Richard B. Codell

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

Abstract 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.

Sensitivity and Uncertainty Analyses Applied to One-Dimensional Radionuclide Transport in a Layered Fractured Rock. Part II: Probabilistic Methods Based on the Limit-State Approach

An uncertainty and probabilistic sensitivity study of a hypothetical underground high-level waste (HLW) repository intersected by a vertical fracture or fault and under saturated conditions is presented. Several recently developed probabilistic methods, including the advanced mean value method and the adaptive importance sampling method, are applied to a previously developed one-dimensional analytical model. These probabilistic methods are based on a limit-state formulation and provide an effective means of computing performance probability distribution and probability-based random parameter sensitivities. A numerical example related to the transport of 237 Np in a system of Iayered fractured rock is used to illustrate the application of these probabilistic methods for efficient uncertainty and probabilistic sensitivity and analyses

Alternative igneous source term model for tephra dispersal at the Yucca Mountain repository

Abstract The Nuclear Regulatory Commission uses the ASHPLUME model in its evaluation of the basaltic volcanism scenario at the possible Yucca Mountain repository. The mixing of magma with the spent-fuel waste form is tied to a reasonable but unverified model that predicts that no dense tephra/fuel particles would form. An alternative model uses a mixing rule that allows the formation of dense tephra/fuel particles that would be transported in the volcanic plume differently. The alternative model shows significant sensitivity to the spent-fuel particle size distribution. However, differences in results between the two models are on average less than a factor of 2.

Independent Postclosure Performance Estimates of the Proposed Repository at Yucca Mountain

Abstract 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.

The Interpretation of Risk and Sensitivity Under the Peak-of-the-Mean Concept

In quantitative performance assessment (PA) for nuclear waste repositories, probabilistic (e.g., Monte Carlo) calculations are frequently used to estimate dose and risk [1], Each Monte Carlo realization represents the uncertain estimate of the future effect of the repository. There are at least two ways to interpret the model output; (1) take the peak doses from the Monte Carlo realizations and draw conclusions from their ensemble, e.g., the mean of the peak doses; and (2) at each instant of time, look at the ensemble of all realizations, and synthesize a representative dose-versus-time curve, e.g., the mean. Method 1 is easy to understand and explain. However, the calculation of the mean of the peak doses allows an additional degree of freedom that may inadvertently overestimate risk, because the peaks occur at different times and therefore the mean may include contributions from peaks outside of a single person’s life span. This dilemma has been discussed previously in connection with the definition of the critical group, e.g., Corbett [2]. The U.S. Nuclear Regulatory Commission (NRC) has adapted Method 2, taking the peak value of the mean curve to represent the dose that the Reasonably Maximally Exposed Individual (RMEI) could receive during the regulatory time period for the purpose of defining risk. We call this the “peak-of-the-mean (POM)” approach, and believe that it is the clearest and fairest definition of risk. However, calculations and sensitivity analyses with the POM must proceed thoughtfully, since there are computational pitfalls and results are sometimes counterintuitive.

Improved Radionuclide Transport Models for a Nuclear Waste Repository

NRC is developing improved models for transport of chain decay radionuclides in porous and fractured/porous media using finite-difference for spatial discretization, and numerical inversion of Laplace transforms for time. This hybrid technique allows great flexibility for spatial variability, flow rate changes and boundary conditions that are difficult with analytical solutions, and are much faster than solutions fully in the time domain. This paper presents the status of NRCs development of a method for transport through fractured porous media at the proposed Yucca Mountain repository, building on the work of a number of authors.