A great deal of effort has been devoted to creating highly durable nuclear waste forms, ranging from borosilicate glasses to crystalline solids, based on the unquestioned assumption that a slow waste-form dissolution rate would effectively limit radionuclide release from a geologic repository. However, we here demonstrate that from the perspective of long-term repository performance this assumption is not entirely valid, and that continued development of incrementally more durable waste forms should be re-evaluated. Radionuclide release from a repository may occur in two stages: stage I, in which the dissolved concentration of a radionuclide has not reached its solubility limit and radionuclide release is controlled by the dissolution rate of the waste form; and stage II, in which the radionuclide concentration has reached the solubility limit imposed by a stable degradation product, at which point radionuclide release becomes independent of the waste-form dissolution rates. Due to high solid/water ratios and slow groundwater flow rates in most proposed repository environments, groundwater contacting waste forms will rapidly (within a few hundred years) reach chemical equilibrium with radionuclide-bearing degradation products. Total release of radionuclides from a repository is thus primarily limited by the solubilities of waste form degradation products, rather than by the dissolution rates of primary waste forms. Therefore, future waste-form development should be increasingly focused on conditioning waste forms and/or repository environments in order to minimize radionuclide solubilities, rather than on striving for marginal improvements in the durability of primary waste forms. The proposed chemical conditioning concept may allow us to design waste forms or backfill materials that include chemical components that can precipitate low-solubility radionuclide-bearing solids that would not form otherwise.

This work was performed at Sandia National Laboratories, which is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the DOE under contract DE-AC04-94AL8500.