The Equilibrium Concept—An Extended Approach to Assess Long-Term Radionuclide Release from Repositories

Abstract

The release of radionuclides from a waste form into an aqueous phase is often assessed using a source term that considers diffusion and/or congruent matrix dissolution as the rate-determining release mechanisms. As an alternative approach, an equilibrium concept is proposed here that can be applied under the condition that there is no appreciable exchange of fluid with the environment of the waste package / form after the water inflow into the near field for a long time. In this case, all reactions that may give rise to radionuclide release will be completed after a certain time and stable final conditions will be established, in which, for each radionuclide, chemical equilibria exist between the dissolved phase and the various coexisting solid phases. Thereafter, a release of radionuclides from the near field is possible only by escape of the aqueous phase into the environment.Release rate predictions on the basis of this concept are of particular interest for the long-lived radionuclides, especially the actinides. Current efforts are aimed at predicting equilibrium concentrations both in theoretical computations and in experimental measurements.Some results available from corrosion studies on cemented waste forms in salt brine are presented. For specimens doped with cesium, strontium, plutonium, or americium these results show that for each radionuclide a partition equilibrium exists between the corrosion products of cement and the surrounding salt brine, which keeps the concentration in solution at a low level.