Diffusive release of radionuclides into saturated and unsaturated tuff

In the proposed Yucca Mountain repository, packages of high-level waste are separated from surrounding unsaturated rock by a 2-cm air-gap annulus. If a waste package contacts the surrounding rock, if the annulus becomes filled with rock and rubble, or if the rock becomes saturated, there can exist pathways for release of dissolved radionuclides by liquid diffusion. We have applied the analytical solutions for time-dependent mass transfer of dissolved species through porous rock to predict the time-dependent release rates of key radionuclides. The expected ground-water velocities are so low that mass-transfer is predicted to occur predominantly by molecular diffusion in pore liquid. Conservatively, we neglect mass-transfer resistance from fuel cladding and failed metal containers. 6 refs., 3 figs.