The Effect of Waste Package and Engineered Barrier System Design Options on Long-Term Waste Package Degradation Characte
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ABSTRACT Sensitivity analyses were conducted to evaluate the impact of a number of waste package and engineered barrier system (EBS) design options on long-term waste package degradation in the potential repository at Yucca Mountain, Nevada. The evaluations were performed by analyzing the waste package degradation in terms of the first breach and first pit-breach profiles with time. Results for each design option were assessed by comparing to results for the reference-case design. An important finding from the analyses is that the waste package lifetime could be extended substantially by limiting and delaying the contact of dripping water with the Alloy 22 corrosion resistant material (CRM) barrier. In the current model, this would keep the CRM barrier outside the conditions that makes the alloy susceptible to localized corrosion. Uncertainty in waste package degradation analysis would be reduced by improved understanding and characterization of the processes that create the local exposure condition on waste package. INTRODUCTION Yucca Mountain, Nevada, is a site being studied for the disposal of the nation's high-level radioactive waste including commercial spent nuclear fuels and vitrified high-level waste. As part of the characterization efforts, the Viability Assessment (VA) of the potential repository has been completed [I. A multi-barrier waste-container design concept provides the primary component of the engineered barrier system (EBS) of the potential repository. The VA reference design specifies a two-layer barrier system with a 10-cm thick corrosion-allowance material (CAM) as the outer barrier and a 2-cm thick corrosionresistant material (CRM) as the inner barrier. The candidate materials are carbon steel (or American Standard Testing and Materials (ASTM) A516) for the CAM, and Alloy 22 (or UNS N06022) for the CRM [2]. Given that the waste package must "fail" (i.e., breach to an extent that the mobile water present in the near-field environment can enter the waste package and any dissolved radionuclides can be transported out of the waste package) before any dissolution of the waste form can occur, it is important to be able to provide reasonable estimates of waste package degradation. Waste package failure is defined as having an opening through the wall of the waste package through which advective or diffusive transport of gaseous or dissolved radionuclides can occur [3]. In this paper a number of waste-package and EBS design options are considered, and their impact on the long-term degradation of the waste package is analyzed with the WAPDEG (Waste PAckage DEGradation) computer code, which employs a stochastic approach to simulate long-term waste package degradation [4,5]. The simulation results are compared with the reference-case waste-package degradation analysis [4,5]. The design options that were analyzed include (1) alternative thicknesses of the CAM barrier, (2) alternative thicknesses of the CRM barrier, (3) backfill, and (4) Alloy 22 drip shield of various thicknesses with backfill. OVERVIEW
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