Progression of performance assessment modeling for the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

• Evolution of the consequence models to simulate physical processes that influence performance of the proposed repository at Yucca Mountain is discussed.
• Evolution of the consequence models between 1984 and 2008 is discussed by noting the addition and linkage of computational modules.
• Evaluation of cumulative release by modeling waste degradation and flow/transport in the saturated and unsaturated zones is depicted for early PAs.
• Elaborate modeling of fluid flow, infiltration, drift seepage, chemical environment, and biosphere transport for the 1998 viability assessment is depicted.
• Refinement of models for the site recommendation and the addition of seismicity for the license application are discussed.

This paper summarizes the evolution of consequence modeling for a repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain in southern Nevada. The discussion includes four early performance assessments (PAs) conducted between 1982 and 1995 to support selection and to evaluate feasibility and three major PAs conducted between 1998 and 2008 to evaluate viability, recommend the site, and assess compliance. Modeling efforts in 1982 estimated dose to individuals 18 km from the site caused by volcanic eruption through the repository. Modeling in 1984 estimated releases via the groundwater pathway because of container corrosion. In combination, this early analysis supported the first environmental assessment. Analysts in 1991 evaluated cumulative release, as specified in the 1985 US radiation protection standards, via the groundwater pathway over 104 yr at a 5-km boundary by modeling waste degradation and flow/transport in the saturated and unsaturated zones. By 1992, however, the US Congress mandated a change to a dose measure. Thus, the 1993 and 1995 performance assessments improved modeling of waste container degradation to provide better estimates of radionuclide release rates out to 106 yr. The 1998 viability assessment was a major step in modeling complexity. Dose at a 20-km boundary from the repository was evaluated through 106 yr for undisturbed conditions using more elaborate modeling of flow and the addition of modules for modeling infiltration, drift seepage, the chemical environment, and biosphere transport. The 2000 assessment for the site recommendation refined the analysis. Seepage modeling was greatly improved and waste form degradation modeling included more chemical dependence. The 2008 compliance assessment for the license application incorporated the influence of the seismicity on waste package performance to evaluate dose at an ~18-km boundary.