General Approach Used in the Performance Assessment for the Waste Isolation Pilot Plant
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General Approach Used in the Performance Assessment for the Waste Isolation Pilot Plant Rob P. Rechard Performance Assessment Department 6852, Sandia National Laboratories, Albuquerque, NM 87185-0776, U.S.A. ABSTRACT This paper discusses the technical approach and rationale of the performance assessments (PAs) conducted for the Waste Isolation Pilot Plant that contributed to the success of the certification in 1998. The PAs were iterated: there were four preliminary PAs between 1989 and 1992 and the certification PA in 1996. Although many changes occurred between the 1992 PA and 1996 PA, the preliminary iterations provided a strong foundation on quality assurance, parameter and model databases, documentation, and peer review. The seven general steps of a PA are used to provide a brief overview of their history. The paper then delves into the rationale used for the most computationally intense step of a PA, the consequence evaluation. For this evaluation, Sandia National Laboratories mostly used detailed models when evaluating the probabilistic performance of the repository under the stylized conditions specified by the U.S. Environmental Protection Agency (EPA). A computational tool, CAMCON, was specifically tailored for this approach. Many advantages were gained by using detailed models directly in the PA, rather than using simplified results of these models. One important advantage was that models and parameters remained fairly unbiased (i.e., the analysis had only a few major conservative assumptions rather than many, unquantified conservatisms). This approach allowed Sandia to faithfully provide a “reasonable expectation” of repository performance, as specified by the EPA. INTRODUCTION In 1973, three years before the Materials Research Society began their special sessions on nuclear waste management, the citizens in southern New Mexico invited the Atomic Energy Commission (AEC), forerunner to the U.S. Department of Energy (DOE), to explore the salt beds in the Delaware Basin for disposal of radioactive waste. By 1976, a suitable site had been found for more detailed characterization. This site became the Waste Isolation Pilot Plant (WIPP), a repository for disposal of transuranic (TRU) waste [1] that was certified by the U.S. Environmental Protection Agency (EPA) in 1998. The EPA’s decision was primarily based on the technical results from a performance assessment (PA) conducted by Sandia National Laboratories and described in the Compliance Certification Application (CCA), submitted in October 1996 [2, 3, 4]. Many technical and programmatic aspects of the project made it possible to successfully certify the WIPP geologic repository. For instance, the bedded salt had excellent properties for isolating radioactive waste (thickness, purity, little evidence of dissolution, and tectonic stability) (Figure 1) and the public in southern New Mexico supported the location of the site (low population density and absence of land use conflicts). Yet, the manner in which Sandia conducted the PA helped generate favorable sci
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