Radionuclide Retention in Geologic Media: Workshop Proceedings, Oskarshamn, Sweden, 7-9 May 2001

Currently, performance assessment (PA) modelling for nuclear waste repositories reliesheavily on the Kd approach which empirically averages contaminant retardation over a range ofconditions in the hope that uncertainties are treated conservatively. Mechanistic approaches arerare or non-existent, often because of lack of information. However, recently developed high-resolution, surface analytical techniques offer direct observation at nanometre scale, meaning weare now able to observe molecular-level processes and describe them conceptually. Thisinformation could be incorporated to make PA models more realistic by considering processes thatimmobilise (on relevant time-scales) rather than simply retard contaminants. Processes responsiblefor contaminant uptake include adsorption, co-precipitation, and the formation of solid solution.Release processes include desorption and dissolution. Exsolution of components of a solidsolution and transformation of a mineral to a more stable phase can make previously incorporatedcontaminants more available for release again. Immobilisation occurs when the rates of the releaseprocesses are negligible compared to the rates of the uptake processes.High-resolution techniques show that: dynamic equilibrium constantly rearranges material,allowing incorporation of trace components by reprecipitation even on dry surfaces; a contaminantcan adsorb to a surface that is actively dissolving; co-precipitated compounds can be liberatedduring mineral transformation; trace components can hide in colloidal-size particles or as thincoatings that are impossible to detect with classical methods; and material adsorbed at surfacescan diffuse into the bulk where it is less available for release and the rate of movement depends onthe defect density of the solid. Examples for illustration were chosen from the work of our groupon the Fe-(oxy)hydroxide and Ca-carbonate systems because of the common occurrence of theseminerals in the environment and because of their ability to take up radioactive components.