REPOSITORY-HEAT-DRIVEN HYDROTHERMAL FLOW AT YUCCA MOUNTAIN .1. MODELING AND ANALYSIS

To safely and permanently store high-level nuclear waste, the potential Yucca Mountain repository site must mitigate the release and transport of radionuclides for tens of thousands of years. In the failure scenario of greatest concern, water would contact a waste package, accelerate its failure rate, and eventually transport radionuclides to the water table. Analyses have demonstrated that (a) the ambient hydrological system will be dominated by repository-heat-driven hydrothermal flow for tens of thousands of years and (b) the only significant source of liquid water is from nonequilibrium fracture flow, driven either by meteoric sources or by the condensation of repository-heat-driven flow of water vapor. For sub-boiling conditions, the infiltration of meteoric water and condensate drainage are controlled by the highly heterogeneous distribution of hydrological properties, while for above-boiling conditions, they are largely determined thermodynamically. In a concept called the ''extended-dry repository, '' the heat of radioactive decay generates a region of above-boiling temperatures around the repository, thereby extending the time before liquid water can contact a waste package. It is also found that the magnitude of repository-heat-driven, buoyant, liquid-phase convection in the saturated zone is more dependent on the total mass of emplaced spent nuclear fuel (SNF) than on the details of SNF emplacement, such as the areal power density (expressed in kilo watts per acre) or SNF age.