Effect of Dry Density on Diffusion of Some Radionuclides in Compacted Sodium Bentonite

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Apparent diffusion coefficients were measured at room temperature under atmospheric condition by one-dimensional non-steady state diffusion for ³H, ⁹⁰Sr, ⁹⁹Tc, ¹²⁹I, ¹³⁷Cs, ²³⁷Np and²⁴¹Am in water-saturated compacted sodium-bentonite whose dry densities were 0.22.0 (×10³ kg/m³). Dry density of bentonite affected the diffusivity for the radionuclide and the apparent diffusion coefficient decreased with increasing dry density of bentonite. Small effect of dry density of bentonite on retardation factor which was derived from the apparent diffusion coefficient was indicated for ¹³⁷Cs, ⁹⁰Sr, ⁹⁹Tc and ¹²⁹I. On the other hand, the retardation factors of ²³⁷Np and ²⁴¹Am showed a tendency of increasing with dry density of bentonite. Dominant chemical species of ²³⁷Np and ²⁴¹Am in the pore water of bentonite were estimated NpO₂(CO)₃^5- and AmOH(CO₃)₂^2-, respectively and they possibly formed large anion. Distribution coefficient derived from the apparent diffusion coefficient showed a tendency of decreasing with increasing dry density of bentonite except for ²³⁷Np and ²⁴¹Am whose coefficients were unchanged and were higher than values from Sorption Data Base of OECD/NEA. The decrease of the distribution coefficient may be caused by the change of the ion concentration in the pore water. The higher distribution coefficients of ²³⁷Np and ²⁴¹Am may be caused by sorption, ion exclusion and molecular filtration.