Diffusion of Radionuclides in Compacted Bentonite: Results from Combined Glass Dissolution and Migration Tests
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DIFFUSION OF RADIONUCLIDES IN COMPACTED BENTONITE : RESULTS FROM COMBINED GLASS DISSOLUTION AND MIGRATION TESTS MASAKI TSUKAMOTO*, T. OHE*, T. FUJITA*, R. HESBOL**, and H-P. HERMANSSON*** * Central Research Institute of Electric Power Industry, 2-11-1 Iwado-Kita, Komae, Tokyo 201, Japan ** Studsvik Radwaste AB, S-611 82 Nykdping, Sweden *** Studsvik Material AB, S-611 82 Nykdping, Sweden ABSTRACT Diffusion experiments of radionuclides in compacted sodium bentonite with a dry density of 1.0 g/cm 3 were performed in nitrogen gas atmosphere at 90 °C for 208 d and 375 d. The corrosion experiments of crushed radioactive glass, JSS-A, carried out simultaneously to provide the source of the radionuclides for the diffusion experiments. The normalized elemental mass losses of cesium isotopes and 38Pu were lower than those of boron (ca. 10 g/m 2) probably because of the difference of sorption and/or precipitation. The apparent diffusion coefficients of 2 38Pu, 23U and 12Sb were determined to be 2x 101 4 m 2/s, 5x10-' 2m 2/s and 2x1012 m 2/s, respectively. The distribution coefficient of Pu estimated from the diffusion data was of the same order as that from batch sorption experiments. The glass corrosion and the plutonium diffusion were described by the geochemical codes PHREEQE, STRAG4 and GESPER. The calculation results well fitted the observed data. INTRODUCTION The radionuclide migration behavior in the engineered barrier of a high-level waste disposal site is known to be governed by the chemical conditions occurring in the geosphere. Many chemical models describing phenomena such as corrosion of the glass waste, radionuclide sorption, etc, have therefore been investigated. We have developed chemical modeling codes to predict glass dissolution and radionuclide migration in porous media, called STRAG4 [11 and GESPER [21, respectively. Experimental data have been tested to examine the validity of these codes [1,2]. However, further tests are required to demonstrate the validity of the codes. In the present paper we describe the examination of radionuclides diffusion in a bentonite combined with glass corrosion experiments. Radioactive glass powder was used as a source of radionuclides migrating into the bentonite bed. The plutonium diffusion and glass corrosion behaviors were modelled using these codes. The surface complexation model has been used to interpret radionuclide sorption [3, 4, 5] and migration [2, 6] and its applicability has been demonstrated. This interpretive model is still in the early stages of development and further application ins needed. The surface complexation model was combined with diffusion to calculate the plutonium migration in the bentonite bed using GESPER. EXPERIMENTAL Figure 1 shows the glass corrosion-migration cell used in the experiments. Japanese sodium bentonite (Kunigel-Vl , Kunimine Co. Ltd., Japan) powder was poured into the migration cell to a packing density of 1 g/cm 3. The pore space of the bentonite bed was filled with preequilibrated bentonite water in the same manner as the pr
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