Technetium Diffusion in Clay-Based Materials Under Oxic and Anoxic Conditions

Home , Hume (soil)

CA9600681 AECL EACL

AECL-1U19,COG-95-428

Diffusion du technetium dans des matériaux à base d'argile dans des conditions oxiques et anoxiques

H.B. Hume

November 1995 novembre AECL EACL

TECHNETIUM DIFFUSION IN CLAY-BASED MATERIALS UNDER OXIC AND ANOXIC CONDITIONS

H.B. Hume

AECL Whiteshell Laboratories Pinawa, Manitoba, Canada ROE 1LO AECL-11419 COG-95-428 AECL EACL

TECHNETIUM DIFFUSION IN CLAY-BASED MATERIALS UNDER OXIC AND ANOXIC CONDITIONS

H.B. Hume

ABSTRACT

Diffusion coefficients were determined for Tc in compacted clay-based materials in both anoxic and oxic environments. The soils were saturated with a synthetic groundwater solution; the principle ions in solution were Ca2+, Na+ and Cl*. Anoxic conditions were established by conducting the experiments in a low-O2 glove box and by mixing 0.5 wt% powdered Fe with the soils. Under anoxic conditions, apparent diffusion coefficients, Da, were <0.01 um2/s for Tc in compacted backfill material (a 1:3 mix by dry mass of Lake Agassiz clay and crushed granite aggregate). Distribution coefficients, Kd, for Tc on Lake Agassiz clay and backfill material in anoxic environments were back-calculated from Da values. Based on the Kd values, Tc strongly sorbs on Lake Agassiz clay and backfill under 2 anoxic conditions. Effective diffusion coefficients, De, for Tc of 10, 16 and 110 um /s were measured in oxic Avonlea bentonite, Lake Agassiz clay and illite-smectite, respectively, at a 3 clay dry bulk density of « 1.2 Mg/m ; the corresponding Da values were 55, 48 and 74 um2/s. Since anoxic conditions are expected in a disposal vault excavated deep in granitic rock in the Canadian Shield, the results suggest the migration of Tc through the backfill will be relatively slow.

AECL Whiteshell Laboratories Pinawa, Manitoba, Canada ROE 1L0 AECL-11419 COG-95-428 AECL EACL

DIFFUSION DU TECHNÉTIUM DANS DES MATÉRIAUX À BASE D'ARGILE DANS DES CONDITIONS OXIQUES ET ANOXIQUES

par

H.B Hume

RÉSUMÉ

On a déterminé les coefficients de diffusion du Te dans des matériaux à base d'argile compactés dans des milieux anoxiques et oxiques. Les sols étaient saturés d'une solution d'eau souterraine synthétique; les ions principaux dans la solution étaient : Ca2+, Na+ et Cl\ On a établi les conditions anoxiques en effectuant des expériences dans une boîte à gants à faible O2 et en mélangeant 0,5 % en poids de Fe en poudre aux sols. Dans des conditions 2 anoxiques, les coefficients de diffusion apparents, Da, étaient <0,01 um /s pour le Te dans du matériau de remblai compacté (un mélange 1/3 par masse sèche d'argile du lac Agassiz et d'agrégats de granit concassés). Les coefficients de distribution, Kd, pour le Te sur l'argile et le matériau de remblai du lac Agassiz dans des milieux anoxiques ont été rétrocalculés à partir des valeurs Dtt. En se fondant sur les valeurs de Kd, le Te est adsorbé fortement sur l'argile et le remblai du lac Agassiz dans des conditions anoxiques. Les coefficients de 2 diffusion efficaces, De, pour le Te de 10, 16 et 110 um /s ont été mesurés respectivement dans la bentonite oxique d'Avonlea, l'argile du lac Agassiz ainsi que l'illite-smectite avec une masse 3 volumique sèche ~ 1,2 Mg/m ; les valeurs correspondantes de Da étaient de 55, 48 et 74 um2/s. Étant donné que des conditions anoxiques sont prévues dans une enceinte de stockage permanent excavée en profondeur dans la roche granitique du Bouclier canadien, les résultats indiquent que la migration du Te dans le remblai sera relativement lente.

EACL Laboratoires de Whiteshell Pinawa (Manitoba) Canada ROE 1L0 AECL-11419 COG-95-428 CONTENTS

Page

INTRODUCTION 1

MATERIALS 2

2.1 SOIL MATERIALS 2 2.2 IRON 3 2.3 SOLUTION 3 2.4 RADIOTRACERS 3

METHODS 4

3.1 CLOSED-CELL DIFFUSION EXPERIMENTS 4 3.2 OPEN-CELL DIFFUSION EXPERIMENTS 8

RESULTS 8

4.1 CLOSED-CELL DIFFUSION EXPERIMENTS 8 4.2 OPEN-CELL DIFFUSION EXPERIMENTS 10

DISCUSSION 14

5.1 ANOXIC CONDITIONS 14 5.2 OXIC CONDITIONS 18

SUMMARY AND CONCLUSIONS 19

ACKNOWLEDGEMENTS 20

REFERENCES 20

APPENDIX 22 1. INTRODUCTION

Deep geological disposal of nuclear fuel waste is being investigated in Canada. The disposal concept involves the use of multiple barriers to limit the movement of radioactive and chemically toxic components of the waste from a disposal vault, excavated SOO to 1000 m deep in plutonic rock of the Canadian Shield, to the surface (AECL 1994). Two of the proposed barrier materials are buffer and backfill. Buffer material (a 1:1 mix by dry mass of Avoniea bentonite and silica sand) would surround the containers holding the nuclear fuel waste, and backfill material (a 1:3 mix by dry mass of Lake Agassiz clay and crushed granite aggregate) would be used to fill much of the remainder of the vault.

Technetium-99 is an important radioisotope in the safety assessment of the disposal concept. For a used fuel burnup of 685 GJ/kg U, about 0.2 g of 99Tc are produced in CANDU™ reactors per kg U (Tait et al. 1989). Technetium-99 decays to the stable isotope "Ru by emission of a p" particle (0.293 MeV). It has a half-life of 2.1 x 105 a and a specific activity of 62.4 GBq/mol.

Technetium is redox sensitive. In moderately oxidizing solutions, such as those exposed to air, the primary aqueous species is TCO4. Clay minerals generally have a net negative surface charge. As a result, most anions are poorly sorbed on most clays, and many anions move relatively rapidly through clay-based materials. This has been observed for TcO^ in bentonite (Sawatsky and Oscarson 1991) and Lake Agassiz clay (Oscarson et al. 1994) under oxic conditions. In moderately reducing environments, low concentrations of aqueous Tc(IV) species are found in equilibrium with sparingly soluble TcO2 (Meyer et al. 1988). At still lower redox potentials Tc(0), which is less soluble than TcO2, may be formed (Lemire and Garisto 1989).

Although anoxic conditions will likely exist in a disposal vault (Johnson et al. 1994), it is important to study the behaviour of Tc in both oxic and anoxic environments. Diffusion coefficients have been measured for Tc under oxic conditions in Lake Agassiz clay and in backfill materials made with various granite particle size fractions (Oscarson et al. 1994). These coefficients have also been determined for Tc in Avoniea bentonite under both oxic and anoxic conditions (Sawatsky and Oscarson 1991). This paper reports the results of Tc closed-cell diffusion experiments under anoxic conditions with Lake Agassiz clay and two backfill materials prepared with different granite particle size fractions. The results of Tc open-cell diffusion experiments conducted under oxic conditions in Avoniea bentonite, Lake Agassiz clay and an illite-smectite interstratified clay are also presented. -2- 2. MATERIALS

2.1 SOIL MATERIALS'

Lake Agassiz clay was obtained from Kildonan Concrete Products Ltd., Winnipeg, Manitoba. The clay was laid down in glacial Lake Agassiz some 23,000 years ago. Its approximate composition is: 35 wt% smectite (montmorillonite), 20% illite, 15% quartz, 10% kaolinite, 10% calcite and minor amounts of feldspar, dolomite and organic matter (Oscarson and Dixon 1989). Lake Agassiz clay has a cation exchange capacity of 0.50 mol/kg, a specific surface area of 300 x 103 m2/kg and Ca2+ is the principle exchangeable cation (Oscarson 1994a).

Granite was obtained from the quarry of Hugh Munro Construction Ltd., which is on the Lac du Bonnet batholith near the Whiteshell Laboratories of Atomic Energy of Canada Limited. It was crushed by Supercrete Division, Lafarge Canada Inc., Winnipeg, Manitoba. After crushing, the particle size distribution of the granite in wt% was: <150 um, 10.1; 150 to 300 pm, 9.1; 300 to 850 um, 21.0; 850 to 2000 um, 19.8; 2000 to 4750 um, 25.7; and >4750 um, 14.2. Both <150-and <4750-um fractions were used in this work. The complete elemental compositions of these size fractions are given by Oscarson et al. (1994).

Avonlea bentonite was supplied by Canadian Clay Products, Wilcox, Saskatchewan. The clay contains approximately 80 wt% smectite (montmorillonite), 10% illite, 5% quartz, and minor amounts of kaolinite, feldspar, gypsum, calcite and organic matter (Oscarson and Dixon 1989). It has a cation exchange capacity of 0.60 mol/kg, a specific surface area of 480 x 103 m2/kg and Na+ is the predominant exchangeable cation (Oscarson 1994a).

Illite-smectite interstratified clay was obtained from the Source Clays Repository, Clay Minerals Society, Columbia, Missouri; it is designated ISMt-2. It is a mixed-layer clay composed of 60% illite and 40% smectite layers and it originates from the Mancos Shale, Montana. The clay has a cation exchange capacity of 0.43 mol/kg and a specific surface area of 290 x 103 m2/kg (Oscarson 1994a). Smectite is not thermodynamically stable in a nuclear fuel waste disposal vault where the temperature ner.r the waste containers could be as high as 100°C; over very long periods of time it may gradually transform to an illite- smectite mixed-layer clay (Oscarson and Hume 1993). The diffusive properties of illite- smectite are, therefore, relevant to the assessment of the disposal concept.

'in this report soil means clay or crushed granite or a mixture of the two (i.e. backfill material). -3- 2.2 IRON

Powdered elemental Fe (Fisher Scientific, Nepean, Ontario) was added to the soils in some experiments to promote anoxic conditions. The Fe has a particle size of < 150 um and a quoted purity of >99%.

2.3 SOLUTION

In this study, a synthetic groundwater (SGW) was used (Table 1). The SGW is similar in composition to WRA-500, which is an average of groundwater compositions found in the Whiteshel! Research Area at a depth of about 500 m (Gascoyne 1988).

TABLE 1

COMPOSITION OF THE SGW

Concentration (mg/L) (mmol/L)

Ca2t 2100 52 Na+ 1900 83 Mg2+ 61 2.5 K+ 14 0.36 cr 6100 170 soj- 1000 10 HCO3 68 1.1

pH = 7.5

2.4 RADIOTRACERS

Two radiotracers were used in these experiments. In the closed-cell diffusion experiments, the gamma-emitting isotope 95mTc was used. Its use enabled the Tc concentrations to be determined while the radiotrac;r was in the soils. The half-life of 95mTc is 61.0 d and the specific activity is 79.2 PBq/mol. It was produced by Los Alamos National Laboratories, Los Alamos, New Mexico by proton irradiation of natural Mo targets and was received as 95ni aqueous NH4 TcO4. A stock solution that contained 507 MBq/L of radioactivity (on the starting date of the closed-cell diffusion experiments) was prepared by diluting the solution received with distilled-deionized water. -4-

99 99 In the open-cell diffusion experiments, Tc as NH4 TcO4 (from Dupont Canada Inc., Mississauga, Ontario) was used as a radiotracer. The solution received was diluted with distilled-deionized water to make a stock solution containing 4.6 GBq/L.

3. METHODS

3.1 CLOSED-CELL DIFFUSION EXPERIMENTS

These experiments were conducted in a low-O2 glove box (Anaerobic Chamber model 855-AC, Plas-Labs, Inc., Lansing, Michigan). The glove box has two polymethyl methacrylate chambers: a main chamber and an attached transfer chamber. Inside the main chamber, a cylinder containing a Pd catalyst on alumina beads is mounted on top of a combination catalyst heater and circulating fan. Three drying columns are mounted outside the glove box and are connected to the main chamber with reinforced polyvinyl chloride tubes. Two of the columns contain Indicating Drierite™ (W.A. Hammond Drierite, Xenia, Ohio) and the third, #13 molecular sieve (Fisher Scientific, Nepean, Ontario).

Initially the glove box was purged overnight with N2. After purging, a mixture of 2% H2/98% N2 was flushed through the glove box for one hour. The atmosphere was then circulated through the heated catalyst. On the catalyst, the reaction

O2 + 2H2 - 2H2O converted most of the residual O2 in the glove box to water vapour; this was removed by simultaneously passing the glove-box atmosphere through the drying columns. The contents of the drying columns were regenerated as required by heating at 160°C overnight.

Normally, the 02-removal system was operated for four hours each day and for five days each week. To determine if air leaked into the glove box, the atmospheric composition was measured by mass spectrometry (model MM8-80, VG Gas Analysis, Middlewich, England) immediately following catalyst operation and after the system had been off for 20 and 92 h. (During the diffusion experiments, 92 h was the longest time the catalyst system was off.) The data are presented in Table 2. There was no apparent correlation between the concentration of N2, H2, Ar or CO2 in the glove box and the length of time the catalyst system was off. However, the O2 concentration in the glove box increased with the duration of the shutdown, which suggests that air leaks occurred.

The transfer chamber was purged three times before transferring materials or equipment between it and the main chamber, or vice versa. It was evacuated to an absolute pressure of 30 kPa using the integrated vacuum pump and then a mixture of 2% H2/98% N2 was added until the transfer-chamber pressure equalled that of the external atmosphere. This procedure -5- 1AB_LE_2

COMPOSITION OF THE GLOVE-BOX ATMOSPHERE

Duration of CONCENTRATION (mol %) Catalyst System

Shutdown N2 H2 Ar CO2 O2 (h)

0 98.7 1.22 0.026 0.009 0.003 20 99.3 0.60 0.041 0.003 0.013 92 98.8 1.06 0.034 0.010 0.043 was repeated twice. Before use, all the materials transferred into the glove box were equilibrated with the low-O2 atmosphere at least overnight.

An illustration of the closed diffusion cell is shown in Figure 1. An outer cylinder encloses two stainless steel inner cylinders (each 2.2-cm inside diameter and 4.0-cm long). One of the inner cylinders contains a soil plug labelled with a radioactive tracer and the other contains an unlabelled soil plug. (A more complete description of the equipment and procedures described in this report was published previously (Hume 1993)). When the two plugs are placed in contact, the radiotracer diffuses from the labelled plug into the initially unlabelled plug. At the end of the experiment, the clay plugs are sliced and the slices analyzed for radioactivity.

To ensure that each soil was well mixed before the plugs were compacted, the components were kneaded in a polyethylene bag inside the glove box until they appeared homogeneous. A mixture of 99.5% Lake Agassiz clay (dry mass basis), 0.5% Fe and SGW was prepared using this method. The mass of SGW added was calculated to be sufficient to saturate the clay when it was compacted to a target dry bulk density of 1.13 Mg/m3. This density was used for the Lake Agassiz clay experiments because it is the approximate effective density of the clay component of reference backfill material (effective clay density is the mass of clay divided by the combined volume of clay and voids). Mixtures of 74.6% granite (<150- or <4750-um fraction), 24.9% Lake Agassiz clay, 0.5% Fe (based on the dry masses of both granite and clay) and SGW were prepared similarly. Enough SGW was added to the backfill mixes to saturate them when compacted to 2.00 Mg/m3, which is close to the density of the reference backfill material.

The same soils were also prepared using SGW solutions spiked with 9SmTc. The 95mTc concentrations in these solutions (on the starting date of the experiments) were 1.35 MBq/L for the experiments with Lake Agassiz clay and 5.36 MBq/L for those with backfill. As -6- DIFFUSION CELL

Stainless steel cap

Stainless steel outer cylinder

Clay

Stainless steel inner cylinders

Clay & tracer

O-ring Teflon plug

2 5cm i

FIGURE 1: Diagram of the Closed Diffusion Cell -7 - only tracer quantities of 95mTc were used, the amount of radiation emanating from these soils was low. Hence it was possible to prepare the labelled soils using the same method as was used with the unlabelled soils.

A manual screw press was used in the glove box to compact the Lake Agassiz clay, and to partially compact the backfill material, into the stainless steel cylinders. The cylinders containing backfill were sealed in a polyethylene bag, removed from the glove box one at a time, and further compacted using a hydraulic press. It was necessary to use a hydraulic press to generate enough force to compact the backfill material to 2.00 Mg/m3, but the press was too large to fit inside the glove box. After compacting, the soil plugs were immediately returned to the glove box. The plugs were placed in pairs into diffusion cells; each pair of plugs was made with the same soil and SGW (with or without the radiotracer). The soils were equilibrated for 13 to 26 d; previous experiments had shown that this was sufficient time for the moisture content to become uniform throughout the plugs. For the labelled plugs, the equilibration period also allowed the tracer to diffuse throughout the Plug- After the equilibration period, a labelled plug was placed into a cell along with an unlabelled plug of the same soil. A small amount of a paste prepared by mixing 99.5% Lake Agassiz clay, 0.5% Fe and an equal mass of SGW was placed at the interface of the two plugs. This ensured a good contact between the labelled and unlabelled soil plugs. The reassembled cells were placed in 1-L polyethylene bottles and SGW was added to the bottles to cover the cells; this prevented the clay plugs from drying during the experiment and also moderated any temperature fluctuations in the laboratory. The experiments were performed in quadruplicate; the reaction period was 217 d.

After the diffusion period, one cell at a time was removed from the glove box and opened. Immediately thereafter, the two plugs were separated and sliced. It is unlikely that significant movement of Tc occurred during the time required to slice each plug-about one hour-particularly since the labelled and initially unlabelled plugs were immediately separated. The plugs were cut into slices about 4 mm thick and each slice was split into two sub-samples. For both the labelled and initially unlabelled plugs, one of the sub- samples from each of the two slices closest to the interface, and a sub-sample of every second slice thereafter, was analyzed for 95mTc by gamma spectroscopy (EG & G Ortec, Oak Ridge, Tennessee). The measured radioactivity was taken to be equal to the average radioactivity at the midpoint of a slice.

A sub-sample of every slice was dried at 110°C to constant mass to delermine the moisture content, W,

J where mw is the mass of wet soil and m0 the mass of oven-dry soil. The masses of oven- dry soil and water in the second sub-sample of each slice were calculated from W. The -8- density of each soil plug was calculated by summing the masses of oven-dry soil in all the slices and dividing by the measured volume of the stainless steel cylinder.

3.2 OPEN-CELL DIFFUSION EXPERIMENTS

Sufficient air-dry clay (Avonlea bentonite, Lake Agassiz clay or illite-smectite) to obtain a target dry bulk density of 1.25 Mg/m3 was packed into a stainless steel ring (4.12-cm inside diameter and 1.28-cm long) using a hydraulic press. The plug was placed in an open diffusion cell and the cell was assembled as shown in Figure 2. Source solution containing about 17 MBq/L (0.27 mmol/L) of 99Tc in SGW was circulated across one end of each clay plug. Tracer-free SGW solution was passed over the other end to flush away any "Tc that had diffused through the plug; this solution was accumulated in a collection reservoir. Every one to four days, samples of the solutions in the source and collection reservoirs were withdrawn, mixed with 18 mL of liquid scintillation cocktail (Insta-Gel XF™, Packard Instrument Company, Inc., Meriden, Connecticut) and analyzed for "Tc by liquid scintillation counting (Beckman LS5801, Beckman Instruments, Inc., Irvine, California). At the end of the experiments, a sample of each plug was dried to constant mass at 110°C and the moisture contents were calculated from Equation (1). The final dry bulk densities of the clay plugs, pb, were calculated from 1 A (2)

3 where pp is the clay particle density (2.75 Mg/m ) and pw the SGW density.

4. RESULTS

4.1 CLOSED-CELL DIFFUSION EXPERIMENTS

Transient diffusive transport is described by Fick's second law,

01 OA

where c is the concentration in the pore solution, t the time, Da the apparent diffusion coefficient and x the space coordinate. The Da value is defined as, Connecting bolts

Stainless steel plate Porous Ni plate Clay Stainless steel ring

-ring

0 12 3 4 5cm • • >

DIFFUSION CELL FIGURE 2: Diagram of the Open Diffusion Cell - 10- where Do is the diffusion coefficient in pure bulk water, T the tortuosity of the soil, ne the effective porosity for diffusive mass transport and Kd the distribution coefficient (the ratio of the concentration of a species sorbed on a soil to the concentration of thai species in the contacting solution).

If there is little or no sorption, as is the case for Tc on Avonlea bentonite and Lake Agassiz clay under oxic conditions (Sawatsky and Oscarson 1991, Oscarson et al. 1994), and hence Kd « 0, Equation (4) reduces to

H = Efcc. (5) Only a small amount of 95mTc moved across the interface of the labelled and initially unlabelled soil plugs in these experiments and, as a result, the concentration profiles in the plugs were poorly developed (Figure 3). Therefore, an approximate solution to Equation (3) was used to calculate Da (Kemper 1986),

Da = —— , (6) where L is the soil plug length and f the fraction of the radiotracer that diffused across the interface of the labelled and initially unlabelled soil plugs. Results close to the exact solution are obtained from Equation (6) when f < 0.3 (Kemper 1986), which is the case here. Data from a typical experiment (that used for Figure 3) are given Table 1A. The Da values are given in Table 3.

4.2 OPEN-CELL DIFFUSION EXPERIMENTS

From Fick's first law, which is applicable to the open-cell diffusion experiments after steady state is established, the flux of a diffusant is proportional to its concentration gradient:

àQ = _Ddc (7) Adt dx where dQ is the quantity of radiotracer that diffused through the clay plug in a time interval dt, A the cross-sectional area of the clay plug and D the diffusion coefficient. An example of a plot of cumulative flux versus time is shown in Figure 4 and the data from which the plot was produced are given in Table 2A in the Appendix. If the concentration gradient is measured across the clay plug, as it was in these experiments, an effective diffusion coefficient, De, is obtained from Equation (7). This coefficient is defined as

De = Dotne . (8)

The De values are presented in Table 4. -11 -

9W632

0.0 -1.0

95m FIGURE 3: Concentration Profile from a 217 d Closed-Cell Experiment with Tc in pb = 1.92 Mg/m3 Backfill (made with <150-jim Granite) under Anoxic Conditions - 12 -

TABLE 3

APPARENT DIFFUSION COEFFICIENTS FOR Te IN ANOXIC SOILS DETERMINED IN CLOSED-CELL EXPERIMENTS

Soil pb Da (Mg/tn3) Qim2/s)

Lake Agassiz Clay 1.04 0.0028 1.09 0.0012 1.11 0.0016 1.07 <0.0017 Mean1 1.08±0.042 0.0019 ± 0.0008

Backfill (<150-um granite) 1.81 0.016 1.88 0.0023 1.79 0.00033 1.79 <0.00019 Mean 1.83 ±0.05 0.0062 ± 0.0085

Backfill (<4750-|jm granite) 1.91 0.0025 1.93 0.0039 1.94 0.0031 1.92 <0.00020 Mean 1.93 ±0.02 0.0032 ± 0.0007

' Less-than values, and the corresponding density values, are not included in the mean values. 2 Mean ± one standard deviation.

These experiments also provided values of Da, which were calculated from (Crank 1975)

where te is obtained by extrapolating from the steady-state region of the curve to the time axis as shown in the enlargement in Figure 4. This technique is not particularly accurate (Helfferich 1962), but it provides additional data without further experimental work. These Da values are given in Table 4. - 13-

15 9&O63.1

2 •

CM 10 - 1 - t. ^#V*

"3 0 ) 5 10

< 5 - a

n 20 40 60 80 Time (d)

99 3 FIGURE 4: Cumulative Flux of Tc from a pb = 1.16 Mg/m Illite-Smectite Plug (length = 1.28 cm) in an Open-Cell Experiment Under Oxic Conditions - 14-

TABLE 4

DIFFUSION COEFFICIENTS FOR Tc IN SELECTED CLAYS UNDER QXIC CONDITIONS DETERMINED IN OPEN-CELL EXPERIMENTS

Clay pb D D (Mg/m3) (um7s) (um2/s)

Avonlea Bentonite - 10 55.

Lake Agassiz Clay 1.27 1.0 nd2 1.25 3.6 nd 1.28 44 48 Mean 1.27 ±0.02' 16 ±24 -

Illite-Smectite . 110 28 1.16 130 85 1.20 76 110 Mean 1.18 ±0.03 110 ±30 74 ±42

1 Mean ± one standard deviation 2 Not determinable from the data.

5. DISCUSSION

5.1 ANOXIC CONDITIONS

The Da values for Tc in soils are much lower under anoxic than oxic conditions. For 2 example, Oscarson et al. (1994) reported a Da value of 59 um /s for Tc in Lake Agassiz clay under oxic conditions; in the same clay at a similar density, a value of 0.0019 um2/s was obtained under anoxic conditions (Table 3). In backfill material the difference in Da between anoxic and oxic conditions is less, but still more than an order of magnitude (Oscarson et al. 1994, Table 3).

In an attempt to explain the differences between diffusion coefficients obtained under anoxic and oxic conditions, the speciation distribution of Tc over a range of redox potentials, Eh, was estimated using data given by Lemire and Garisto (1989). Ideally the actual pore-solution composition should be used in the calculations. However, since the composition of this solution is unknown, the SGW composition (Table 1) was used. It is - 15- apparent from the results of the calculations (Table 5) that as Eh drops from 0.072 to -0.44 V, the major solution species of Tc changes from TcO4 to TcOH(CO3)j. At an Eh slightly greater than 0 V, the concentrations of TcO4 and TcOH(CO3)2' are equal; at lower Eh the concentration of TcO4" is less than that of TcOH(CO3)j. This has several important consequences for diffusive transport in soils.

TABLE 5

ESTIMATED MOLAR CONCENTRATION OF Tc SPECIES IN SOLUTION AS A FUNCTION OF REDOX POTENTIAL fnH = 7.51

Species Eh(V) 0.072 0.00 -0.10 -0.20 -0.30 -0.40 -0.44

TcO4 9E-5 2E-8 2E-13 9E-22 1E-33 2E-45 3E-50 TcO(OH)+ 1E-13 1E-13 1E-13 8E-17 1E-23 2E-30 5E-33 TcO(OH)? 3E-9 3E-9 3E-9 2E-12 3E-19 5E-26 9E-29 TcO(OH)3 8E-11 8E-U 8E-11 5E-14 8E-21 1E-27 2E-30 (TcO(OH)2)2 2E-11 2E-11 2E-11 8E-18 2E-31 7E-45 3E-50 TcOH(CO3)2 3E-6 3E-6 3E-6 2E-9 3E-16 6E-23 1E-25 Tc(OH)2(CO3£ 5E-19 8E-18 4E-16 1E-17 9E-23 8E-28 7E-30 Total Solubility 9E-5 3E-6 3E-6 2E-9 3E-16 6E-23 1E-25

Perhaps most importantly, Kd values > 0 have been reported for Tc sorption on granite under reducing conditions. For example, in deaerated artificial groundwater containing 2+ 3 20 mg/L Fe , Allard et al. (1979) reported Kd = 50 m /Mg for Tc on granite. Similarly, Ito 3 and Kanno (1988) found that Kd = 46 m /Mg for Tc on granite in a solution containing 0.16 mol/L NaNO3 md 0.1 mol/L NaBH4. Since the conditions in these studies differ significantly from those used here, the results are not directly comparable, but they show that Tc sorbs on some soils under reducing conditions.

Also, it is likely the hydrated radius of TcOH(CO3)2 is greater than that of TcO4. The rate of diffusion in porous media is inversely related to the size of the diffusant (for example, fewer pores are available for diffusion as the diffusant size increases). Therefore, the larger size of TcOH(CO3)j, compared to TcO4, probably also contributes to the lower Da values in anoxic environments.

The reduction in the solubility of Tc with decreasing Eh (Table 5) is not a cause of the corresponding change in Da. Nevertheless, the change in Tc solubility is significant with respect to diffusive transport from a nuclear fuel waste disposal vault. From Equation (7), - 16- it is apparent that the diffusive flux (dQ/Adt) is proportional to dc. From 0.072 to -0.44 V, the solubility of Tc decreases and, therefore, the maximum possible diffusive flux also decreases. Depending on the conditions present in a disposal vault, the decreased solubility of Tc may further reduce the already low diffusive flux that results from TcOH(CO3)2 being the predominant aqueous Tc species throughout most of the Eh range examined.

It is difficult to quantify the Eh of these complex systems. There are several possible reactions of elemental iron that could control Eh (Pourbaix 1966):

8H+- " E (pH=',.5; V) = -0.528 3Fe-h4H20 -Fe 3o4 + h

+ Fe + H20 * FeO + 2H + 2e Eh(pH=7.5; V) = -0.490

2+ 2+ Fe Fe + 2e" Eh(V) = -0.440 + 0.0295 log[Fe ]

If the concentration of Fe2+ in the pore solution of the soils was <1 mol/L (it was likely several orders of magnitude less), and if Fe controlled Eh, then it was <-0.440 V, provided the systems were at equilibrium.

The low diffusion coefficients for Tc under ancxic conditions have practical implications for nuclear fuel waste disposal in Canada. Due to the presence of reductants like Fe(II) in the backfill and the surrounding host rock, the environment in a disposal vault in the Canadian Shield will likely be anoxic (Johnson et al. 1994). Under these conditions, the rate of migration of Tc in the backfill would be very slow, and the probability of "Tc reaching the biosphere before decaying to insignificant levels would be reduced accordingly.

At the 95% confidence level, there is no statistically significant difference among the means of the Da values for Tc under anoxic conditions in the clay and the two backfill materials studied (Table 3). In contrast, in similar experiments performed without adding Fe to the soils or using a low-O2 glove box, the Da values in backfill material made with <150- or <4750-um granite were much lower than the value in Lake Agassiz clay alone (Oscarson et al. 1994). The results under anoxic conditions are consistent with the hypothesis put forward by Oscarson et al. (1994) that, in the experiments with backfill materials, Tc(VII) was reduced to Tc(IV) by components of the granite, such as magnetite and biotite, and subsequently sorbed.

There is more variability in the data than is usually observed in closed-cell experiments (Oscarson et al. 1992; Cho et al. 1993), particularly in the backfill made with <150-pm granite (Table 3). This is probably due to the slow migration of Tc in these experiments. As a result, little Tc moved across the interface of the labelled and initially unlabelled soil plugs. Therefore, if the plugs were not separated exactly at the interface at the end of the experiments, substantially more or less 95mTc could have been apportioned to the initially unlabelled soil plug. - 17-

Schwochau and Astheimer (1962) reported that Do = 1480 um*/s for TcO;. From this value 2 3 and Da = 59 nm /s for Tc in Lake Agassiz clay at pb = 1.1 Mg/m under oxic conditions (Oscarson et al. 1994), T is calculated from Equation (5) to be 0.040.

Values of Kd were back-calculated for Tc on Lake Agassiz clay and backfill under anoxic conditions from Equation (4). (The values of Do and T given above were used for the calculation, along with the pb and Da values in Table 3 and a value of 0.33 for ne (determined in Section 5.2)). The results are presented in Table 6. The following assumptions were made: (1) both Do and x are the same in oxic and anoxic environments; 3 (2) ne of Lake Agassiz clay at pb = 1.27 Mg/m under oxic conditions is equal to that of 3 3 both Lake Agassiz clay at pb = 1.1 Mg/m and backfill material at pb = 1.9 Mg/m in 3 anoxic environments; and (3) x is the same for Lake Agassiz clay at pb = 1.1 Mg/m and 3 for backfill at pb = 1.9 Mg/m .

TABLE 6

CALCULATED Kd AND r VALUES FOR Tc ON LAKE AGASSIZ CLAY AND BACKFILL MATERIALS UNDER ANOXIC CONDITIONS

Soil Kd (Mg/m3) (um!/s) 3 (m3/Mg) Lake Agassiz Clay 1.08 0.0019' 9500 10000 Backfill (<150-um granite) 1.83 0.0062 1700 3100 Backfill (<4750-nm granite) 1.93 0.0032 3200 6200

1 Mean values from Table 3.

The non-zero Kd values that were calculated imply that there was sorption of Tc in the anoxic diffusion experiments. However, the results of these calculations do not distinguish between sorption and precipitation. If precipitation occurred in these experiments, the Da values that were measured would be lower, and the calculated Kd values would be higher, than the true values. Oscarson et al. (1994) reported no sorption of Tc on Lake Agassiz clay or several granite particle size fractions under oxic conditions. The suggestion made earlier that sorption of Tc on Lake Agassiz clay and backfill is likely one of the reasons that lower Da values were obtained under anoxic than oxic conditions is supported by the Kd results. - 18- The denominator of Equation (4) is often termed the capacity factor and is ususally designated r:

r = ne+PbKd. (10)

The value of r is a measure of the ability of the solution and clay to hold a diffusing species and, hence, slow its movement. Values of r were calculated for Tc on Lake Agassiz clay and backfill material from the Kd data, and the results are presented in Table 6.

Johnson et al. (1994) describe a mathematical model for radionuclide transport in a nuclear fuel waste disposal vault and specify ranges of values for the model parameters, including r. The values of r for Tc listed in Table 6 are greater than the mean, but less than the upper bound, used by Johnson et al. (1994). Assuming that anoxic conditions will exist in the vault, the model for radionuclide transport is conservative with respect to r for Tc in backfill material. Therefore, the vault model described by Johnson et al. (1994) overestimates the rate of Tc movement and the possible adverse consequences.

5.2 OXIC CONDITIONS

2 The Da value of 55 |im /s from the open-cell experiment with Avonlea bentonite under oxic conditions (Table 4) can be compared to those in the literature. Sawatsky and Oscarson 2 (1991), for example, reported mean Da values of 180 um /s for Tc in Avonlea bentonite at 3 2 3 pb = 1.13 Mg/m and 89 pm /s at 1.35 Mg/m . The difference in the results of the two studies is not large, especially considering that the technique that was used here is not particularly accurate.

2 Oscarson et al. (1994) reported a mean Da value of 59 um /s in oxic Lake Agassiz clay at 1.1 Mg/m3 using closed cells. The value of 48 pm2/s given in Table 4 was obtained at a higher density of 1.28 Mg/m3 and is, therefore, reasonable.

No value of Da for Tc in illite-smectite was found in the literature. However, the hydrated radius of "TcO; is » 0.35 nm (the mean of the hydrated radii of MnOâ and ReO;) and that of 125I" is 0.331 nm (Nightingale 1959). Therefore, since 99Tc0; and 12*T have similar sizes and the same charge, they would be expected to have comparable diffusivities. Oscarson 2 125 3 (1994b) reported a value of 140 um /s for I" in illite-smectite at pb = 1.25 Mg/m . The 2 99 3 value of 74 um /s reported here for Tc in illite-smectite at pb = 1.18 Mg/m is slightly lower, but this may be partly due to the greater size of "TCO4 relative to I2ÎI".

The total porosity of a soil plug, n, can be obtained from

n«l-£ . (11) - 19- The effective porosity is less than n; dead-end pores, for example, do not contribute to diffusive transport. Therefore, since ne is < 1, De for a non-sorbing diffusant should be less than Da (Equations (5) and (8)). The De values for Tc in Avonlea bentonite and Lake Agassiz clay under oxic conditions are 10 and 16 nm2/s, respectively (Table 4), which are indeed less than the corresponding Da values.

The ratio of DJDa for a non-sorbing diffusant gives an estimate of ne (Equations (5) and (8)). The calculated ne values for Avonlea bentonite and Lake Agassiz clay are 0.18 and 0.33, respectively (Table 4), or 40 and 60% of n. The ne value for Avonlea bentonite is close to that obtained on the same clay, at a similar density, from I" diffusion data (Oscarson et al. I992).

At the 95% confidence level, there is no statistically significant difference between the mean Dc and Da values for Tc in oxic illite-smectite (Table 4). This implies either Kd > 0 for Tc on illite-smectite under oxic conditions or, more likely, that the Da value reflects the inaccuracy of the time-lag technique.

The Da values for Tc obtained for the three clays tested in this study are similar (Table 4). However, there is a difference of about an order of magnitude between the Dc value for illite-smectite and those for the other two clays. Oscarson (1994b) reported that De values for several diffusants—Sr2*, Ca2+, Na+ and I"--were greater by a factor of about two to four in illite-smectite than in Avonlea bentonite and Lake Agassiz clay. The difference is not great, however, and unlikely to affect the performance assessment of clay-based materials as barriers to radionuclide migration (Johnson et al. 1994).

6. SUMMARY AND CONCLUSIONS

Both apparent, Da, and effective, De, diffusion coefficients were measured for Tc in clays and clay-based materials under anoxic and oxic conditions. The Da values for Tc in these materials are much lower in anoxic than oxic environments. The calculated Kd values for Tc on anoxic soils are higher than the values reported for oxic soils. The capacity factor calculated for Tc on backfill material is greater than the mean value used in the model used to describe radionuclide transport in a nuclear fuel waste disposal vault. Under oxic 2 conditions, Da values for Tc are about 50 to 70 um /s in the three clays, namely Avonlea 3 bentonite, Lake Agassiz clay and an illite-smectite interstratified clay, at pb « 1.2 Mg/m . The De value for Tc in illite-smectite, however, is about an order of magnitude greater than that in either Avonlea bentonite or Lake Agassiz clay. The greater De value in illite- smectite is consistent with results reported for other diffusants. The results from this study have practical implications for the disposal of radioactive waste in Canada. Anoxic conditions are expected to exist in an underground repository in the granite rock present in the Canadian Shield. Under these conditions, sorption of Tc would occur on the clay-based barriers and diffusion of Tc would be slow. -20- ACKNOWLEDGEMENTS

I thank D.W. Oscarson, F. King and A.W.L. Wan for constructive reviews of the manuscript. The Canadian Nuclear Fuel Waste Management Program is jointly funded by AECL and Ontario Hydro under the auspices of the CANDU Owners Group.

REFERENCES

AECL. 1994. Environmental impact statement on the concept for disposal of Canada's nuclear fuel waste. Atomic Energy of Canada Limited Report, AECL-10711.

Allard, B., H. Kigatsi and B. Torstenfelt. 1979. Technetium: reduction and sorption in granitic bedrock. Radiochemical and Radioanalytical Letters 37, 223-230.

Cho, W.J., D.W. Oscarson and P.S. Hahn. 1993. The measurement of apparent diffusion coefficients in compacted clays: an assessment of methods. Applied Clay Science 8, 283-294.

Crank, J. 1975. The Mathematics of Diffusion. 2nd ed. Clarendon Press, Oxford, England.

Gascoyne, M. 1988. Reference groundwater composition for a depth of 500 m in the Whiteshell Research Area - Comparison with synthetic groundwater WN-1. Atomic Energy of Canada Limited Technical Record, TR-463.*

Helfferich, F. 1962. Ion Exchange. McGraw-Hill, New York, NY.

Hume, H.B. 1993. Procedures and apparatus for measuring diffusion and distribution coefficients in compacted clays. Atomic Energy of Canada Limited Report, AECL-10981.

Ito, K. and T. Kanno. 1988. Sorption behaviour of carrier-free technetium-95m on minerals, rock and backfill materials under both oxidizing and reducing conditions. Journal of Nuclear Science and Technology 25, 534-539.

Johnson, L.H., D.M. LeNeveu, D.W. Shoesmith, D.W. Oscarson, M.N. Gray, R.J. Lemire, and N.C. Garisto. 1994. The disposal of Canada's nuclear fuel waste: the vault model for postclosure assessment. Atomic Energy of Canada Limited Report, AECL-10714.

Kemper, W.D. 1986. Solute Diffusivity. In Methods of Soil Analysis, Part 1, Second edition. (A. Klute, editor), American Society of Agronomy, Madison, WI.

Lemire, R.J. and F. Garisto. 1989. The solubility of U, Np, Pu, Th and Tc in a geological disposal vault for used nuclear fuel. Atomic Energy of Canada Limited Report, AECL-10009. -21 - Meyer, R.E., W.D. Arnold, F.I. Case and G.D. O'Kelley. 1988. Thermodynamic properties of Tc(IV) oxides: solubilities and the electrode potential of the Tc(VII)/Tc(IV)-oxide couple. Oak Ridge National Laboratory Report, ORNL-6480.

Nightingale, E.R. Jr. 1959. Phenomenological theory of ion solvation. Effective radii of hydrated ions. Journal of Physical Chemistry 63, 1381-1387.

Oscarson, D.W. 1994a. Comparison of measured and calculated diffusion coefficients for iodide in compacted clays. Clay Minerals 29, 145-151.

Oscarson, D.W. 1994b. Surface diffusion: is it an important transport mechanism in compacted clays? Clays and Clay Minerals 42, 534-543.

Oscarson, D.W. and D.A. Dixon. 1989. Elemental, mineralogical, and pore solution composition of selected Canadian clays. Atomic Energy of Canada Limited Report, AECL-9891.

Oscarson, D.W. and H.B. Hume. 1993. On the smectite-to-illite reaction. Atomic Energy of Canada Limited Report, AECL-10842.

Oscarson, D.W., H.B. Hume and J.-W. Choi. 1994. Diffusive transport in compacted mixtures of clay and crushed granite. Radiochimica Acta 65, 189-194.

Oscarson, D.W., H.B. Hume, N.G. Sawatsky and S.C.H. Cheung. 1992. Diffusion of iodide in compacted bentonite. Soil Science Society of America Journal 56, 1400-1406.

Pourbaix, M. 1966. Atlas of Electrochemical Equilibria. Pergamon Press, New York, NY.

Sawatsky, N.G. and D.W. Oscarson. 1991. Diffusion of technetium in dense bentonite under oxidizing and reducing conditions. Soil Science Society of America Journal 55, 1261-1267.

Schwochau, K. and L. Astheimer. 1962. Conductometric determination of the diffusion coefficient of pertechnetate ions in aqueous solution. Zeitschrift Fuer Naturforschung A. 17a, 820.

Tait, J.C., I.C. Gauld and G.B. Wilkin. 1989. Derivation of initial radionuclide inventories for the safety assessment of the disposal of used CANDU fuel. Atomic Energy of Canada Limited Report, AECL-9881.

* Internal report available from SDDO, AECL, Chalk River Laboratories, Chalk River, ON, Canada KOJ 1J0 -22-

TABLE 1A

EX AMPLE OF DATA FROM A 217 DAY ANOXIC CLOSED-CELL DIFFUSION 9îm 3 EXPERIMENT WITH Tc IN ph = 1.93 Mg/m BACKFILL MATERIAL MADE WITH <150-wn GRANITE

5 9 Wet Dry Water Calc. Slice x/L Radio- Cone. Calc. Calc. c/c0 Clay1 Clay Content2 Dry Location activity (c)s Cone.7 Radio- (g) (g) (%) Clay3 (x)4 (Bq) (Bq/g) (Bq/g) activity8 (g) (cm) (Bq)

Initially Unlahelled Soil Plug

1.694 1.484 14.2 0 2.390 2.093 3.75 0.938 0 0 0.000

1.541 1.350 14.1 0 0 1.161 1.018 0 0

2.204 1.918 14.9 0 1.726 1.502 2.96 0.740 0 0 0.000

1.074 0.924 16.2 0 0 1.616 1.391 0 0

0.971 0.830 17.0 0 1.690 1.444 2.25 0.563 0 0 0.000

2.272 1.988 14.3 0 0 1.678 1.468 0 0

1.220 1.047 16.5 0 1.799 1.544 1.45 0.363 0 0 0.000

1.522 1.326 14.8 0 0 1.654 1.441 0 0

1.837 1.602 14.7 0 1.467 1.279 0:70 0.175 0 0 0.000

2.280 2.005 13.7 4.49 1.923 1.691 0.25 0.063 3.79 2.24 0.055

33.719 14.474 14.871 3.79 4.49

continued.. -23-

TABLE 1A (continued)

J 9 Wet Dry Water Calc. Slice x/L Radio- Cone. Calc. Calc. c/c0 Clay1 Clay Content2 Dry Location activity (c)6 Cone.7 Radio- (g) (g) <%) Clay5 (x)4 (Bq) (Bq/g) (Bq/g) activity8 (g) (cm) (Bq)

Labelled Soil Plug

1.170 0.986 18.7 41.9 0.776 0.654 -0.12 -0.030 27.8 42.5 1.039

1.985 1.711 16.0 65.9 1.782 1.536 -0.49 -0.124 59.1 38.5 0.941

1.870 1.614 15.9 40.5 65.4 1.411 1.217 40.5 49.3

1.349 1.165 15.8 49.6 1.415 1.222 -1.35 -0.341 52.0 42.6 1.042

1.278 1.105 15.7 40.3 44.5 1.929 1.667 40.3 67.2

1.592 1.378 15.5 52.5 1.514 1.311 -2.15 -0.543 49.9 38.1 0.932

1.228 1.056 16.3 40.8 43.1 1.891 1.626 40.8 66.3

1.107 0.957 15.7 41.7 2.046 1.768 -2.97 -0.750 77.1 43.6 1.066

0.935 0.800 16.9 41.9 33.5 1.872 1.601 41.9 67.1

1.970 1.738 13.3 69.9 1.468 1.296 -3.77 -0.952 52.1 40.2 0.983

30.588 12.510 13.898 318.0 757.9

continued.. -24-

TABT.R 1A (concluded)

1 The numbers below are paired because each soil slice was split into two sub-samples. One sub- sample was dried to determine the moisture content, and the other sub-sample was used for radioactivity analysis. 2 Calculated from Equation (1). 3 Calculated Dry Clay = Wet Clay / (1 + (Water Content (%) / 100)). 4 Slice location (x) is the distance between the interface and the centre of the slice. The initially unlabelled soil plug is on the positive side of the interface and the labelled plug is on the negative side of the interface. 5 L is the plug length. The initially unlabelled plug was 3.998 cm long and the labelled plug was 3.9*0 cm long. 6 Concentration (c) = Radioactivity / Calculated Dry Clay. 7 The calculated concentration for a slice is the mean of the concentrations of the two adjacent slices. 8 Calculated Radioactivity = Calculated Concentration x Dry Clay, or Calculated Radioactivity = Calculated Concentration x Calculated Dry Clay, as appropriate. 9 c0 is the mean of those c results for the labelled soil plug that are similar in value. In the example presented here, all the c values for the labelled soil plug were included in the calculation of c0 (c0 = 40.9 Bq/g). -25-

TABLE 2A EXAMPLF OF DATA FROM AN OXIC OPEN-CEIX DIFFUSION EXPERIMENT 99 3 WITH Tc IN pb - 1. 16 Mg/m ILLÏTR-SMECTÏTI« rPLTJG LENGTH = 1 9.8 cmï

Time «Tc Volume "Tc1 Cumulative Q/A2 (Days) Concentration (mL) (kBq) "Tc (mmol/m2) (Bq/mL) (Q) (kBq)

0 0 0 0 0 0 1 15 10 0.15 0.15 0.0018 2 114 44 5.0 5.2 0.062 5 355 96 34 39 0.47 6 424 37 16 55 0.66 7 411 35 14 69 0.83 8 465 33 15 84 1.0 9 539 27 15 99 1.2 12 527 102 54 153 1.84 13 530 35 19 172 2.06 14 538 46 25 197 2.36 15 518 27 14 211 2.53 16 580 30 17 228 2.74 19 704 87 61 289 3.47 20 784 30 23 312 3.75 21 911 21 19 331 3.97 22 1140 15 17 348 4.18 23 1320 15 20 368 4.42 26 1400 39 55 423 5.08 27 1980 10 20 443 5.32 28 2900 4.0 12 455 5.46 29 3450 1.5 5.2 460 5.52 30 1860 17 32 492 5.91 33 2440 9.0 22 514 6.17 34 3590 1.5 5.4 519 6.23 35 674 41 28 547 6.57 36 570 35 20 567 6.81 37 468 35 16 583 7.00 41 516 117 60 643 7.72 42 591 26 15 658 7.90 43 495 37 18 676 8.11 44 433 39 17 693 8.32 47 487 111 54 747 8.97 48 557 26 14 761 9.14

continued. -26- TABLE 2A (concluded)

Time "Tc Volume "Tc1 Cumulative Q/A2 (Days) Concentration (mL) (kBq) ^c (mmol/m2) (Bq/mL) (Q) (kBq)

49 543 30 16 111 9.33 50 542 34 18 795 9.54 54 644 140 90 885 10.62 55 725 15 11 896 10.76 56 737 28 21 917 11.01 57 789 24 19 936 11.24 58 884 20 18 954 11.45 61 973 54 53 1007 12.09 62 1160 14 16 1023 12.28 63 1450 10 14 1037 12.45

_ 99Tc concentration x Volume. 2 A is the cross-sectional area of the clay plug (1.335 x 10'3 m2). The specific activity of 99Tc is 62.4 GBq/mol. AECL-1H19 COG-95-428

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