Total-System Performance Assessment for the Yucca Mountain Site
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Total-System Performance Assessment for the Yucca Mountain Site Michael L. Wilson,1 Peter N. Swift,1 Jerry A. McNeish,2 S. David Sevougian2 1 Total System Performance Assessment Department, Sandia National Laboratories, Albuquerque, NM 87185-0778, U.S.A. 2 Duke Engineering and Services, Inc., 1180 Town Center Drive, Las Vegas, NV 89144, U.S.A. ABSTRACT Yucca Mountain, Nevada, is under consideration as a potential site for a repository for high-level radioactive waste. Total-system performance-assessment simulations are performed to evaluate the safety of the site. Features, events, and processes have been systematically evaluated to determine which ones are significant to the safety assessment. Computer models of the disposal system have been developed within a probabilistic framework, including both engineered and natural components. Selected results are presented for three different total-system simulations, and the behavior of the disposal system is discussed. The results show that risk is dominated by igneous activity at early times, because the robust waste-package design prevents significant nominal (non-disruptive) releases for tens of thousands of years or longer. The uncertainty in the nominal performance is dominated by uncertainties related to waste-package corrosion at early times and by uncertainties in the natural system, most significantly infiltration, at late times. INTRODUCTION The U.S. Department of Energy (DOE) is studying Yucca Mountain, Nevada, as a potential site for disposal of high-level radioactive waste. The site has been the subject of an extensive site-characterization effort, and a series of total-system performance assessments (TSPAs) has been conducted over the past decade, with increasing complexity and detail in the models used for the assessments. The general approach for conducting a TSPA is to (1) identify and screen potentially relevant features, events, and processes to develop scenarios, (2) develop models, (3) estimate parameter ranges and uncertainties, (4) perform calculations, and (5) interpret results. Some of these steps can be carried out in parallel, and the procedure generally must be repeated iteratively as knowledge is gained. The TSPA model for Yucca Mountain includes numerous submodels for natural systems, engineered systems, and the interactions between them. Disruptive events are also modeled (primarily igneous activity, but with consideration of other possible disruptive events as well). As implied by step (3), a probabilistic approach is used, in which uncertainties are propagated through the system so that the effects of uncertainties on the final results can be analyzed. Two major TSPA studies have been completed recently. They are documented in Total System Performance Assessment for the Site Recommendation (TSPA-SR) [1] and Supplemental Science and Performance Analyses (SSPA) [2]. This paper provides a brief summary of those studies. Regulatory context is important for TSPA since one of the purposes of TSPA is to determine whether the site complies with th
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