Prediction of waste performance in a geologic repository

The rate of dissolution of low-solubility species from waste forms in a geologic repository can be calculated from a theoretical analysis of the time-dependent rate of mass transfer by diffusion and convection into the groundwater surrounding the waste, assuming a concentration at the waste-form surface equal to the solubility of the radioelement. The predicted steady-state dissolution rates are considerably below those observed in laboratory leaching experiments with borosilicate glass and with other waste forms, indicating that the solid-liquid chemical reaction rates measured in the laboratory experiments are greater than the rates of diffusive-convective mass transfer in the concentration boundary layer surrounding the waste form in a geologic repository. The steady-state mass transfer rate can be increased, and the time to reach steady state decreased, by sufficiently short half lives of the dissolving species. The mass-transfer theory has been extended to include the effect of time-dependent solubilities, diffusion coefficients, and retardation coefficients, which provides a means of calculating the time-dependent dissolution of low-solubility species from waste exposed to groundwater during the period of repository heating. The transient and steady-state diffusion of radionuclides through a finite backfill layer separating a finite waste solid and porous rock has been analyzed, including the effects of radioactive decay. The results show that the break-through time and rate of radionuclide release depend on properties of the backfill and surrounding rock and on the waste form dimensions. Peak far-field concentrations of more soluble radionuclides such as cesium-135, with suitably long radionuclide transport times and sufficiently large axial dispersion, are shown to be insensitive to dissolution rate. Equivalent phenomena occur in fracture-flow radionuclide transport. 36 references, 16 figures, 2 tables