Study on Retardation Mechanism of 3 H, 99 Tc, 137 Cs, 237 Np and 241 AM in Compacted Sodium Bentonite
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STUDY ON RETARDATION MECHANISM OF 'H,' 9Tc,' 3 7 Cs, IN COMPACTED SODIUM BENTONITE
2 3
7Np AND
241AM
H.Sato,T.Ashida,Y.Kohara and M.Yui Power Reactor and Nuclear Fuel Development Corporation, Tokai-mura, Ibaraki-ken, 319-11 JAPAN ABSTRACT The apparent diffusion coefficients were measured at room temperature (about 23*C ) under atmospheric condition by the one-dimensional non-steady state 2 in compacted sodium-bentonite diffusion method for 3 H,99Tc, '3 7 Cs, 37 Np and "Am saturated with water. Sodium-bentonite, which is commercially available as KunigelVl 3 , was used in this study. Experiments were carried out in the density range of 0.4-2.0( xlO3 kg/m3 ). Bentonite in the cell was prepared to be saturated with distilled water. The measured apparent diffusion coefficient decreases with increasing dry density of bentonite. That the apparent diffusion coefficient of 3 H decreased as a function of dry density of bentonite appears to be the effect of the change of porous structure with dry density of bentonite. 99Tc may be retarded by anion-exclusion because dominant diffusion specie of 59Tc is pertechnetate ion under atmospheric condition. Retardation for 37 1 Cs may be caused by ion-exchange on bentonite. The sorption, anion-exclusion and molecular filtration are considered as a retardation mechanism for 2 3 7 Np and 2 4 'Am because those dominant species are negatively charged and of large ionic size. INTRODUCTION Sodium-bentonite has been considered as a candidate buffer material for the geological isolation of high level radioactive waste[I]. Many laboratory experiments concerning the diffusivities of radionuclides in bentonite have been carried out[21,[4]--[11]. In those studies, it was indicated that the apparent diffusion coefficients(Da) depend on density of bentonite and pore water chemistry. The apparent diffusion coefficients(Da) were also measured for 3 H,' 0 Sr,''Tc,1 2 5 I, 3 7Cs, 2 3 7 Np and 2 4'Am under the variation of density of purified bentonite by authors and it was indicated that the diffusivities decreased with increasing dry density of bentonite[20]. To identify the performance of the buffer material in the transient period, a fundamental understanding of the diffusion of radionuclides in compacted bentonite is essential. Therefore, in this study, the apparent diffusion coefficients for 1H, 24 9 9 Tc, 1 3 7 Cs, 2 3 7 Np and 'Am in bentonite were measured in the density range of 0.4-2.0(xlOlkg/m3 ). Since 3H is a non-sorbing radionuclide on bentonite, 1H was included as a referential radionuclide to investigate porous structure of bentonite. This paper presents dependency of the apparent diffusion coefficient (Da) on dry density of bentonite, retardation mechanism derived from Da and distribution coefficients(Kd) measured by batch method. EXPERIMENTAL Sodium-bentonite used in this study is KunigelVl 0 which is commercially available. Major composition of this bentonite is Na-montmorillonite. and its content is 50-55wt%. The composition of KunigelVi( 0 is shown in Table 1 [1]. One-dimention
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