7 Method for Determining Leach Rates of Simulated Radioactive Waste Forms
1
K. F. FLYNN, L. J. JARDINE, and M. J. STEINDLER Argonne National Laboratory, Chemical Engineering Division, 9700 South Cass Avenue, Argonne, IL 60439
One of the more important factors affecting the isolation of radioactive waste i s the rate of release of the radioactivity from the solid waste form to the environment. The most probable mechanism for release and transport of radioactivity from a solid waste form i s by leaching of radioactive isotopes with groundwater. The objective of leach-testing various waste forms i s to evaluate the rate at which specific hazardous radionuclides migrate from waste if and when the waste form comes i n contact with groundwater. In this paper, measurement of leach rates of radioactive waste by a method which incorporates neutron activation i s described. The radioactive isotopes of most concern i n high level waste (HLW) and their estimated toxicities after several decay periods are given i n Table I. These data are largely taken from Wallace(1); supplemental estimates from calculations based on ORIGEN code information(2) are included. Other isotopes of the transuranic elements (e.g., Pu, Am, Cm), while present, have not been included since their relative toxic i t i e s are negligible compared to the toxicities of the isotopes shown i n Table I. Of the isotopes included i n Table I , the greatest initial hazard to the public i s from 9 0 S r and 137Cs. These isotopes represent the greatest hazard during the relative short term (i.e., ~500 y ) . Plutonium and americium tend to predominate during the next phase (i.e., 500 y to 10 y) with 129I and 9 9 T c becoming of comparable importance toward the latter part of this period. Because of either h a l f - l i f e or product yield considerations, the other isotopes have significantly less toxicity and w i l l not be considered here. Considerable uncertainty exists about the mechanism of leaching of activity from solidified waste by water. The process of dissolution of the solid matrix i s important, but ion migration through the matrix to the interface may also play a role. Since 240,241,242
243
245,246,247
5
1
Work performed under the auspices of the U.S. Department of Energy.
0-8412-0498-5/79/47-100-115$05.00/0 © 1979 A m e r i c a n C h e m i c a l Society
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Table I . R a d i o a c t i v e Isotopes i n Stored High L e v e l Wastes Having H a l f - L i v e s Greater than One Year and T h e i r Estimated R e l a t i v e T o x i c i t i e s (Revision of Wallace Data) 3
Relative Toxicity Half-Life, Isotope
y
10-y
90 r Cs l" Pm 125 106 2 38p
28.8 30.0 2.6 2.7 1.0 89 4.3 E+2 18.1 90 5.0 2.4 E+4 1.6 E+7 2.1 E+5 7 E+4 2 E+6 1 E+5 9.5 E+5 2 E+4 2.1 E+6 7 E+6
1,.0 1,.5 E-2 5 E-4 6 E-5 5 E-5 5 E-5 4 E-5 3 E-5 2 E-5 7 E-6 5 E-6 8 .5 E-7 1,.5 E-7 6 E-8 5 E-8 5 E-8 1,.5 E-8 6 .5 E-9 6 E-9 1.5 E-9
S
1 3 7
7
sb
Ru
u
^lAm ^Cm Sm 155 2 39p 129j 2
151
E u
u
"Tc Se Cs l^Sn Zr *m> Np 107 7 9
1 3 5
9 3
9t
2 3 7
pd
3
10 -y
5
10 -y ___
8 E-6
5 E-6 8.5 E-7 1.5 E-7 6 E-8 5 E-8 5 E-8 1.5 E-8 6.5 E-9 6 E-9 1.5 E-9
3 E-7 8.5 E-7 1 E-7 2 E-8 5 E-8 2.5 E-8 1.4 E-8 6 E-9 1.5 E-9
R e l a t i v e t o x i c i t y i s defined as the r a t i o of the concent r a t i o n of a given isotope i n the waste to i t s maximum p e r m i s s i b l e c o n c e n t r a t i o n (MPC) i n p u b l i c zone water (Réf. 1 ) . R e l a t i v e t o x i c i t y values are a r b i t r a r i l y normalized to S r a t 10 y as u n i t y . Values may vary from those l i s t e d depending on the f i s s i o n product content o f the f u e l . The plutonium values are based on the assumption that 98% of the plutonium has been removed by r e p r o c e s s i n g of the fuel. R e l a t i v e t o x i c i t i e s smaller than 1 E-9 have been a r b i t r a r i l y assumed to be n e g l i g i b l e and hence are not included. 9 0
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the l e a c h r a t e can vary f o r d i f f e r e n t elements, i t i s most important that the leach r a t e be determined f o r s p e c i f i c l i m i t s of interest. I t i s a l s o d e s i r a b l e to measure the leach r a t e s of samples i n an undisturbed s t a t e since crushing or other d e s t r u c t i v e measures ( u s u a l l y employed to o b t a i n p r a c t i c a l r a t e s ) can s i g n i f i c a n t l y a l t e r the p h y s i c a l c h a r a c t e r i s t i c s and hence the leach r a t e of the m a t e r i a l . Neutron a c t i v a t i o n a n a l y s i s (NAA) provides a method whereby measurements of s p e c i f i c leached elements can be made i n a reasonable time i n t e r v a l without the n e c e s s i t y of p h y s i c a l l y a l t e r i n g the waste matrix p r i o r to leaching. Leach r a t e s are g e n e r a l l y reported i n u n i t s of grams of t o t a l mass leached per square centimeter of surface area per day of l e a c h i n g (g/cm day). These u n i t s can be misleading i n waste management s t u d i e s since they have been adapted from m e t a l l u r g i c a l t e s t s f o r c o r r o s i o n r a t e s . Loss of volume (or weight) from a surface corresponds to an e r o s i o n (or depth) expressed as g/cm day). In waste management, i n c o n t r a s t , the primary i n t e r e s t i s the r a t e at which a s p e c i f i c isotope i s separated from the s o l i d matrix i n t o the l e a c h i n g medium. However, as long as the u n i t s measured i n any experimental arrangement are c l e a r l y defined, at l e a s t confusion can be avoided. Leach r a t e s f o r b o r o s i l i c a t e and phosphate glasses (among the most l e a c h - r e s i s t a n t of a l l commercially produced glasses) are g e n e r a l l y i n the range of about 10" to 1CT** g/cm day. The very low leach r a t e s are a s s o c i a t e d with s p e c i a l experimental g l a s s e s . The most l i k e l y range of waste g l a s s l e a c h r a t e s center around 10"" g/cm day. Hence, i t i s important i n a l l r a d i o a c t i v e waste forms (i.e. to minimize the surface to volume r a t i o ) . The most appropriate shape f o r t h i s c o n d i t i o n i s an impervious c y l i n d e r . While a s p h e r i c a l shape has the lowest surface-to-volume r a t i o , the ease of f a b r i c a t i o n and handling of a c y l i n d r i c a l shape makes i t more d e s i r a b l e . 2
2
5
6
2
2
3
Laboratory s t u d i e s of leach r a t e s would most conveniently be done on samples of about 1 g (i.e., 0.1 to 10 g range) of wasteHowever the measurement of very low leach r a t e s (e.g., ^10~ g/cm day) on unground samples of t h i s small s i z e would give tenuous r e s u l t s . A 1-g sphere with a d e n s i t y of 2 would have a surface area of 3 cm . A leach r a t e of 10" g/cm day would r e s u l t i n a t o t a l weight l o s s of 3 χ 10" g per day. Assuming a 30 wt % mixture of HLW (see Table I I ) i n s t a b i l i z i n g a d d i t i v e s , (i.e., g l a s s f r i t , etc.), r e a l waste would c o n t a i n about 3 wt % cesium. I f the t e s t m a t e r i a l was r e p r e s e n t a t i v e HLW, the cesium l o s s would be about 10"" g per day. The l e a c h i n g times necessary to o b t a i n s u f f i c i e n t m a t e r i a l i n s o l u t i o n so that the concentration exceeds the d e t e c t i o n l i m i t f o r the a n a l y s i s would be p r o h i b i t i v e (i.e., vL-y). Neutron a c t i v a t i o n a n a l y s i s not only provides the increased s e n s i t i v i t y required f o r l e a c h r a t e measurements but a l s o when a p p l i e d i n the manner described i n t h i s r e p o r t , circumvents the background problems caused by the d i s s o l u t i o n of container materials. 6
2
2
6
6
7
2
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Table I I .
Fission Products
Actinides
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Composition of Simulated High L e v e l R a d i o a c t i v e Waste: R e v i s i o n of Bonner's Data(13)
Elements i n Waste
Relative Molar Abundance
Rb Cs Zr Mo Tc Ru Rh Pd Ag Cd Sb Te Sr Bà Y La Ce Pr Nd Pm Sm Eu Gd U Np Pu Am Cm
0.015 0.079 0.155 0.139 0.032 0.086 0.015 0.047 0.003 0.003 0.010 0.018 0.039 0.040 0.020 0.035 0.075 0.034 0.104 0.003 0.020 0.004 0.003 0.016 0.012 0.000 15 0.002 6 0.000 59
A c t i v a t i o n Product (and Half L i f e ) of N a t u r a l l y Occurring Elements 18.7 d 2.05 ^ 65.5 d 2.8 d
86
Rb *Cs Zr "Mo
13l
9 5
t
—1 103
39.5 d
Ru
11 Om^g 115m cd
12l
*Sb 129m 85
Te
S r
1 3 1
Ba
255 d 43 d 60 d 34.1 d 64 d 11.5 d
°La ^Ce
1.7 d 32.5 d
152
13.2 y
1I+
2 37 2 39
E u
Π
N p
6.8 d 2.4 d
7.
F L Y N N
ET
AL.
Leach
Rates
119
The b a s i c technique e s t a b l i s h e d f o r leach t e s t i n g i n v o l v e s exposure of a s o l i d sample of known composition and having a premeasured surface area to a l e a c h i n g medium f o r a f i x e d time i n t e r v a l . This i s followed by a n a l y s i s of the l e a c h i n g medium f o r the mass of a s p e c i f i c m a t e r i a l which has d i s s o l v e d i n the l e a c h i n g medium. D i f f i c u l t i e s a r i s e i n t r y i n g to d u p l i c a t e i n the l a b o r a t o r y the leaching medium and c o n d i t i o n s present i n the n a t u r a l environment. The p h y s i c a l c o n d i t i o n s under which the measurements are made d i f f e r widely f o r the v a r i o u s methods. A survey of leach r a t e measurements has been made by Mendel(3). As examples of r e l a t i v e l y simple procedures, three methods are described below. A l l such methods r e q u i r e e i t h e r analyses of bulk l i q u i d s f o r i o n i c c o n s t i t u e n t s (at low concentrations) a f t e r l e a c h i n g or a d r a s t i c increase i n the surface area p r i o r to l e a c h i n g to get higher concentrations that are e a s i e r to measure. Leaching with Soxhlet Apparatus. A Soxhlet l e a c h i n g apparatus has been used e x t e n s i v e l y ( 4 ) i n r a d i o a c t i v e waste measurements. The sample i s contacted with continuously r e p l e n ished d i s t i l l e d water. Leaching r a t e s have been shown to be i n h i b i t e d by ions p r e v i o u s l y leached from the s o l i d m a t e r i a l ( 5 ) . This technique e l i m i n a t e s i n t e r f e r e n c e from ions i n s o l u t i o n . Leaching with A i r l i f t R e c i r c u l a t o r . In a s i m i l a r method (6) used by other i n v e s t i g a t o r s , 50 mL of d i s t i l l e d water at room temperature i s r e c i r c u l a t e d over a sample at a r a t e of 250 mL/min by means of an a i r l i f t r e c i r c u l a t o r . The leach water i s sampled and i s changed p e r i o d i c a l l y . Leaching onto Ion-Exchange Resins. A more s o p h i s t i c a t e d procedure f o r determining l e a c h a b i l i t i e s of r a d i o a c t i v e waste forms has been developed Ç7). Flowing deionized water i s c o n t i n uously c i r c u l a t e d across the sample surface and then through i o n exchange r e s i n s where the leached ions are adsorbed. The adsorbed ions are subsequently e l u t e d from the r e s i n columns f o r atomic absorption analyses. L e a c h a b i l i t i e s measured by t h i s procedure are claimed to be lower and more c o n s i s t e n t than those made i n stagnant water i n the absence of continuous i o n i c c o n t r o l ( 7 ) . Measurement of Surface Area. The l e a c h a b i l i t y determined by these methods i s u s u a l l y reported as g/cm day. The t o t a l surface area of p a r t i c u l a t e m a t e r i a l can be assessed: 1) by assuming a p a r t i c l e shape (e.g. s p h e r i c a l ) and estimating the number of p a r t i c l e s , or 2) by measurements using the Brunauer-Emmett-Teller (BET) n i t r o g e n adsorption technique(8). Unfortunately, the BET method measures the area of surfaces to which n i t r o g e n has access; t h i s i s not n e c e s s a r i l y the same as the area to which a s o l u t i o n has access. Access by s o l u t i o n s r e q u i r e s much l a r g e r pore areas, 2
3
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and even then f r e e transport of ions from the surface to the bulk s o l u t i o n i s necessary. Hence, the BET can overestimate the surface area a v a i l a b l e f o r l e a c h i n g . Accurate determination of the surface area exposed to l e a c h i n g remains a major u n c e r t a i n t y i n the conversion of e x p e r i mental r e s u l t s to true l e a c h r a t e s . I f a simple geometric shape i s assumed i n determining a lower l i m i t f o r the surface area, conservative estimates (i.e., upper l i m i t s ) of the true leach r a t e s are obtained. When there are u n c e r t a i n t i e s i n the l e a c h r a t e data, upper l i m i t s are the most u s e f u l f o r cautious e x t r a p o l a t i o n s to long term storage c o n d i t i o n s . In c o n t r a s t s u r f a c e areas measured by BET technique lead to e r r o r s i n the other direction. Surface area measurements by l i q u i d p e n e t r a t i o n i s not a well-developed technique. Form of Reported Data. The mass of m a t e r i a l leached can be determined e i t h e r by any standard g r a v i m e t r i c method or, i n the case of r a d i o a c t i v e samples, by measurement of the r a d i o a c t i v i t y d i s s o l v e d i n the l e a c h i n g medium. Since d i f f e r e n t ions are o f t e n leached at d i f f e r e n t r a t e s , i t i s important to s p e c i f y the i o n when quoting leach r a t e r e s u l t s . To avoid confusion on t h i s matter, i t has been suggested(9) that long-term l e a c h i n g r e s u l t s be reported as ( f r a c t i o n of A l e a c h e d ) ( c m / g ) ( d a y ) , where A i s the s p e c i f i c i o n analyzed f o r . A standard method f o r l e a c h - t e s t i n g immobilized r a d i o a c t i v e waste s o l i d s and f o r r e p o r t i n g r e s u l t s has been proposed(10). Other workers, however, consider t h i s proposed method of t e s t i n g unduly tedious and o f f e r c o n s t r u c t i v e suggestions f o r a c c e l e r a t i n g l e a c h t e s t s ( 4 ) . Many of these proposed methods i n v o l v e p h y s i c a l d e s t r u c t i o n of the waste form i n order to increase the surface area exposed to the leachant. Such a procedure not only can a l t e r the p h y s i c a l c h a r a c t e r i s t i c s of the surface exposed to l e a c h i n g but a l s o can r e q u i r e the measurement of the s u r f a c e areas of f i n e l y d i v i d e d s o l i d s with the concomitant d i f f i c u l t i e s mentioned above. 2
- 1
- 1
F i e l d Leach Tests. Two f i e l d leach t e s t s have been successf u l l y conducted(11) , using g l a s s blocks c o n t a i n i n g aged f i s s i o n products and b u r i e d below the surface water t a b l e . The groundwater composition and flow r a t e were a c c u r a t e l y determined, the surface area of the block was measured, and the concentration of S r i n the groundwater was monitored at r e g u l a r i n t e r v a l s over a 15-y p e r i o d . T h i s type of experiment i s very u s e f u l i n addressing the questions involved i n s a f e , long-term waste d i s p o s a l . However, the extensive e f f o r t and the need to do f i e l d t e s t i n g f o r each m a t e r i a l make t h i s approach undesirable f o r development of l a r g e s c a l e t e s t i n g programs. 9 0
7.
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ET
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Neutron A c t i v a t i o n Method. Neutron a c t i v a t i o n a n a l y s i s f o r the measurement of t r a c e elements has been used e x t e n s i v e l y and the l i t e r a t u r e i s r e p l e t e with papers on t h i s s u b j e c t . A recent review of t h i s technique as a p p l i e d to environmental samples has been published by the N a t i o n a l Academy of Sciences(12). The experimental technique devised i n t h i s work i s based on a NAA method. Neutron i r r a d i a t i o n of s o l i d waste forms of simulated HLW (see Table I I ) produces a c t i v a t i o n of the elements i n the sample. The a c t i v a t i o n products can be r e a d i l y measured before and a f t e r l e a c h i n g by radiochemical and/or instrumental techniques. In order to be u s e f u l f o r these purposes, the a c t i v a t i o n product must have a s u f f i c i e n t l y e n e r g e t i c and abundant r a d i a t i o n ( e i t h e r 3~ or γ) to be e a s i l y detected, as w e l l as a s u f f i c i e n t l y long h a l f - l i f e (i.e., s e v e r a l days or more) to be u s e f u l f o r r e l a t i v e l y long-term l e a c h s t u d i e s . The elements i n HLW of most concern (see Table I) and t h e i r a s s o c i a t e d measurable r a d i o a c t i v e isotopes (see Table II) are strontium ( S r ) , cesium ( C s ) , the t r i v a l e n t actinides ( E u as a s t a n d i n ) , and the t e t r a v a l e n t a c t i n i d e s ( C e as a s t a n d i n ) . T r i v a l e n t r a r e earths ( E u ) have been shown to be a s a t i s f a c t o r y s u b s t i t u t e f o r the t r i v a l e n t a c t i n i d e s , and t e t r a v a l e n t cerium (*^*Ce) s u b s t i t u t e s f o r the t e t r a v e l e n t a c t i n i d e s i n waste s t u d i e s ( 1 3 ) . However, i f there i s uranium present i n the waste form, ^ U produced by (n,2n) on u and N p from (η,γ,β) on 2 38|j become u s e f u l a c t i n i d e t r a c e r s . In the work described here, S r , S r , Ba, C e , and E u in a s p r a y - c a l c i n e d simulated waste and S b i n l e a d were measured. 8 5
1 3 l f
1 5 2
lifl
1 5 2
7
2 3 8
2 3 9
8 5
8 9
1 3 1
1 4 1
1 5 2
1 2 i +
a n a l y s i s using a l i t h i u m - d r i f t e d germanium (GeLi) d e t e c t o r provides an assay of the gamma-emitting r a d i o i s o t o p e s present. A f t e r l e a c h i n g of the s o l i d matrix f o r a s u i t a b l e time i n t e r v a l , the l e a c h i n g medium can be assayed e i t h e r by gamma ray (i.e., GeLi) or ( i f greater s e n s i t i v i t y i s required) by radiochemical a n a l y s i s , using conventional radiochemical s e p a r a t i o n techniques(14) followed by gamma ray a n a l y s i s (GeLi d e t e c t o r ) and/or low-back ground beta counting of the separated s p e c i e s . From these measurements, the f r a c t i o n of the element of i n t e r e s t which has been leached from the matrix can be determined with great s e n s i tivity. Because of the s e n s i t i v i t y , these measurements r e q u i r e no d e s t r u c t i o n of the s o l i d matrix p r i o r to l e a c h i n g i n order to increase the l e a c h r a t e ; a l s o , the concentrations of the ions i n s o l u t i o n remain very low, minimizing t h e i r e f f e c t on the d i s s o l u tion reaction. The t o t a l mass of the s o l i d matrix can be determined q u i t e a c c u r a t e l y by weighing on a standard a n a l y t i c a l balance. The surface area of a waste granule exposed to the leachant i s more d i f f i c u l t to determine. A conservative estimate (lower l i m i t ) can be made by assuming smooth surfaces and measuring the apparent geometric area (i.e., assume an impervious sphere c o n f i g u r a t i o n ) . More accurate measurements of the surface area exposed to the leachant are d i f f i c u l t to o b t a i n with c u r r e n t l y a v a i l a b l e
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techniques. On the other hand, s i n c e glass monolithic shapes are planned f o r HLW d i s p o s a l , t e s t samples that simulate t h i s form, e.g., g l a s s c y l i n d e r s or beads, can be described by the simple geometric area i f the leach r e a c t i o n does not proceed to change the surface area over the course of the experiment. I t i s recognized that leach r a t e s of p a r t i a l l y reacted surfaces may d i f f e r s i g n i f i c a n t l y from those determined on smooth, f r e s h surfaces. Experimental Results Leach r a t e measurements have been made on s e v e r a l waste forms, using the NAA technique. Some r e s u l t s are presented here as examples o f the a p p l i c a t i o n o f t h i s method. Simulated wastes used i n these s t u d i e s c o n s i s t e d o f two types of granules obtained from B a t t e l l e P a c i f i c Northwest L a b o r a t o r i e s . The d i f f e r e n t i a l leach r a t e s of the bulk waste matrix were c a l c u l a t e d with the equation: W Ο where A^ = amount of isotope A removed i n time t A
ο
= i n i t i a l amount o f isotope A i n s o l i d 2
S = surface area of the s o l i d m a t e r i a l (cm ) W
Q
= i n i t i a l weight of the s o l i d m a t e r i a l (g)
t = leach period
(days) 2
L = leach r a t e (g/cm day) A /A t
Q
= f r a c t i o n of isotope
leached.
Gross d i s s o l u t i o n of the waste can be measured from the l o s s of weight of the sample or by determination of the appearance i n the l e a c h i n g medium of the major matrix c o n s t i t u e n t (e.g., glass). However, the s e l e c t i v e l e a c h i n g of important f i s s i o n product elements has been observed i n t h i s work to be s i g n i f i c a n t l y d i f f e r e n t from that of the bulk waste matrix. The u n i t s normally used to d e s c r i b e leach r a t e s , i.e., g/cm day, appear to imply d i s s o l u t i o n of the waste. Results quoted as f r a c t i o n leached per cm day(9) where the f r a c t i o n leached r e f e r s to an i d e n t i f i e d element or n u c l i d e [i.e., ( A / A ) / S t ] may be more d e s c r i p t i v e of a c t u a l l e a c h i n g . However i t must be remembered that the f r a c t i o n leached depends on the s p e c i f i c surface area (cm /g) of the monolith. Conversion of the data given i n these terms to the more conventional form shown i n E q ( l ) can be e a s i l y made i f the s p e c i f i c surface area (cm /g) i s known. 2
2
t
0
2
2
7.
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123
Leaching of S i n t e r e d Granules. The i n i t i a l s t u d i e s i n t h i s work(13) were done i n granules c o n s i s t i n g of 80% PW-4b sprayc a l c i n e d simulated waste, 10% s i l i c a , and 10% b o r o s i l i c a t e g l a s s f r i t s i n t e r e d at 1100°C. The granules were about 0.6 cm i n diameter and weighed about 370 mg each. The surface area, based on the assumption of an impervious (i.e., "hard") s p h e r i c a l shape, was c a l c u l a t e d to 1.1 cm per 370-mg bead or 3 cm /g. D i s t i l l e d water (at 25°C) was the only leachant used i n these s t u d i e s s i n c e the wide v a r i a b i l i t y i n groundwaters makes i t impossible to o b t a i n a s i n g l e meaningful generic groundwater composition. Several of these granules were i r r a d i a t e d i n the isotope t r a y ( f l u x *v>6 χ 1 0 n/cm s) of the Argonne CP-5 Research Reactor f o r 24 h ( t o t a l neutron f l u e n c e , ^5 χ 1 0 n/cm ). A f t e r ^3 weeks c o o l i n g to allow unwanted s h o r t - l i v e d a c t i v i t i e s to decay, neutroninduced r a d i o a c t i v i t i e s present i n the granules were determined by gamma-ray a n a l y s i s , using a GeLi detector and a computer-programmed gamma spectrum a n a l y s i s . Isotopes of p a r t i c u l a r i n t e r e s t that were i d e n t i f i e d were S r (514-keV γ ) , B a (496-keV γ ) , Ce (145-keV γ ) , and E u (1408-keV γ ) . A f t e r gamma ray a n a l y s i s was complete, a granule was immersed i n 100 mL of room-temperature d i s t i l l e d water f o r s e v e r a l days with no s t i r r i n g . ( S t i r r i n g was g e n e r a l l y avoided i n these experiments s i n c e i t was concluded that stagnant water c l o s e l y approximates slow-moving groundwater i n n a t u r a l s i t u a t i o n s . ) The water was then decanted from the granule and analyzed radiochemically(14) f o r the elements barium, strontium, cerium, and r a r e earths (RE). The barium and strontium contents were determined i n order to measure l e a c h r a t e s of the a l k a l i n e e a r t h metals. Because of chemical (and valence) s i m i l a r i t y , the measured cerium l e a c h r a t e s should r e f l e c t the l e a c h i n g c h a r a c t e r i s t i c s of the t e t r a v a l e n t a c t i n i d e s . The other RE l e a c h r a t e s should r e f l e c t the l e a c h i n g c h a r a c t e r i s t i c s of the t e t r a v a l e n t a c t i n i d e s and of the RE themselves. The r a d i o chemically separated samples were counted i n a low-background (background ^0.8 count/min) beta p r o p o r t i o n a l counter, as w e l l as with a GeLi d e t e c t o r . The r e s u l t s with the two d i f f e r e n t counting methods agreed w i t h i n experimental e r r o r (i.e., about 10%). Two successive seven-day l e a c h t e s t s were performed on each granule. The agreement between these two t e s t s i n d i c a t e d no s i g n i f i c a n t change i n l e a c h r a t e with time on t h i s short time s c a l e , f o r the p a r t i c u l a r elements s t u d i e d (i.e., barium, strontium, cerium, and rare earths). I t was not p o s s i b l e to determine the cesium content i n these granules because, f o r economy reasons, cesium was not included i n the c a l c i n e production. The r e s u l t s of these measure ments are given i n Table I I I . S i g n i f i c a n t d i f f e r e n c e s i n the leach r a t e s of the a l k a l i n e earths (barium, strontium) the RE (europium), and cerium are observable. Leach r a t e measurements using t h i s technique can be made using h i g h - r e s o l u t i o n GeLi d e t e c t o r s f o r gamma counting e x c l u s i v e l y (i.e., without radiochemical separations from the l e a c h i n g medium). T h i s method, while much more r a p i d , r e s u l t s i n a s i g n i f i c a n t 2
1 2
2
2
1 7
8 5
1 5 2
2
1 3 1
lkl
124
RADIOACTIVE
Table I I I .
WASTE
IN GEOLOGIC
STORAGE
Leach Rates o f Waste Form PW-4 i n D i s t i l l e d Water at 25°C (7 day l e a c h i n g period)
Leach Rate
Isotope
Counting Method
8 5
Sr Sr Ba C e
F r a c t i o n Leached per day
γ 3
8 9
1 3 1
3
m
γ
1.1 9.8 1.0 3.3 2.9 1.3
3
Rare Earths ( Eu)
γ
a
b Equivalent Mass g/cm 'day 2
E-4° E-5 E-4 E-6 E-6 E-5
3.8 E-5 3.3 E-5 3.5 E-5 1.1 E-6 1.0 E-6 4.4 E-6
1 5 2
a
F r a c t i o n of i n d i c a t e d i s o t o p e .
^Mass of s o l i d waste that would have been leached i f the t o t a l mass had been leached a t the same r a t e as the i n d i c a t e d isotope. C
Read 1.1 χ 1 0
_lf
.
decrease i n s e n s i t i v i t y , as w e l l as some increase i n u n c e r t a i n t y , as a consequence of a n a l y z i n g extremely complex gamma s p e c t r a c o n t a i n i n g many overlapping gamma rays. Low-enerey gamma rays such as those a s s o c i a t e d with N d (91-keV) and l'°Tm (84-keV) are p a r t i c u l a r l y d i f f i c u l t to r e s o l v e as i s S r (514-keV) whose gamma ray energy i s very c l o s e to the u s u a l l y abundant a n n i h i l a t i o n r a d i a t i o n (511-keV). Low-abundance gamma rays a l s o tend to get l o s t i n the Compton continuum from the more abundant high-energy gamma rays which are always present. For these reasons, and to improve s e n s i t i v i t y , radiochemical separations, though timeconsuming, represent an important p a r t of the procedure f o r q u a n t i t a t i v e measurements. For q u a l i t a t i v e measurements, a p p l i c a t i o n of the technique without radiochemical separations i s simple, s t r a i g h t f o r w a r d , and quick. The counting techniques described i n t h i s paper are a l s o r e a d i l y a p p l i c a b l e t o s t u d i e s o f "hot" r a d i o a c t i v e waste (i.e., r a d i o a c t i v e waste from reprocessed nuclear f u e l ) . With t h i s type of m a t e r i a l , the cesium can be analyzed as 30-y C s (662-keV γ ) , the RE as 13-y E u (964-keV and 1408-keV γ ) , the strontium as 28-y S r ( a f t e r chemical s e p a r a t i o n and beta counting), and the a c t i n i d e s by group s e p a r a t i o n and alpha counting. Leaching of Lead Matrix M a t e r i a l s . The use of a metal matrix f o r waste encapsulation i s being s t u d i e d . The neutron a c t i v a t i o n method was used to evaluate the l e a c h i n g c h a r a c t e r i s t i c s of p o t e n t i a l metal matrix m a t e r i a l s . Lead was s e l e c t e d f o r i n i t i a l s t u d i e s . Reagent grade l e a d beads (2.2-mm diameter, weighing 75 mg llf7
8 5
1 3 7
1 5 2
9 0
7.
FLYNN
ET
Leach
AL.
125
Rates
2
each) having a s p e c i f i c s u r f a c e area of about 2 cm /g were i r r a d i a t e d (as described above) f o r 24 h at a thermal neutron f l u x of about 6 χ 1 0 n/cm s. A f t e r an appropriate c o o l i n g time to allow s h o r t - l i v e d gamma a c t i v i t i e s to decay (about 2 weeks), the gamma r a d i a t i o n from the beads was measured. S u f f i c i e n t Sb a c t i v i t y was produced from neutron capture by the antimony impurity i n the lead to serve as a t r a c e r f o r the bulk lead during the l e a c h s t u d i e s . The i r r a d i a t e d lead beads were immersed i n 25°C a i r - s a t u r a t e d quiescent d i s t i l l e d water (pH, ^6) f o r one week, a f t e r which the l e a c h i n g medium was analyzed f o r S b by gamma counting. The lead beads were a l s o weighed before and a f t e r l e a c h i n g to assess the t o t a l weight l o s s . F i f t y beads were used i n order to i n c r e a s e the s e n s i t i v i t y of the measurement. The leach r a t e s (grams of lead/cm day) as c a l c u l a t e d from E q ( l ) were 3.6 E-4 from the weight l o s s and 3.2 E-4 from the *Sb determination. These two numbers agree w i t h i n estimated experimental u n c e r t a i n t i e s and hence i n d i c a t e that S b i s a r e p r e s e n t a t i v e t r a c e r to measure the t o t a l mass leach r a t e s f o r l e a d . A l s o , t h i s agreement serves to v e r i f y that t h i s method of leach r a t e determination gives r e s u l t s comparable to those obtained w i t h more standard methods such as weight l o s s measurements. 1 2
2
12lf
1 2 I +
2
12l
12If
D i s c u s s i o n and
Conclusions
The neutron a c t i v a t i o n t e s t i n g procedure i s most u s e f u l f o r the measurement of very low leach r a t e s . The l e a c h r a t e s d e l i n e a t e d i n Table I I I were each based on one granule weighing 370 mg and having 1.1 cm (^3 cm /g) surface area. These c o n d i t i o n s e a s i l y permitted the measurement of l e a c h r a t e s of the order of 10~ g/cm day f o r one-week l e a c h t e s t s without radiochemical separations being necessary. I f gamma counting i s supplemented with radiochemistry and low-background beta counting, at l e a s t two orders of magnitude more s e n s i t i v i t y (i.e., 10~ g/cm day) can be achieved. T h i s s e n s i t i v i t y can be increased f u r t h e r by i n c r e a s i n g the sample s i z e and/or neutron f l u e n c e (each can r e a d i l y be increased by a f a c t o r of 10). With such m o d i f i c a t i o n s , measure ment of l e a c h r a t e s of l e s s than l O " ^ g/cm day would be r e a d i l y available. Leach r a t e s f o r elements other than those l i s t e d i n Table I I can a l s o be determined by t h i s method. In f a c t , any element i n the p e r i o d i c t a b l e that i s s o l i d at room temperature and has an a c t i v a t i o n product with a h a l f - l i f e s u f f i c i e n t l y long to allow l e a c h t e s t i n g can be studied with t h i s technique. T h i s method can a l s o be a p p l i e d to the study of the l e a c h r a t e s of alpha e m i t t i n g a c t i n i d e s present i n waste. In t h i s case, standard c a r r i e r - f r e e radiochemical procedures, coupled with low background alpha counting, would be invoked. The c l a s s i c a l leach t e s t methods c u r r e n t l y i n use, (the Soxhlet, Paige, K e l l e y , procedures described above i n D e s c r i p t i o n of Leach Rate Measurement Procedures as w e l l as those d e s c r i b e d i n 2
6
2
2
8
1
2
2
RADIOACTIVE
126
WASTE
IN GEOLOGIC
STORAGE
Ref 3) give a t best q u a l i t a t i v e r e s u l t s . Although these r e s u l t s may be adequate f o r comparing the leach r a t e s of v a r i o u s waste forms, they tend t o be u n s u i t a b l e f o r e x t r a p o l a t i o n to f i n a l r e p o s i t o r y or accident c o n d i t i o n s . This d e f i c i e n c y has been pointed out by Mendel(3) and S h e f f l e r et at. (15), among others. The technique described i n t h i s paper allows leach r a t e s t o be measured without p h y s i c a l l y a l t e r i n g the waste form being studied hence, the r e s u l t s are more amenable t o e x t r a p o l a t i o n t o f i n a l conditions. Our method f o r measuring leach r a t e s i s thought superior t o other methods c u r r e n t l y i n use. Meaningful leach r a t e data can be obtained using r e l a t i v e l y simple l a b o r a t o r y s c a l e equipment coupled with standard NAA techniques. More d e t a i l e d information can be procured by applying radiochemical separations and more s o p h i s t i c a t e d counting methods. The experimental technique described here i s a p p l i c a b l e t o the measurement of leach r a t e s f o r the elements of i n t e r e s t , from any s o l i d waste form, i n any p o t e n t i a l storage environment.
Literature Cited 1.
Wallace, R. M. et al., "High Level Radioactive Waste Management," Advan. Chem., Ser. 153, M. H. Campbell (ed.), American Chemical Society, Washington, D. C. (1976).
2.
B e l l , M. J., "ORIGEN - The ORNL Isotope Generation and Depletion Code," Oak Ridge National Laboratory Report ORNL-4628 (1972).
3.
Mendel, J. Ε., "A Review of Leaching Test Methods and the Leachability of Various Solid Media Containing Radioactive Wastes," Battelle Pacific Northwest Labora tories Report BNWL-1765 (1973).
4.
Mendel, J. E. and Warner, I. Μ., "Waste Glass Leaching Measurements," Battelle Pacific Northwest Laboratories Quarterly Report BNWL-1741 (1973).
5.
Weyl, W. A. and Marloe, E. C., "The Constitution of Glasses, a Dynamic Interpretation," V o l . I I , Part 2, pp. 1118-1119, Interscience Publishers, New York (1964).
6.
Paige, Β. Ε., "Leachability of Glass Prepared from Highly Radioactive Calcined Alumina Waste," P h i l l i p s Petroleum C o . , Atomic Energy Division, Report IDO-14672 (1966).
7.
Kelley, J. A. and Wallace, R. M. Nucl. Technol. (1976), 30, 47.
7.
FLYNN ET AL.
Leach Rates
8.
Brunauer, S., Emmett, P. H . , and T e l l e r , E., J . Am. Chem. Soc. (1938), 60, 309.
9.
Godbee, H. W., Clark, C. W., and Fitzgerald, C. L., Physical Properties of Solids Incorporating Simulated Radioactive Wastes, p. 565, "Proceedings of the Symposium on the Solidification and Long Term Storage of Highly Radioactive Wastes," Richland, Washington, CONF-660208 (1966).
10.
Hespe, E. D., Atomic Energy Review (1971), 9, 195.
11.
Merritt, W. F., Nucl. Technol. (1977), 32(1), 88.
12.
Robertson, D. E. and Carpenter, R . , "Neutron Activation Techniques for the Measurements of Trace Metals in Environmental Samples," National Academy of SciencesNational Research Council Report NAS-NS-3114 (1974).
13.
Bonner, W. F., B l a i r , H. T., and Romero, L . S., "Spray Solidification of Nuclear Waste," Battelle Pacific Northwest Laboratories Report BNWL-2059 (August 1976).
14.
Flynn, K. F., "Radiochemical Procedures and Techniques," Argonne National Laboratory Report ANL-75-24 (1975).
15.
Scheffler, K., Riege, U., Louwrier, Κ., Mabzke, Hj., Ray, I . , and Thiele, Η . , "Long Term Leaching of Silicate Systems: Testing Procedure, Actinides Behavior and Mechanism," Karlsruhe Report KFK-2456 (1977).
RECEIVED January 16, 1979.
