Radionuclide sorption and diffusion studies

Experimental and theoretical analyses of radionuclide sorption by and diffusion in abyssal red clays are being conducted in support of activity C-2, Far-Field Ion Transport, and more specifically in support of subactivities C.2.1.1, Property Acquisition and Model Development and C.2.1.2, Laboratory and/or Field Verification. During the current reporting period, priority of effort has been given to conducting and interpreting data from one-dimensional diffusion experiments involving smectite-rich sediment and radionuclides such as 22Na, 137Cs, 133Ba, 144Ce, and 152Eu. Most data from the diffusion experiments appear consistent with results predicted using appropriate mathematical models and independently measured sorption equilibrium data. In particular, experiments with 22Na indicated that when small effects due to sorption were accounted for, the diffusion coefficient for sodium in the sediment samples was on the order of 3 x 10-6 cm2/sec (at about 230C), which appears consistent with previously reported results. Experiments with 137Cs; 133Ba, and 152Eu indicated effective diffusion coefficients of 7 x 10-10, 8 x 10-11, and 7 x 10-11 cm2/sec, respectively, for those nuclides in the sediment. Such results are consistent with the effective diffusion coefficients which would be expected from the sorption equilibrium data for those nuclides at the solution-phase concentrations involved. However, some deviations from expected behavior have been observed and are currently under investigation to determine if such deviations represent significant phenomena or are artifacts of the experimental technique