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

Abstract

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 and 241Am in water-saturated compacted sodium-bentonite whose dry densities were 0.2–2.0 (x103kg/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)3 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.