Preliminary assessment of radionuclide vapor phase transport in unsaturated tuff

The possibility of radionuclide migration in the vapor phase for unsaturated tuff has been investigated. Radionuclide movement could be the result of either aerosol migration or convection/diffusion of volatile species. A diffusion model for supersaturation of air in tuff groundwater indicates that there is no possibility of aerosol formation under expected repository conditions. An assessment of migration due to convection/diffusion requires solution of the coupled heat transfer-vapor flow-liquid flow problem around a waste canister. The TOUGH code has been used to calculate spatial and temporal distributions of liquid and vapor velocity, saturation and temperature for typical tuff formation properties. These distributions have been used to calculate the relative transport rates due to convection in the vapor phase as compared to the liquid phase. An analogous mobility ratio when diffusion dominates radionuclide transport has also been calculated. As a first order approximation, the maximum ratio of transport in the vapor phase to that in the liquid phase (irregardless of transport mechanism) is given by 1000 K/sub w/ where K/sub w/ is the vapor-liquid distribution coefficient (i.e., vapor concentration/liquid concentration) for the particular radionuclide. 13 refs., 3 figs