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 3 H, 90 Sr, 99 Tc, 129 1, 137 Cs, 237 Np and 241 Am in water-saturated compacted sodium-bentonite whose dry densities were 0.2-2.0(X 10 3 kg/m 3 ).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 137 Cs, 90 Sr, 99 Tc and 129 1.On the other hand, the retardation factors of 237 Np and 241 Am showed a tendency of increasing with dry density of bentonite.Dominant chemical species of 237 Np and 241 Am in the pore water of bentonite were estimated Np0 2 (C0) 3 5 -and Am0H(C0 3 )l-, respectively and thty 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 237 Np and 241 Am 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 237 Np and 241 Am may be caused by sorption, ion exclusion and molecular filtration.