Center For Nuclear Waste Regulatory Analyses Research Articles

Understanding long-term corrosion of Alloy 22 container in the potential Yucca Mountain repository for high-level nuclear waste disposal

Alloy 22 (Ni–22Cr–13Mo–3W–4Fe) is the candidate material for the waste package outer container in a potential geologic repository for high-level nuclear waste disposal at Yucca Mountain, Nevada. This alloy exhibits very low corrosion rates in the absence of environmental conditions promoting crevice corrosion. However, there are uncertainties regarding Alloy 22’s corrosion performance when general corrosion rates and susceptibility to crevice corrosion are extrapolated to a geological time period (e.g. 10 5 years). This paper presents an analysis of available literature information relevant to the long-term extrapolation of general corrosion processes and the crevice corrosion behavior of Alloy 22, under potential repository environments. For assessment of general corrosion rates, potential degradation processes causing the loss of the long-term persistence of passive film formed are considered. For crevice corrosion, induction time, and the extent of susceptibility and opening area, are considered. Disclaimer: The US Nuclear Regulatory Commission (NRC) staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed nor of the acceptability of a license application for a geologic repository at Yucca Mountain. The paper describes work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) for NRC under Contract Number NRC-02-02-012. The activities reported here were performed by CNWRA on behalf of the NRC office of Nuclear Material Safety and Safeguards, Division of High Level Waste Repository Safety. This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC.

Independent Postclosure Performance Estimates of the Proposed Repository at Yucca Mountain

The key findings from a suite of independent analyses of the performance of the proposed repository at Yucca Mountain, conducted by the Center for Nuclear Waste Regulatory Analyses (CNWRA) and the U.S. Nuclear Regulatory Commission (NRC), are summarized. The analyses are geared toward obtaining risk insights from deterministic and probabilistic calculations of potential exposure to people in a down-gradient community, determining the capability of barriers to reduce flow of water and prevent or delay radionuclide transport, and identifying models, parameters, and subsystems that have the most influence on repository performance through the use of sensitivity and uncertainty analyses. The analyses have allowed the CNWRA and NRC to focus on the most critical aspects of estimating postclosure repository performance.

CDF sensitivity analysis technique for ranking influential parameters in the performance assessment of the proposed high-level waste repository at Yucca Mountain, Nevada, USA

Abstract A cumulative distribution function (CDF)-based method has been used to perform sensitivity analysis on a computer model that conducts total system performance assessment of the proposed high-level nuclear waste repository at Yucca Mountain, and to identify the most influential input parameters affecting the output of the model. The performance assessment computer model referred to as the TPA code, was recently developed by the US nuclear regulatory commission (NRC) and the center for nuclear waste regulatory analyses (CNWRA), to evaluate the performance assessments conducted by the US department of energy (DOE) in support of their license application. The model uses a probabilistic framework implemented through Monte Carlo or Latin hypercube sampling (LHS) to permit the propagation of uncertainties associated with model parameters, conceptual models, and future system states. The problem involves more than 246 uncertain parameters (also referred to as random variables) of which the ones that have significant influence on the response or the uncertainty of the response must be identified and ranked. The CDF-based approach identifies and ranks important parameters based on the sensitivity of the response CDF to the input parameter distributions. Based on a reliability sensitivity concept [AIAA Journal 32 (1994) 1717], the response CDF is defined as the integral of the joint probability-density-function of the input parameters, with a domain of integration that is defined by a subset of the samples. The sensitivity analysis does not require explicit knowledge of any specific relationship between the response and the input parameters, and the sensitivity is dependent upon the magnitude of the response. The method allows for calculating sensitivity over a wide range of the response and is not limited to the mean value.

An approach to the assessment of high-level radioactive waste containment. I: Waste package degradation

A description is presented of the various models (i.e. thermal, chemical environment, humid-air and aqueous corrosion) used in the engineered barrier system performance assessment code (EBSPAC) to represent processes that govern the failure of waste packages (WPs) and, ultimately, the release of radionuclides from the engineered barrier system (EBS). These models are specifically adapted to the US Department of Energy (DOE) WP design, adopted in 1996, for the proposed repository at Yucca Mountain (YM), NV. The design consists of a double-wall overpack composed of two concentric containers of different metallic materials in a horizontal drift emplacement. EBSPAC was developed to deterministically evaluate the performance of the engineered barriers and to be used as the source term module incorporated in the Center for Nuclear Waste Regulatory Analyses (CNWRA)/Nuclear Regulatory Commission (NRC) total performance assessment (TPA) code. EBSPAC essentially consists of two separate codes. The code dealing with WP failure calculations is the focus of this paper and the other, dealing with radionuclide release, is described in a companion paper. An example problem is presented to illustrate the results obtained with the code analyzing the influence of several critical input parameters, related to the repository and EBS designs and the resulting environmental conditions, on WP failure.

An approach to the assessment of high-level radioactive waste containment — II: radionuclide releases from an engineered barrier system

This paper is the second in a series describing the models used in the Engineered Barrier System Performance Assessment Code (EBSPAC) to represent processes that govern the failure of waste packages (WPs) and the release of radionuclides from the engineered barrier system (EBS). These models are specifically adapted to the US Department of Energy (DOE) WP design, adopted in 1996, for the proposed high-level radioactive waste (HLW) repository at Yucca Mountain (YM). The design consists of a double-wall overpack composed of two concentric containers of different metallic materials in a horizontal drift emplacement. EBSPAC was developed to deterministically evaluate the performance of the engineered barriers and to be used as the source term module in the Center for Nuclear Waste Regulatory Analyses (CNWRA)/Nuclear Regulatory Commission (NRC) Total-system Performance Assessment (TPA) code. EBSPAC has two distinct parts. The part dealing with the radionuclide release subsequent to WP failure is the focus of this paper in which various models (i.e. dry-air oxidation and aqueous dissolution of spent fuel (SF), gaseous and aqueous release of radionuclides) are presented, whereas modeling of the WP failure is described in a companion paper. An example problem is presented to illustrate computational results obtained with the code analyzing the influence of several critical input parameters for the source term related to the repository and EBS designs and resulting environmental conditions. The source term calculations are confined to the radionuclides being released just outside of the WP. Both gaseous and aqueous release calculations are performed using models in which radionuclide decay, in-growth of daughter products in the chains, degradation process of SF, temporal variation of inventory in the WP, and spatial variations in the properties of the surrounding material are included. The degree of complexity varies from model to model as necessary simplifications are made, while ensuring conservatism.

Prediction of Waste Package Life for High-Level Radioactive Waste Disposal at Yucca Mountain

The U.S. Department of Energy (DOE) has released a Viability Assessment (VA) of the proposed high-level waste (HLW) repository at Yucca Mountain (YM), Nevada. The proposed standard for the repository requires the evaluation of the predicted —over 10,000 yr— performance of the system. In the VA, it is argued that long waste package (WP) lifetimes are responsible for significant containment and delay in release of the HLW. This paper examines the DOE WP lifetime prediction by combining the VA parameters modeling the corrosion behavior of the WP materials with the repository performance assessment code developed by the U.S. Nuclear Regulatory Commission (NRC) and the Center for Nuclear Waste Regulatory Analyses (CNWRA). The objective of the analysis is to exercise and strengthen the NRC review capabilities in preparation for receipt of the DOE license application for the proposed HLW repository. The results indicate that during the first 10,000 yr, radionuclide release depends almost entirely on the fraction of initially defective waste packages (IDWP). Radionuclide release resulting from corrosion of the WPs occurs only beyond 10,000 yr. The long WP lifetimes (>10,000 yr), reported in the VA and in this study, primarily depend on corrosion rates developed from an expert elicitation consensus. Given the importance in the estimation of the onset of radionuclide release and the lifetime of the WP materials, it is suggested the need for a stronger technical basis to assess the fraction of IDWP and to support the low corrosion rates of the WP materials.

2.1.1 THE APPLICATION OF SYSTEMS ENGINEERING TECHNIQUES IN A REGULATORY ENVIRONMENT: NRC HIGH‐LEVEL WASTE REGULATORY PROGRAM

AbstractThe Nuclear Regulatory Commission (NRC) and its Federally Funded Research and Development Center, the Center for Nuclear Waste Regulatory Analyses (CNWRA) are streamlining the NRC high‐level radioactive waste (HLW) regulatory program by applying systems engineering (SE) techniques. This effort is partially in response to a statutory mandate requiring a licensing decision for construction of a geologic repository for HLW within three years after license application submission. The tailored use of SE presented herein has evolved from experience gained attempting to apply classical SE techniques to evaluating regulatory compliance. This compliance evaluation is made against the criterion of “reasonable assurance.” The resulting subjectivity prevents a clear definition of a requirements hierarchy and development of specifications. Efforts have been made to streamline the HLW program in the following areas: (i) definition of a regulatory structure for program execution, (ii) provision of regulatory guidance to the license applicant, (iii) provision of guidance to the NRC staff in the form of compliance determination strategies and methods, and (iv) establishment of criteria to support prioritization of technical and regulatory work.

The use of probability elicitation in the high-level nuclear waste regulation program

Expert judgement elicitation is expected to be used in the performance assessments (PA) of the long-term behavior of high-level waste (HLW) geologic repositories. As a preparation for an effective review of the U.S. Department of Energy (DOE) PA, the Nuclear Regulatory Commission (NRC) is evaluating the mechanics of eliciting expert judgements. One of the objectives of this evaluation is to explore techniques for generating and aggregating probabilistic judgements of future conditions at the proposed HLW repository at Yucca Mountain, Nevada. An actual elicitation was conducted as an aid to these evaluations. This paper documents this probabilistically centered elicitation and subsequent activities to explore aggregation of opinion techniques. Future climate in the Yucca Mountain, Nevada vicinity was selected as the topic for elicitation. Personnel from the NRC and Center for Nuclear Waste Regulatory Analyses (CNWRA) defined the climatic parameters of interest in conjunction with a panel of five expert climatologists. Individual elicitations were performed with each climatologist to produce probabilistic estimates of each parameter at seven points of time in the future. The elicitations employed the fractile technique to generate cumulative probability distributions representing the uncertainty in the predictions. After the individual elicitations, a group session was conducted to explore aggregation and consensus methods.