Far-field radionuclide transport investigations

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. Previously, sorption equilibrium data for rubidium, cesium, strontium, barium, cadmium, silver, technetium, cerium, promethium, europium, gadolinium, uranium, plutonium, americium, and curium were reported, and concentration profiles from diffusion experiments with sodium, cesium, barium, cerium, and europium were determined. In general, the results from the diffusion experiments were consistent with predictions based on solutions to the diffusion equation using appropriate boundary conditions and the sorption equilibrium data. However, a small fraction, usually much less than one percent, of the total amount of the nuclide used in each diffusion experiment migrated much further than predicted. The anomalies in the diffusion data for europium were considered to be of most concern, and priority of effort during the current reporting period was given to investigating those europium anomalies. Analogous experiments with other sorbents also were performed. Although a single mechanism for accelerated transport may not be appropriate for all nuclides. It appears that the anomalous europium concentration profiles represent the effects of competing chemical reactions, probably hydrolysis, which produce small amounts of transient, weakly sorbing species. It further appears that the decay times for such transient europium species are sufficiently small so that such competing reactions will have negligible impact on long-term, far-field radionuclide transport