Risk-Based Evaluation of Long-Term Safety for a Yucca Mountain Repository using the IMARC TSPA Code
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Risk-Based Evaluation of Long-Term Safety for a Yucca Mountain Repository Using the IMARC TSPA Code John Kessler1, Michael Apted2, Matthew Kozak2, and Wei Zhou2 1 Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto CA 94304, U.S.A [email protected]. 2 Monitor Scientific LLC, 3900 South Wadsworth Boulevard, Denver CO 80235, U.S.A [email protected]. ABSTRACT The Electric Power Research Institute (EPRI) has conducted independent total system performance assessments (TSPAs) of the proposed Yucca Mountain spent fuel and high-level radioactive waste repository for 15 years. EPRI uses its TSPA code, IMARC, currently in its eighth version. The major results of the IMARC-8 analyses are presented in this paper. In all of the situations evaluated by EPRI using IMARC-8, the results for the reference repository concept for Yucca Mountain are well within the regulatory criteria established in the applicable regulations, 10 CFR 63 and 40 CFR 197. Further analyses indicate that at least two of the repository barriers must fail to function as anticipated for dose risks to rise as much as one millirem per year (1/15th of the regulatory limit). INTRODUCTION AND DESCRIPTION OF THE IMARC-8 TOTAL SYSTEM PERFORMANCE ASSESSMENT CODE Site characterization activities related to determining the suitability of the Yucca Mountain site to serve as a geologic repository for the disposal of spent nuclear fuel and high-level radioactive wastes have been ongoing, in one manner or another, since 1982 [1]. Throughout most of that period, the Electric Power Research Institute (EPRI) has been performing its own scientific and technical investigations and reviews to provide an independent perspective on site suitability and issues that could potentially have a significant impact on the eventual licensing of the facility [2-3]. The main features of the Yucca Mountain repository system are shown in Figure 1. The proposed repository is located within a thick unsaturated zone (UZ) approximately 300 meters below the ground surface and 300 meters above the water table. The spent fuel and high-level radioactive waste will be stored in thick-walled waste packages placed underneath titanium “drip shields”. Once the waste packages fail, any waste that escapes must pass through the UZ and then 18 kilometers downstream in the saturated zone (SZ) before it is assumed to enter the biosphere. Regulations [4-5] set forth by the U.S. Environmental Protection Agency (40 CFR Part 197) and the U.S. Nuclear Regulatory Commission (10 CFR Part 63) specify that probability-weighted mean doses to a “reasonably maximally exposed individual” living 18 km downstream must not exceed 15 millirem per year for the first 10,000 years after repository closure. Total system performance assessment (TSPA) models [6] will be used by the U.S. Department of Energy to assess compliance with these regulations [1].
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Figure 1. Main features of the Yucca Mountain repository system. EPRI’s probabilistic TSPA code for Yucca Mountain, IMARC-8, uses a simplified lo
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