THE ROLE OF DIFFUSION AND ITS TECHNICAL BASES IN THE ASSESSMENT OF PERFORMANCE OF THE POTENTIAL HIGH-LEVEL WASTE REPOSITORY AT YUCCA MOUNTAIN, NEVADA

Advection and diffusion are used in the Department of Energy's (DOE's) performance assessment of the potential Yucca Mountain high-level radioactive waste repository to describe transport of radionuclides. Both physical characteristics of the potential repository site and the proposed repository design influence the extent to which diffusion occurs. Radionuclide movement via diffusion is modeled in three main areas by the DOE: transport through the engineered barrier system; transport in the unsaturated zone; and transport in the saturated zone. In the DOE's performance assessment radionuclide release from waste packages is diffusive until waste packages and drip shields are sufficiently degraded to allow advective flow into and out of waste packages. Currently the DOE models the diffusive release in a manner that maximizes the diffusive release from the package. Radionuclide transport through the welded tuffs in the unsaturated and saturated zones at Yucca Mountain is modeled by the DOE using an active fracture model. As a result of their current approach, matrix diffusion is an important radionuclide retardation mechanism in the unsaturated zone.

The Nuclear Regulatory Commission (NRC) is an independent regulatory agency and evaluates the sufficiency of the DOE's performance assessments, including the supporting data. Where the NRC judges, based upon a risk-informed, performance-based approach, that the DOE's approach and technical bases is inadequate, the NRC requires that the DOE provide additional information. For each of the three main areas where diffusion is used by the DOE to model radionuclide transport the NRC has determined that additional technical bases are needed. Because the degradation geometry of waste packages controls whether diffusive or advective transport is dominant, the NRC has requested that the DOE provide additional data to support their characterization of the waste package degradation processes and the resultant geometry of the degradation. The NRC has requested additional information from the DOE to support its approach to matrix diffusion and retardation in the unsaturated and saturated zones. For instance, pre-test predictive analyses and test plans of proposed DOE field tests, which are designed to assess the extent of matrix diffusion, have been requested by the NRC.