Integrated Analysis for Long-Term Degradation of Waste Package at the Potential Yucca Mountain Repository for High-Level
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Integrated Analysis for Long-Term Degradation of Waste Package at the Potential Yucca Mountain Repository for High-Level Nuclear Waste Disposal Joon H. Lee1, Kevin G. Mon2, Dennis E. Longsine3, Bryan E. Bullard2, and Ahmed M. Monib4 1 Sandia National Laboratories, 1180 Town Center Drive, Las Vegas, NV 89144 2 Duke Engineering & Services, 1180 Town Center Drive, Las Vegas, NV 89144 3 Duke Engineering & Services, 9111 Research Blvd., Austin, TX 78758 4 Bechtel-SAIC, 1180 Town Center Drive, Las Vegas, NV 89144 ABSTRACT The technical basis for Site Recommendation (SR) of the potential repository for high-level nuclear waste at Yucca Mountain, Nevada has been completed. Long-term containment of the waste and subsequent slow release of radionuclides from the engineered barrier system (EBS) into the geosphere will rely on a robust waste package (WP) design, among other EBS components as well as the natural barrier system. The WP and drip shield (DS) degradation analyses for the total system performance assessment (TSPA) baseline model for the SR have shown that, based on the current corrosion models and assumptions, both the DSs and WPs do not fail within the regulatory compliance time period (10,000 years). From the perspective of initial WP failure time, the analysis results are encouraging because the upper bounds of the baseline case are likely to represent the worst case combination of key corrosion model parameters that significantly affect long-term performance of WPs in the potential repository. The estimated long life-time of the WPs in the current analysis is attributed mostly to the following two factors that delay the onset of stress corrosion cracking (SCC): (1) the stress mitigation to substantial depths from the outer surface in the dual closure-lid weld regions; and (2) the very low general-corrosion rate applied to the closure-lid weld regions to corrode the compressive stress zones. Uncertainties are associated with the current WP SCC analysis. These are stress mitigation on the closure-lid welds, characterization of manufacturing flaws applied to SCC, and general corrosion rate applied to the closurelid weld regions. These uncertainties are expected to be reduced as additional data and analyses are developed. INTRODUCTION The technical basis for Site Recommendation (SR) of the potential repository for high-level nuclear waste at Yucca Mountain, Nevada has been completed. Long-term containment of the waste and subsequent slow release of radionuclides from the engineered barrier system (EBS) into the geosphere will rely on a robust waste package (WP) design, among other EBS components as well as the natural barrier system. The SR WP design has two layers: an Alloy 22 outer barrier and a Type 316NG stainless steel inner shell. A thickness of 20 mm is assumed for the WP outer barrier in the current analysis. A Titanium Grade 7 drip shield (DS) (15-mm thick) is placed over the WP at the time of closure of the repository. No backfill is used. The stainless steel inner shell is intended to provide the structura
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