TSPA Model for the Yucca Mountain Unsaturated Zone

Yucca Mountain, Nevada, is being considered as a potential site for a repository for spent nuclear fuel and high-level radioactive waste. Total-system performance-assessment (TSPA) calculations are performed to evaluate the safety of the site. Such calculations require submodels for all important engineered and natural components of the disposal system. There are five submodels related to the unsaturated zone: climate, infiltration, mountain-scale flow of water, seepage into emplacement drifts, and radionuclide transport. For each of these areas, models have been developed and implemented for use in TSPA. The climate model is very simple (a set of climate states have been deduced from paleoclimate data, and the times when climate changes occur in the future have been estimated), but the other four models make use of complex process models involving time-consuming computer runs. An important goal is to evaluate the impact of uncertainties (e.g., incomplete knowledge of the site) on the estimates of potential repository performance, so particular attention is given to the key uncertainties for each area. Uncertainties in climate, infiltration, and mountain-scale flow are represented in TSPA simulations by means of discrete high, medium, and low cases, Uncertainties in seepage and radionuclide transport are represented by means of continuous probability distributions for several key parameters