Long-Term Performance Assessment for a Proposed High-Level Radioactive Waste Disposal Site at Yucca Mountain - Explanati
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ABSTRACT The Nuclear Regulatory Commission (NRC) Total-system Performance Assessment (TPA) code is a tool to independently evaluate the long-term performance of the proposed Yucca Mountain (YM) high-level radioactive waste (HLW) repository by modeling processes such as the dissolution of the spent nuclear fuel (SNF) and the subsequent transport through unsaturated and saturated highly heterogeneous fractured porous media to a hypothetical release into the biosphere. The release rates of radionuclides into the biosphere corresponding to some of the SNF dissolution models show a sinusoidal trend with an overall decrease in the rate with time; for other models the sinusoidal behavior is nonexistent. This study identifies two key mechanisms contributing to these trends: the spatial discretization of the repository into subareas and radionuclide-specific sorption properties in the saturated zone alluvium. Other mechanisms which might affect the release rates, such as solubility limits, radioactive decay and ingrowth, inventory depletion, and transport properties of the unsaturated zone (UZ), do not significantly contribute to these trends. INTRODUCTION The NRC has the responsibility to review the license application for the HLW repository site at YM. In support of its regulatory review activities, the NRC staff has focused on detailed technical evaluation [1,2] to understand and quantify the isolation characteristics and capabilities of the proposed YM repository system. To support these technical assessments, the NRC and the Center for Nuclear Waste Regulatory Analyses (CNWRA) recently developed Version 3 of the TPA code [3]. YM is located in a semi-arid environment on the Nevada Test Site in southern Nevada, and rises several hundred meters above the surrounding land. The current design for the YM repository is to dispose of SNF in waste packages (WPs) emplaced in drifts approximately 300 m below the top of YM and 300 m above the water table. Following emplacement in the repository, the WPs will eventually fail, and the infiltrating water will contact the SNF in the WP and cause SNF dissolution. After the SNF dissolves into the contacting water, the flowing water will transport radionuclides out of the engineered barrier system (EBS), which is comprised of the WP, concrete invert that supports the WP, and possibly shields and backfill, through the hydrologically UZ and saturated zone (SZ) to a hypothetical release into the biosphere 20 km from the repository footprint. Because of the high level of uncertainty in projecting the probable evolution of the repository, alternative conceptual models are used to evaluate the performance of the YM repository. To represent uncertainty in predicting the SNF dissolution rate, which is one of several key factors in assessing repository performance, four alternative conceptual models have been proposed [3] to characterize the nature of the time evolution of releases from the EBS. While the EBS releases show an expected decreasing trend for each of the SNF dissolution rate models,
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