Peter N. Swift

Uncertainty and sensitivity analysis results obtained in the 1992 performance assessment for the waste isolation pilot plant

Uncertainty and sensitivity analysis results obtained in the 1992 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) are presented. The primary performance measure under study is the complementary cumulative distribution function (CCDF) used in assessing compliance with the U.S. Environmental Protection Agencys (EPAs) standard for the geologic disposal of radioactive waste (40 CFR 191, Subpart B). The analysis considers releases to the accessible environment initiated by exploratory drilling for natural resources and models cuttings removal to the surface due to drilling intrusions, brine and gas flow in the vicinity of the repository and through drilling intrusions away from the repository, radionuclide transport by the flow of brine through intruding boreholes, and brine flow and radionuclide transport in permeable formations overlying the repository (i.e., the Culebra Dolomite). The effects of 49 imprecisely known variables are assessed with techniques based on Latin hypercube sampling and regression analysis. In addition, the effects of several alternative conceptual models for radionuclide transport in the Culebra Dolomite are investigated. Important issues identified in the analysis include (1) the importance of characterizing retardations and solubilities for individual elements, (2) the impact of assumptions involving human activities, including the rate and properties of drilling intrusions, and (3) the need to resolve the question of whether a single-porosity or dual-porosity transport model is appropriate for use in the Culebra Dolomite.

Uncertainty and Sensitivity Analysis Results Obtained in a Preliminary Performance Assessment for the Waste Isolation Pilot Plant

Uncertainty and sensitivity analysis results obtained in a preliminary performance assessment for the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico are presented. The most appropriate conceptual model for performance assessment at the WIPP is believed to include gas generation due to corrosion and microbial action in the repository and a dual-porosity (matrix and fracture porosity) representation for solute transport in the Culebra Dolomite Member of the Rustler Formation. Under these assumptions, complementary cumulative distribution functions (CCDFs) summarizing radionuclide releases to the accessible environment due to both cuttings removal and groundwater transport fall substantially below the release limits promulgated by the U.S. Environmental Protection Agency (EPA). This is the case even when the current estimates of the uncertainty in analysis inputs are included. Performance assessment results are dominated by cuttings removal. The releases to the accessible environment due to groundwater transport make very small contributions to the total release. The variability in the distribution of CCDFs that must be considered in comparisons with the EPA release limits is dominated by the rate constant in the Poisson model for drilling intrusions. The variability in releases to the accessible environment due to cuttings removal is dominated by drill bit diameter.

Deep Borehole Disposal of High-Level Radioactive Waste.

Preliminary evaluation of deep borehole disposal of high-level radioactive waste and spent nuclear fuel indicates the potential for excellent long-term safety performance at costs competitive with mined repositories. Significant fluid flow through basement rock is prevented, in part, by low permeabilities, poorly connected transport pathways, and overburden self-sealing. Deep fluids also resist vertical movement because they are density stratified. Thermal hydrologic calculations estimate the thermal pulse from emplaced waste to be small (less than 20 C at 10 meters from the borehole, for less than a few hundred years), and to result in maximum total vertical fluid movement of ~100 m. Reducing conditions will sharply limit solubilities of most dose-critical radionuclides at depth, and high ionic strengths of deep fluids will prevent colloidal transport. For the bounding analysis of this report, waste is envisioned to be emplaced as fuel assemblies stacked inside drill casing that are lowered, and emplaced using off-theshelf oilfield and geothermal drilling techniques, into the lower 1-2 km portion of a vertical borehole ~ 45 cm in diameter and 3-5 km deep, followed by borehole sealing. Deep borehole disposal of radioactive waste in the United States would require modifications to the Nuclear Waste Policy Act and to applicable regulatory standards for long-term performance set by the US Environmental Protection Agency (40 CFR part 191) and US Nuclear Regulatory Commission (10 CFR part 60). The performance analysis described here is based on the assumption that long-term standards for deep borehole disposal would be identical in the key regards to those prescribed for existing repositories (40 CFR part 197 and 10 CFR part 63).

Effect of alternative conceptual models in a preliminary performance assessment for the waste isolation pilot plant

Abstract The most appropriate conceptual model for performance assessment (PA) at the waste isolation pilot plant (WIPP) is believed to include gas generation resulting from corrosion and microbial action in the repository, and a dual-porosity (matrix and fracture porosity) representation for the solute transport in the Culebra dolomite member of the Rustler formation. Under these assumptions, complementary cumulative distribution functions (CCDFs) which summarize the radionuclide releases to the accessible environment, resulting from both cuttings removal and groundwater transport, fall substantially below the release limits promulgated by the US Environmental Protection Agency (EPA), with the releases being dominated by cuttings removal. To provide additional views, the following alternative conceptual models were considered as part of a preliminary PA for the WIPP: (1) no gas generation in the repository and a dual-porosity transport model in the Culebra; (2) gas generation in the repository and a single-porosity (fracture porosity) transport model in the Culebra; (3) no gas generation in the repository and a single-porosity transport model in the Culebra; (4) gas generation in the repository and a dual-porosity transport model in the Culebra, without chemical retardation; (5) gas generation in the repository, chemical retardation in the Culebra, and extremes of climatic variation. These variations relate to groundwater transport, so do not affect the releases resulting from cuttings removal. Several of these variations substantially increase the importance of releases resulting from groundwater transport relative to releases resulting from cuttings removal. However, the total amount of releases generally remained small, with the CCDFs which summarize the releases to the accessible environment falling below the EPA release limits.

Scenario development for the Waste Isolation Pilot Plant compliance certification application

Abstract Demonstrating compliance with the applicable regulations for the Waste Isolation Pilot Plant (WIPP) requires an assessment of the long-term performance of the disposal system. Scenario development is one starting point of this assessment, and generates inquiry about the present state and future evolution of the disposal system. Scenario development consists of four tasks: (i) identifying and classifying features, events and processes (FEPs); (ii) screening FEPs according to well-defined criteria; (iii) forming scenarios (combinations of FEPs) in the context of regulatory performance criteria, and (iv) specifying of scenarios for consequence analysis. The development and screening of a comprehensive FEP list provides assurance that the identification of significant processes and events is complete, that potential interactions between FEPs are not overlooked, and that responses to possible questions are available and well documented. Two basic scenarios have been identified for the WIPP: undisturbed performance (UP) and disturbed performance (DP). The UP scenario is used to evaluate compliance with the Environmental Protection Agencys (EPAs) Individual Dose (40 CFR § 191.15) and Groundwater Protection (40 CFR § 191.24) standards and accounts for all natural and waste- and repository-induced FEPs that survive the screening process. The DP scenario is required for assessment calculations for the EPAs cumulative release standard (Containment Requirements, 40 CFR § 191.13) and accounts for disruptive future human events, which have an uncertain probability of occurrence, in addition to the UP FEPs.

Computational implementation of a systems prioritization methodology for the Waste Isolation Pilot Plant: a preliminary example

Abstract A systems prioritization methodology (SPM) is under development for the Waste Isolation Pilot Plant (WIPP). The SPM is based on a large numerical integration problem that must be repeatedly evaluated to determine compliance probabilities associated with different experimental programs and design modifications. Due to the complexity and computational cost of the underlying integration problem, the implementation of the SPM must be planned very carefully. This presentation describes a preliminary application of the SPM, designated SPM-l, performed to provide insights to facilitate the development and implementation of the methodology. Topics illustrated by SPM-1 include definition of probability spaces on which the SPM is based, use of Latin hypercube sampling and simple random sampling to integrate over different probability spaces, selection of mechanistic calculations to be performed, efficient use of the limited number of mechanistic calculations that can be performed, and assembly of many individual calculations into a complete analysis.

Uncertainty and sensitivity analysis for gas and brine migration at the Waste Isolation Pilot Plant: permeable shaft without panel seals

Abstract Latin hypercube sampling, partial correlation analysis, stepwise regression analysis and examination of scatterplots are used in conjunction with the BRAGFLO model in an uncertainty and sensitivity analysis of gas and brine movement at the Waste Isolation Pilot Plant. Topics investigated include (1) gas production due to corrosion of steel and microbial degradation of cellulosics, (2) gas saturation and pressure in the repository, (3) gas movement in anhydrite marker beds, and (4) gas and brine movement through a short, low permeability shaft seal to the Culebra Dolomite. Important variables identified in the analysis include initial brine saturation of the waste, stoichiometric terms for corrosion of steel and microbial degradation of cellulosics, corrosion and microbial degradation rates, porosity of the Salado Formation, transition (disturbed) zone porosity, marker bed (anhydrite) permeability, and seal permeability. Published by Elsevier Science Limited.

Uncertainty and sensitivity analysis for gas and brine migration at the Waste Isolation Pilot Plant: Permeable shaft with panel seals

Abstract Uncertainty and sensitivity analysis techniques based on Latin hypercube sampling, partial correlation analysis, stepwise regression analysis and examination of scatter plots are used in conjunction with the BRAGFLO model to examine two-phase (i.e., gas and brine) flow at the Waste Isolation Pilot Plant (WIPP), which is being developed by the US Department of Energy as a disposal facility for transuranic waste. The following topics are investigated to develop insights on factors that are potentially important in establishing compliance with applicable regulations of the US Environmental Protection Agency (i.e., 40 CFR 191, Subpart B; 40 CFR 268): (1) gas production due to corrosion of steel; (2) gas production due to microbial degradation of cellulosics; and (3) gas migration through a sealed shaft to the Culebra Dolomite. Important variables identified in the analysis include initial brine saturation of the waste, stoichiometric terms for corrosion of steel and microbial degradation of cellulosics, and seal permeabilities.

Summary discussion of the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

A deep geologic repository at Yucca Mountain (YM), Nevada, for the disposal of spent nuclear fuel and high-level radioactive waste was proposed by the U.S. Department of Energy (DOE). This paper summarizes the historical development of the 2008 YM performance assessment (PA), and explains how the methods and results of the 2008 PA address regulatory requirements specified by the United States Environmental Protection Agency (EPA) and the United States Nuclear Regulatory Commission (NRC). Topics covered include (i) screening of features, events and processes, (ii) development of scenario classes, (iii) descriptions of barrier capability, and (iv) compliance with applicable quantitative standards for individual protection, individual protection following human intrusion, and ground water protection. This article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA and provides a brief summary of information presented in detail in multiple articles in this issue and interprets the results in the context of applicable EPA and NRC regulations.

International Approaches for Nuclear Waste Disposal in Geological Formations: Report on Fifth Worldwide Review:

Author(s): Faybishenko, Boris; Birkholzer, Jens; Persoff, Peter; Sassani, David; Swift, Peter