Prediction of Waste Performance in a Geologic Repository

ABSTRACTThe 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 eanal to the solubility of the radioelement. The analytical solutions for time-dependent mass transfer are reduced to asymptotic steady-state approximations valid over specified ranges of repository conditions. 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. It is shown that the time to reach steady-state dissolution can be as short as a few years, if convective mass transfer in the concentration boundary layer is important, to many thousands of years, If mass transfer is mainly by diffusion in the groundwater. 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 theory has been extended to Include the effect of backfill on the steady-state mass transfer from the waste-form surface into moving groundwater.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 lowsolubility species from waste exDosed to groundwater during the Period of repository heating. The effects of timetemperature transient dissolution on far-field concentrations and cumulative release are calculated.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. Previous backfill analyses have neglected radioactive decay and have assumed an infinite amount of backfill. 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.Laboratory tests have been designed to validate these theoretical analyses of diffusive-convective mass transfer with solubility-limit boundary conditions. Preliminary tests are now underway at the Battelle Northwest Laboratory.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 he insensitive to dissolution rate. Equivalent phenomena occur in fracture-flow radionuclide transport.