r a t i o was constant f o r a l l the f i s s u r e s used.


2

F i s s u r e Adsorption Experiments. To perform f i s s u r e s o r p t i o n experiments, one end of each f i s s u r e was connected v i a small bore tubing to a m i c r o l i t e r p i p e t t e r as i s shown i n Figure 2. Rocke q u i l i b r a t e d water, described below, was i n j e c t e d i n t o each f i s sure and the f i s s u r e w a l l s were allowed to e q u i l i b r a t e with the s o l u t i o n f o r a p e r i o d of two days before the a d s o r p t i o n e x p e r i ments were performed. Rock-équilibrated water was produced by a l l o w i n g d i s t i l l e d water to be i n contact w i t h granulated gray hornblende s c h i s t f o r a period of s e v e r a l days. The s o l u t i o n produced was f i l t e r e d through a 0.4 \im NUCLEPORE f i l t e r before use. A n a l y s i s of the r e s u l t i n g s o l u t i o n showed that rock e q u i l i b r a t e d - w a t e r contained 48 mg/L t o t a l s o l i d s (dried at 180°C). The E^ of the s o l u t i o n was measured to be 0.275 V. The pH of the gray hornblende s c h i s t e q u i l i b r a t e d water was measured to be 7.6 at the time of the f i r s t a d s o r p t i o n experiment. Thereafter the s o l u t i o n pH was maintained at 7.5-7.7. Adsorption experiments were performed by removing rocke q u i l i b r a t e d water from the f i s s u r e s and i n j e c t i n g stock s o l u t i o n which was made by d i s s o l v i n g t r a c e r amounts of americium-241 i n rock e q u i l i b r a t e d water. The stock s o l u t i o n was allowed to e q u i l i b r a t e w i t h i n the f i s s u r e s f o r d i f f e r e n t p e r i o d s of time and was then removed from each f i s s u r e . The stock s o l u t i o n was assayed before i n j e c t i o n and a f t e r removal from the f i s s u r e s so that the change i n americium c o n c e n t r a t i o n was determined f o r a d i f f e r e n t time i n each f i s s u r e . Ten a d s o r p t i o n experiments were performed i n t h i s manner and the r e s u l t s are presented i n t a b l e I . F i g u r e 3 i s a g r a p h i c a l r e p r e s e n t a t i o n of the i n i t i a l part of the adsorpt i o n curve. E q u i l i b r i u m P a r t i t i o n i n g Experiments. The e q u i l i b r i u m p a r t i t i o n i n g of americium-III between gray hornblende s c h i s t and rock e q u i l i b r a t e d water was determined i n batch p a r t i t i o n i n g e x p e r i ments w i t h r e c t a n g u l a r blocks of gray hornblende s c h i s t ( 5 ) . The surface area s o r p t i o n c o e f f i c i e n t , K , was determined to be 4.5 ± .5 mL/cm where g

2

Κ s α from tion 3 from tion

— volume of s o l u t i o n β surface area of sample

,gv

equation 1 i s the f r a c t i o n of n u c l i d e concentra adsorbed from s o l u t i o n . equation 1 i s the f r a c t i o n of n u c l i d e concentra remaining i n s o l u t i o n ; 1 - a.

Solving equation 8 f o r the geometric surface area to volume r a t i o



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ADSORPTION TIME, hr Figure 1. Sorption of plutonium IV and americium III from solution by a tablet of granite. Experiment No. 16; granite LI-6152; (•), Pu; (O), Am. 237

241

FISSURE WITH WAX GASKET -ROCK FISSURE

MICROLITER PIPETTE

-MICROBORE TUBING

Figure 2.

Apparatus used in static fissure adsorption experiments


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found i n the f i s s u r e s , the f r a c t i o n of the n u c l i d e concentration p r e d i c t e d to be adsorbed by the f i s s u r e surfaces at e q u i l i b r i u m i n f i s s u r e s o r p t i o n experiments i s 0.997. F i s s u r e Desorption Experiments. Desorption experiments were performed w i t h two f i s s u r e s by i n j e c t i n g rock e q u i l i b r a t e d water i n t o the f i s s u r e s immediately a f t e r removing the americium-bearing s o l u t i o n from the f i s s u r e s i n adsorption experiments. A f t e r i n j e c t i o n , the r o c k - e q u i l i b r a t e d water was allowed to r e a c t w i t h i n the f i s s u r e s f o r a p e r i o d of time before removal. The rocke q u i l i b r a t e d water was assayed a f t e r removal from the two f i s s u r e s and the amount of n u c l i d e desorbed as a f u n c t i o n of contact time was determined. Between f i v e and seven desorption experiments were performed f o r each of s i x time p e r i o d s between ten seconds and s i x t y minutes f o r each of the two f i s s u r e s . The r e s u l t s of the desorption experiments are presented i n t a b l e 2 as the percent of 3 (at e q u i l i b r i u m ) that desorbed from the f i s s u r e s i n a period of time. Because most of the americium remains on the rock even a f t e r d e s o r p t i o n i s completed, the r a t i o of a c t i v i t y desorbed to that expected at e q u i l i b r i u m i s subject to l a r g e u n c e r t a i n t y . The r e s u l t s of the desorption experiments are so v a r i e d that no q u a n t i t a t i v e c o n c l u s i o n can be drawn about the r a t e f o r d e s o r p t i o n . However, the r e s u l t s do i n d i c a t e that desorption may occur with a r a t e equal to or l e s s than the r a t e f o r a d s o r p t i o n . F i s s u r e E l u t i o n Experiments. The m i g r a t i o n c h a r a c t e r i s t i c s of americium by water transport i n f i s s u r e s f a b r i c a t e d from gray hornblende s c h i s t were determined. F i s s u r e s not used i n the previous s o r p t i o n experiments were used f o r these e l u t i o n e x p e r i ments. A diagram of the experimental apparatus i s shown i n Figure 4. S o l u t i o n r e s e r v o i r s were attached above the f i s s u r e s and the small bore tubes a f f i x e d to the bottom o f the f i s s u r e s were connected to s o l u t i o n metering pumps. Before the e l u t i o n experiments were performed, the " s c h i s m e q u i l i b r a t e d water was pumped through the f i s s u r e s f o r two days to a l l o w the f i s s u r e w a l l s to i n t e r a c t w i t h the r o c k - e q u i l i b r a t e d water. Two sets of f i s s u r e e l u t i o n experiments were performed. In the f i r s t set of experiments, the rock e q u i l i b r a t e d water contained i n the r e s e r v o i r s above three f i s s u r e s was f i r s t replaced with americium b e a r i n g stock s o l u t i o n . The s o l u t i o n metering pumps were l e f t running i n order not to i n t e r r u p t the flow w h i l e the exchange was made. The flow was slow enough that a i r was not drawn i n t o the f i s s u r e s during the exchange. Flow was continued u n t i l an a r b i t r a r y volume, 0.67 f i s s u r e volumes, of stock s o l u t i o n were drawn i n t o each f i s s u r e and then the s o l u t i o n metering pumps were turned o f f . T h i s volume was chosen to permit the observance of a l e a d i n g edge of a c t i v i t y , i f i t e x i s t e d , moving w i t h the water f r o n t . Stock s o l u t i o n was drawn i n t o the f i s s u r e s at l i n e a r


RADIOACTIVE W A S T E

Table I .

241 Am Adsorption by Gray Hornblende S c h i s t : F i s s u r e Experiments ( s t a t i c )

Adsorption Time, Seconds


IN GEOLOGIC STORAGE

Fissure Number

15 54 150 300 600 3600 60120 60120 86400 250200

1 5 8 2 7 4 2* 8* 5* 6

Percent Remaining in Solution

Activity Applied i n 152 λ

84 .6 71 63 .1 50 .7 36 .3 32 .3 25 .0 17 .4 2.20 0. 05

65 n C i 0.98 n C i 0.97 n C i 1.1 n C i 1.7 n C i 56 n C i 173 n C i 184 n C i 111 n C i 0.84 n C i

± .1 + 1 + .9 ± .8 ± .5 ± .1 ± .1 ± .1 ± .02 ± .25

*Cores #2, #5, and #8 were used twice f o r adsorption ex periments. The a c t i v i t y a p p l i e d during the f i r s t ad s o r p t i o n experiments was i n s i g n i f i c a n t when compared to the a c t i v i t y a p p l i e d i n the second experiment. The reported e r r o r s are f o r counting s t a t i s t i c s . +3 Table I I . Desorption Time, Seconds 10 20 60 360 1200 3600

Am

Desorption from Gray Hornblende S c h i s t : F i s s u r e Experiments ( s t a t i c ) Desorption Runs 5 7 7 6 5 7, 6 (core #8)

Percent of 3* (at e q u i l i b r i u m ) Desorbed Core #2 Core #8 8 13 31 8 13 14

+ 5

± ± ± ±

10 47 6 8 + 19

4 13 19 13 6 22

± ± ± ± ±

2 4 6 5 2 + 16

*3 i s the f r a c t i o n of a n u c l i d e i n s o l u t i o n a t e q u i l i b r i u m . The desorption values were determined assuming that 0.003 of the n u c l i d e would be i n s o l u t i o n a t e q u i l i b r i u m . The values that are presented are the a r i t h m e t i c mean and standard d e v i a t i o n s found f o r the number of experiments performed f o r each time i n t e r v a l .


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IN GEOLOGIC

STORAGE

flow r a t e s of 1.13, 2.29, and 4.77 cm/hr r e s p e c t i v e l y . After stopping the flow of stock s o l u t i o n i n t o the f i s s u r e s , i t was r e moved from the r e s e r v o i r s above the f i s s u r e s and the s o l u t i o n i n each f i s s u r e was l e f t stagnant f o r a period of twenty-four hours. Based on the k i n e t i c data i n t a b l e 1, i t i s b e l i e v e d that the americium i n the stock s o l u t i o n becomes e q u i l i b r a t e d with the surface of the f i s s u r e s d u r i n g the twenty-four hour p e r i o d that the stock s o l u t i o n remained stagnant. At e q u i l i b r i u m 0.997 of the americium i s adsorbed; i t i s t h e r e f o r e expected that the americium d i s t r i b u t i o n on the f i s s u r e w a l l s , a f t e r the twenty-four hour period, represents the m i g r a t i o n of americium i n t o the f i s s u r e s during the 0.67 f i s s u r e volume e l u t i o n s . A f t e r the twenty-four hour period of contact, the s o l u t i o n i n the f i s s u r e s was r a p i d l y withdrawn. The s u r f a c e s of the f i s s u r e s were d r i e d under vacuum at 25°C and then dismantled. The d i s t r i butions of americium on the f i s s u r e surfaces were determined f i r s t q u a l i t a t i v e l y using autoradiographs of the f i s s u r e s u r f a c e s with POLAROID LAND black and white 3000 ASA type 47 f i l m . The autoradiographs are shown i n F i g u r e s 5, 7, and 9. The l i g h t e r areas on the autoradiographs represent areas of americium a d s o r p t i o n . The d i s t r i b u t i o n s o f americium on the f i s s u r e s u r f a c e s were then q u a n t i t a t i v e l y determined by scanning the face of each f i s sure with a Nal s c i n t i l l a t i o n c r y s t a l through a 0.3 cm s l i t i n lead s h i e l d i n g . The 59 keV gamma ray emitted by Am was moni t o r e d . Histograms of the americium d i s t r i b u t i o n s on the f i s s u r e surfaces were produced and a r e presented i n F i g u r e s 5, 7, and 9. The second s e t of f i s s u r e - e l u t i o n experiments was performed i n the same manner as described f o r the f i r s t set of f i s s u r e e l u t i o n experiments. The d i f f e r e n c e i n the second set o f e x p e r i ments was that a f t e r 0.67 f i s s u r e volumes of stock s o l u t i o n were drawn i n t o the f i s s u r e s a t t h e i r r e s p e c t i v e flow r a t e s , the stock s o l u t i o n i n the r e s e r v o i r s was replaced with r o c k - e q u i l i b r a t e d water. The s o l u t i o n metering pumps were not turned o f f during the exchange. Subsequently, a t o t a l of twenty f i s s u r e volumes of sol u t i o n was drawn through each f i s s u r e before the metering pumps were turned o f f . A f t e r the metering pumps were stopped, the rocke q u i l i b r a t e d water was removed from the r e s e r v o i r s and the s o l u t i o n i n the f i s s u r e s was r a p i d l y drawn o f f . The f i s s u r e s were d r i e d under vacuum at 25°C and were d i s mantled. The d i s t r i b u t i o n s of americium on the f i s s u r e s u r f a c e s i n the second set of experiments were determined as they were i n the f i r s t s e t , that i s : f i r s t q u a l i t a t i v e l y by autoradiography and then q u a n t i t a t i v e l y by gamma scanning the f i s s u r e faces through a 0.3 cm s l i t . Autoradiographs of the f i s s u r e surfaces from the second set of experiments are presented i n Figures 6, 8, and 10. Histograms r e p r e s e n t i n g the q u a n t i t a t i v e l y determined d i s t r i b u t i o n of americium on the f i s s u r e s u r f a c e s are a l s o presented i n F i g u r e s 6, 8, and 10. 2lfl


10.

Nuclide

RICKERT E T A L .

177

Migration

60η 50ζ 40»— 30ζ 20L U 10L U

Α.

ERC

Ο rvi


I '2 V 4 ' 5 ^ >

7 8 9Ίθ'ΐΐΊ2Ί3Ί4Ί5Ί6' Ι

,

Z0NES(0.3I75 cm)

ιοο

Ί

90L U 80Ζ Ο 70ΓνΙ Ζ 6050L U 40CC 30L U •_ 20-

ιο-

Ι 2Τ4 5 6 7 8 9 Ι0 || Ι2 Ι3 Ι4 Ι5 Ι6' Z0NES(0.3l75cm) ,

,

,

,

,

,

,

,

,

,

,

Ι

,

Figure 5. Distribution of americium on fissure surface, experimental and pre dicted, after 0.67 fissure volume elution. Flow rate of 1.13 cm/hr. (A), Americium distribution on surface of fissure; (B), model prediction of americium on sur face of fissure; (C) autoradiograph of fissure surface showing americium dis tribution. y


178


Ο

IN GEOLOGIC

STORAGE

6050H 403020i OH


5 6 7 β^ΊοΊΓιζΊδΊ^ΐδΊθ ZONES (0.3175cm

1

β'θΊΟ'ΙΙ'^'β'^Ίδ'Ιο' ZONES (0.3175cm)

Figure 6. Distribution of americium on fissure surface, experimental and pre dicted, after 20 fissure volume elution. Flow rate of 1.13 cm/hr. (A), Americium distribution on surface of fissure; (B), model prediction of americium on sur face of fissure; (C), autoradio graph of fissure surface showing americium dis tribution.


10.

RiCKERT E T A L .

Nuclide

179

Migration

I '2


ZONES (0.3175 cm)

90η

Β.

LU 8 0 Ο 70-

(VI

605040ο 30ce UJ 2 0 10-

5 " V 7 8 9 Ï Ô i I ' 12 ' 13 ' 14 Ί 5 ' 16 ' T

T

T

r

ZONES (0.3175 cm)

Figure 7. Distribution of americium on fissure surface, experimental and pre dicted, after 0.67 fissure volume elution. Flow rate of 2.29 cm/hr. (A), Americium distribution on surface of fissure; (B), model prediction of americium on surface of fissure; (C), autoradio graph of fissure surface showing americium distribution.



180

IN GEOLOGIC

STORAGE

A.

5040i 302010-

i^3V5V7V9 io iM2 m4 i5 i6' ZONES (0.3175 cm)


,

,

,

,

,

5 6 7 8 9 10 II 12 13 14 15 16 Z0NES(0.3l75cm)

Figure 8. Distribution of americium on fissure surface, experimental and predicted, after 20 fissure volume elution. Flow rate of 2.29 cm/hr. (A), Americium distribution on surface of fissure; (B), model prediction of americium on surface of fissure; (C), autoradio graph of fissure surface showing americium distribution.


10.

RiCKERT E T A L .

Nuclide

Migration

181

504030i 20ce 10ο

7 8 9 io ii i2 i3 i4 i5 i6 ZONES (0.3175 cm) Downloaded by FUDAN UNIV on February 26, 2017 | http://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch010

,

,

,

,

,

,

,

,

,

LU

Z0NES(0.3l75cm)

Figure 9. Distribution of americium on fissure surface, experimental and pre dicted, after 0.67 fissure volume elution. Flow rate of 4.77 cm/hr. (A), Americium distribution on surface of fissure; (B), model prediction of americium on surface of fissure; (C ), autoradio graph of fissure surface showing americium distribution.


182

RADIOACTIVE

WASTE

IN GEOLOGIC

STORAGE

60-, 50-4030201012' 13' 14' 1516' Downloaded by FUDAN UNIV on February 26, 2017 | http://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch010

ZONES (0.3175 cm

U J

ZONES (0.3175cm)

Figure 10. Distribution of americium on fissure surface, experimental and predicted, after 20 fissure volume elution. Flow rate of 4.77 cm/hr. (A), Americium distribution on surface of fissure; (B), model prediction of americium on surface of fissure; (C), autoradio graph of fissure surface showing americium distribution.


10.

RICKERT E T A L .

Nuclide

Migration

183


Model P r e d i c t i o n s . The r a t e f o r desorption of americium from the f i s s u r e surfaces i n t o s o l u t i o n was assumed to equal the r a t e f o r the adsorption of americium from s o l u t i o n by the f i s s u r e sur f a c e s . The s o r p t i o n r a t e and the e q u i l i b r i u m f r a c t i o n a t i o n of americium that were determined i n the s t a t i c experiments were used to determine input parameters to the ARDISC model. The ARDISC model p r e d i c t i o n s f o r the d i s t r i b u t i o n s of americium on the f i s sure surfaces i n both sets of experiments are presented i n F i g u r e s 5 through 10 along with the autoradiographs and the experimental histograms r e p r e s e n t i n g the v a r i o u s d i s t r i b u t i o n s of americium on the f i s s u r e s u r f a c e s . Strontium M i g r a t i o n Experiments i n Glauconite The work that was performed i n t h i s set of experiments was an extension of work performed by Inoue and Kaufman (7). In the previous work, the m i g r a t i o n of strontium i n g l a u c o n i t e was modeled using c o n d i t i o n s of l o c a l e q u i l i b r i u m f o r flows up to 6.3 kilometers per year (72 cm/hr). The d i f f e r e n c e s between the pre d i c t e d and experimental r e s u l t s i n the experiments performed by Inoue and Kaufman may be due to the existence of n o n - e q u i l i b r i u m behavior. Column I n f i l t r a t i o n Experiments. Six i n f i l t r a t i o n e x p e r i ments, each at a d i f f e r e n t flow, were performed with one column of g l a u c o n i t e . The apparatus used i n the i n f i l t r a t i o n experiments c o n s i s t e d of a m i n e r a l column contained i n a s t a i n l e s s - s t e e l tube connected to a s a m p l e - i n j e c t i o n v a l v e and a solution-metering pump. G l a u c o n i t e [a hydrous s i l i c a t e , nominally (K,Na)(Al,Fe , Mg)2 (Al,Si)i*Oio ( 0 H ) 2 ] was prepared by s i f t i n g g l a u c o n i t e sand to o b t a i n a f r a c t i o n of uniform s i z e (70 mesh s i z e ) . The sand was washed repeatedly i n d i s t i l l e d water to remove dust ad h e r i n g to the p a r t i c l e s and was packed i n a s t a i n l e s s - s t e e l tube to form a column 15 cm long by 1.0 cm diameter w i t h 53 percent porosity. The column was conditioned by e l u t i n g i t with a s o l u t i o n of 0.01 M CaSOi* and 0.0001 M SrC03 . The s o l u t i o n used i n t h i s set of experiments was produced to simulate the s o l u t i o n used by Inoue and Kaufman. Strontium-85 with SrCl2 c a r r i e r was added to an a l i q u o t of the calcium-strontium s o l u t i o n to form a r a d i o a c t i v e s o l u t i o n with Μ . χ 10 M S r . At the s t a r t of an experiment, a small q u a n t i t y (20 y£) of the r a d i o a c t i v e s o l u t i o n was i n j e c t e d i n t o the s o l u t i o n stream above the g l a u c o n i t e column and the column was eluted w i t h s o l u t i o n u n t i l a l l the r a d i o a c t i v e strontium had moved through the column. The eluent was c o l l e c t e d i n f r a c t i o n s and each f r a c t i o n was analyzed to determine the m i g r a t i o n c h a r a c t e r i s t i c s of the strontium i n the column. The r a d i o a c t i v e strontium i n s o l u t i o n v s . the e l u a t e f r a c t i o n s f o r each flow r a t e i s p l o t t e d i n Figure 11. The strontium 6

8 5


184



Figure 11. Elution of strontium from the column of glauconite. Peak width increases with flow (top to bottom) indicating nonequilihrium behavior. Slowest flow rate is 0.1 mL/min or 14 cm/hr; fastest flow rate is 5 mL/min or 720 cm/hr.

150

200

250

VOLUME, mL


STORAGE


10.

RICKERT E T

AL.

Nuclide

Migration

185

peaks have a shape that i s dependent on the flow r a t e of the eluent. The sharpest peak i s produced with the slowest flow r a t e ; the widest peak i s produced with the f a s t e s t flow r a t e . The strontium peaks were delayed r e l a t i v e to the advancing water f r o n t (which would t r a v e r s e the column i n the f i r s t 6.5 mL of e l u e n t ) . Moreover, each of the strontium peaks seen i n F i g u r e 11, even the one produced with the slowest flow i s wider than p r e d i c t e d by the peak width (2 mL f u l l width at h a l f maximum) that was seen f o r peaks of t r i t i a t e d water that were e l u t e d through the column ( 8 ) . Therefore, the strontium has i n t e r a c t e d with the g l a u c o n i t e column and, as i n d i c a t e d by the d i f f e r e n t peak shapes, i n t e r a c t e d d i f f e r e n t l y at each flow. The flows given i n Figure 11 were measured by c o l l e c t i n g eluate over s p e c i f i c periods of time. In each ex periment, a l l the r a d i o a c t i v e strontium was eluted i n the s i n g l e peak seen i n the f i g u r e . To r e l a t e the s i x curves obtained experimentally with the ARDISC model, the r a t e f o r adsorption, the r a t e f o r desorption and the e q u i l i b r i u m d i s t r i b u t i o n of the n u c l i d e between g l a u c o n i t e and s o l u t i o n had to be determined. The r e l a t i v e m i g r a t i o n r a t e of the peak c o n c e n t r a t i o n of strontium at a s o l u t i o n flow r a t e of 0.1 mL per minute was used to c a l c u l a t e the e q u i l i b r i u m f r a c t i o n a t i o n of strontium between g l a u c o n i t e and s o l u t i o n . I t was determined that at e q u i l i b r i u m 0.958 of the strontium would be adsorbed by the g l a u c o n i t e and 0.042 of the strontium would be i n s o l u t i o n (mobile phase). The r a t e f o r a d s o r p t i o n and the r a t e f o r desorption were determined by curve f i t t i n g with ARDISC. I t was assumed that the r a t e f o r d e s o r p t i o n equaled the r a t e f o r a d s o r p t i o n . The α and β parameters were h e l d constant at 0.958 and 0.042 r e s p e c t i v e l y . The F, G, and zone length input parameters to the ARDISC model were v a r i e d to f i t the strontium migration data at a flow r a t e of 2 mL per minute. Assuming f i r s t order r e a c t i o n k i n e t i c s , the s o r p t i o n r a t e that was determined f o r adsorption and d e s o r p t i o n was 0.187 sec"* . A r e a c t i o n r a t e of 0.187 sec" i m p l i e s a h a l f time of r e a c t i o n of 3.7 seconds. The curves p r e d i c t e d by the model f o r four of the s i x flow r a t e s are presented i n F i g u r e 12. D i s c u s s i o n and

Conclusions

Figures 5, 7, and 9 show the americium d i s t r i b u t i o n s on f i s sure surfaces a f t e r the a d m i n i s t r a t i o n of 0.67 f i s s u r e volumes of stock s o l u t i o n a t flow r a t e s of 1.13, 2.29, and 4.77 cm/hr. The peak concentrations of americium were adsorbed at the top of the f i s s u r e s and l e a d i n g edges of americium were extended i n t o the f i s s u r e s . In general, a t f a s t e r flow r a t e s the r e l a t i v e q u a n t i t i e s of americium sorbed at the top of each f i s s u r e decreased and the l e n g t h and r e l a t i v e amount of the l e a d i n g edge extending i n t o each f i s s u r e i n c r e a s e d .




186


STORAGE


10.

RiCKERT E T A L .

Nuclide

Migration

187

The ARDISC model p r e d i c t i o n s f o r 0.67 f i s s u r e volume a d d i t i o n s of stock s o l u t i o n i n t o f i s s u r e s a t the three flow r a t e s a l s o show that the peak concentrations of americium w i l l be sorbed at the top of each f i s s u r e . The l e a d i n g edges of the n u c l i d e are p r e d i c t e d to extend i n t o the f i s s u r e s . The ARDISC model a l s o pred i c t s that at f a s t e r flow r a t e s , the r e l a t i v e amount of americium sorbed i n the peak concentration at the top of each f i s s u r e should decrease. A l s o , the length and r e l a t i v e amount of the l e a d i n g edge extending i n t o each f i s s u r e i s shown to i n c r e a s e . Figures 6, 8, and 10 show that the d i s t r i b u t i o n s of americium on the f i s s u r e surfaces a f t e r the a d d i t i o n of 0.67 f i s s u r e volumes of stock s o l u t i o n followed by the e l u t i o n of 20 f i s s u r e volumes of " s c h l s t - e q u i l i b r a t e d water through the f i s s u r e s at flow r a t e s of 1.13, 2.29, and 4.77 cm/hr. A comparison was made between the americium d i s t r i b u t i o n s found on the f i s s u r e surfaces a f t e r the a d d i t i o n of americium stock s o l u t i o n and that found a f t e r e l u t i o n of the americium by 20 f i s s u r e volumes of s c h i s t - e q u i l i b r a t e d water. I t was found that a f t e r the i n i t i a l loading of americium i n t o the f i s s u r e s i n the f i r s t 0.67 f i s s u r e volumes of s o l u t i o n , the peak concentrations of americium that were sorbed at the top of the f i s s u r e s decreased i n t h e i r r e l a t i v e c o n c e n t r a t i o n . The l e a d i n g edges of the detectable n u c l i d e concentration extending i n t o the f i s s u r e s had increased i n length and r e l a t i v e concentrat i o n with subsequent e l u t i o n by rock e q u i l i b r a t e d water through the f i s s u r e s . The ARDISC model p r e d i c t e d the same r e l a t i o n s h i p s . The ARDISC model q u a l i t a t i v e l y p r e d i c t e d the shape of the curves r e p r e s e n t i n g the d i s t r i b u t i o n of americium on the f i s s u r e s u r f a c e s . However, the model was not able to p r e d i c t a c c u r a t e l y the d i s t r i b u t i o n of americium on the f i s s u r e surfaces i n each experiment. A p l o t of the n a t u r a l logarithm of the f r a c t i o n of the n u c l i d e remaining i n s o l u t i o n v s . the absorption time does not produce a s t r a i g h t l i n e (Figure 13) f o r the f i r s t f i v e data p o i n t s obtained i n the s t a t i c f i s s u r e absorption experiments. A p l o t of the n a t u r a l logarithm of the f r a c t i o n remaining i n s o l u t i o n v s . time should produce a s t r a i g h t l i n e i f the r e a c t i o n were f i r s t order. The ARDISC model assumes that the s o r p t i o n r e a c t i o n s are f i r s t order. The discrepancy between the experimental and p r e d i c ted r e s u l t s may be due to higher order s o r p t i o n k i n e t i c s . In add i t i o n , d i f f e r e n c e s between the p r e d i c t e d and the experimental r e s u l t s may a l s o be due to a number of f a c t o r s such as non-homogeneous media, non uniform flow through the f i s s u r e s , small pH f l u c t u a t i o n s , p r e c i p i t a t i o n r e a c t i o n s , and c o l l o i d formation by some of the n u c l i d e . In the column i n f i l t r a t i o n experiments with strontium, the model p r e d i c t i o n s c l o s e l y resemble the experimental curves f o r the four flow r a t e s compared. The input parameters to the ARDISC model were derived from experimental data obtained i n i n f i l t r a t i o n experiments. The model p r e d i c t i o n s were based on the assumptions that the r a t e f o r adsorption and the r a t e f o r desorption were equal and that the s o r p t i o n r e a c t i o n s were both f i r s t order. l,

f f

f l



188


-ι

-I 9I " 0

1

ι

ι

1 60

ι

ι

ι

I

1

120

ι

ι

I

1

180

ι

ι

I

ι

240

1

1

I

ι

300

1

1

1

1

1

1

STORAGE

p—r

1 1 1 1 1 « 1 360 420 480 540

Λ

600

seconds

Figure 13.

Plot of In fraction of americium in solution vs. time for the data ob tained in the static fissure absorption experiments



10.

RICKERT

E TA L .

Nuclide

Migration

189

The r e s u l t s o f the experiments presented i n t h i s paper demon s t r a t e that the m i g r a t i o n o f n u c l i d e s i n geologic media can be studied experimentally and t r e a t e d i n a t l e a s t a s e m i q u a n t i t a t i v e f a s h i o n u s i n g k i n e t i c and p a r t i t i o n i n g data. The ARDISC model i s a u s e f u l a i d i n a n a l y z i n g n u c l i d e m i g r a t i o n data obtained i n l a b o r a t o r y experiments. But the ARDISC model i s l i m i t e d to f i r s t order s o r p t i o n k i n e t i c s . Phenomena such as n u c l i d e transport by p a r t i c l e s or n u c l i d e t r a n s p o r t i n c o l l o i d a l form w i l l i n t e r f e r e with a k i n e t i c approach to p r e d i c t i n g n u c l i d e m i g r a t i o n . The r e s u l t s i n d i c a t e that the measurement o f k i n e t i c parameters may be as important to under standing the migration of a n u c l i d e through a geologic media as the measurement of the e q u i l i b r i u m - s o r p t i o n value ( K ^ ) .

Literature Cited 1.

2.

3. 4.

5. 6.

7.

8.

Seitz, M. G . , Rickert, P. G . , Fried, S. M . , Friedman, Α. Μ., and Steindler, M. J., "Studies of Nuclear-Waste Migration in Geologic Media," Annual Report November 1976 - October 1977, ANL-78-8 (1978). Strickert, R., Friedman, A. M . , and Fried, S., "The Sorption of Technetium and Iodine Radioisotopes by Various Minerals," Nuclear Technology, in press (1978). Friedman, A. M . , E d . , "Actinides in the Environment," Am. Chem. Soc. Symposium Series No. 35, Washington, D . C . , 1976. Strickert, R. G . , Friedman, Α. Μ., and Fried, S. M . , "ARDISC: A Program to Calculate the Distribution of a Radioisotope in Pores of Partially Equilibrated Rock," Argonne National Lab oratory Report, in print (1978). Snyder, L . R., Principles of Adsorption Chromatography, Marcel Dekker, Inc., New York, pp. 12-16, (1968). Rickert, P. G . , Seitz, M. G . , Fried, S. M . , Friedman, A. M . , and Steindler, M. J., "Transport Properties of Nuclear Waste in Geologic Media," report to the Waste Isolation Safety Assessment Program, March, ANL program code 85249-00 (1978). Inoue, Y . , and Kaufman, W., "Prediction of Movement of Radio nuclides in Solution Through Porous Media," Health Physics, 9, pp. 705-715 (1963). Seitz, M. G . , Rickert, P. G . , Fried, S. M . , Friedman, A. M . , and Steindler, M. J., "Transport Properties of Nuclear Waste in Geologic Media," report to the Waste Isolation Safety Assessment Program, A p r i l , ANL program code 85249-00 (1978).

RECEIVED January 16, 1979.


Nuclide Migration in Fractured or Porous Rock - ACS Symposium

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