Stochastic Modeling of Long-Term Waste Package Degradation Incorporating Expert Elicitation on Corrosion Processes
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package degradation model is very significant in the overall TSPA of the potential repository, because it significantly contributes to determining the rate and time of release of radionuclides from the waste package. This paper describes the stochastic simulation model and the analysis results for the VA reference case. Detailed descriptions of the model and analyses are found elsewhere [6]. SUMMARY OF THE WASTE PACKAGE DEGRADATION EXPERT ELICITATION (WPDEE) Data for corrosion and other degradation processes of the candidate waste package materials and their process models are being developed under the project's comprehensive corrosion testing and modeling programs [7]. However, long-term testing data were not yet available under prototypical conditions for the VA analysis. In addition, there was limited field experience for the candidate corrosion resistant alloy (Alloy 22) because of a short field performance history. Consequently, a panel of six highly qualified engineers and scientists in corrosion was convened, and an expert elicitation was conducted to develop additional information and models that were needed for waste package degradation modeling for TSPAVA. Another major objective of the expert elicitation was to capture the uncertainties involved in assessing waste package degradation processes, including uncertainty both in the conceptual and numerical models used to represent corrosion processes and the parameter values in those models. In the elicitation, the experts provided an overview of the processes for waste package degradation in the potential repository, taking into consideration potential evolutions of the exposure conditions in the emplacement drifts. Then, the experts developed assessments, which included uncertainties, for the models of various key corrosion processes and the parameters of the corrosion models for the CAM (carbon steel) and CRM (Alloy 22) barriers. Those parameters for which assessments were provided include:
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Temperature threshold for the initiation of corrosion of the CAM outer barrier Relative humidity threshold for the initiation of humid-air corrosion of the CAM outer barrier Relative humidity threshold for the initiation of aqueous corrosion of the CAM outer barrier Humid-air "roughness" factor (or localization factor) model for the CAM outer barrier Aqueous "roughness" factor (or localization factor) model for the CAM outer barrier Aqueous pitting corrosion model and pit density for the CAM outer barrier in alkaline conditions (pH>10) Local exposure conditions on the CRM inner barrier (after breach of the outer barrier) in the presence of drips and the probability of these conditions to occur General corrosion model for the CRM inner barrier in the presence of drips General corrosion model for the CRM inner barrier without drips Critical temperature threshold for the initiation of localized corrosion of the CRM inner barrier Localized corrosion (pitting and crevice corrosion) model for the CRM inner barrier Pit density and pit dia
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