Summary discussion of the 1996 performance assessment for the Waste Isolation Pilot Plant

The Waste Isolation Pilot Plant (WIPP) is under development by the US Department of Energy (DOE) for the geologic disposal of transuranic waste. The construction of complementary cumulative distribution functions (CCDFs) for total radionuclide release from the WIPP to the accessible environment is described. The resultant CCDFs (i) combine releases due to cuttings and cavings, spallings, direct brine release, and long-term transport in flowing groundwater; (ii) fall substantially to the left of the boundary line specified by the US Environmental Protection Agency's (EPA's) standard 40 CFR 191 for the geologic disposal of radioactive waste; and (iii) constitute an important component of the DOE's successful Compliance Certification Application to the EPA for the WIPP. Insights and perspectives gained in the performance assessment (PA) that led to these CCDFs are described, including the importance of: (i) an iterative approach to PA; (ii) uncertainty and sensitivity analysis; (iii) a clear conceptual model for the analysis; (iv) the separation of stochastic (i.e. aleatory) and subjective (i.e. epistemic) uncertainty; (v) quality assurance procedures; (vi) early involvement of peer reviewers, regulators, and stakeholders; (vii) avoidance of conservative assumptions; and (viii) adequate documentation.     

Characterization of stochastic uncertainty in the 1996 performance assessment for the Waste Isolation Pilot Plant

The 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) maintains a separation between stochastic (i.e. aleatory) and subjective (i.e. epistemic) uncertainty, with stochastic uncertainty arising from the possible disruptions that could occur at the WIPP over the 10,000-yr regulatory period specified by the US Environmental Protection Agency (40 CFR 191, 40 CFR 194) and subjective uncertainty arising from an inability to uniquely characterize many of the inputs required in the 1996 WIPP PA. The characterization of stochastic uncertainty is discussed, including drilling intrusion time, drilling location, penetration of excavated/nonexcavated areas of the repository, penetration of pressurized brine beneath the repository, borehole plugging patterns, activity level of waste, and occurrence of potash mining. Additional topics discussed include sampling procedures, generation of individual 10,000-yr futures for the WIPP, construction of complementary cumulative distribution functions (CCDFs), mechanistic calculations carried out to support CCDF construction, the Kaplan/Garrick ordered triple representation for risk, and determination of scenarios and scenario probabilities.     

Characterization of subjective uncertainty in the 1996 performance assessment for the Waste Isolation Pilot Plant

The 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) maintains a separation between stochastic (i.e. aleatory) and subjective (i.e. epistemic) uncertainty, with stochastic uncertainty arising from the possible disruptions that could occur at the WIPP over the 10,000 yr regulatory period specified by the US Environmental Protection Agency (40 CFR 191, 40 CFR 194) and subjective uncertainty arising from an inability to uniquely characterize many of the inputs required in the 1996 WIPP PA. The characterization of subjective uncertainty is discussed, including assignment of distributions, uncertain variables selected for inclusion in analysis, correlation control, sample size, statistical confidence on mean complementary cumulative distribution functions, generation of Latin hypercube samples, sensitivity analysis techniques, and scenarios involving stochastic and subjective uncertainty.     

Regulatory basis for the Waste Isolation Pilot Plant performance assessment

The Waste Isolation Pilot Plant (WIPP) is the first operational repository designed for the safe disposal of transuranic (TRU) radioactive waste from the defense programs of the US Department of Energy (DOE). The US Environmental Protection Agency (EPA) is responsible for certifications and regulation of the WIPP facility for the radioactive components of the waste. The EPA has promulgated general radioactive waste disposal standards at 40 CFR Part 191, and WIPP-specific criteria to implement and interpret the generic disposal standards at 40 CFR Part 194. In October 1996, the DOE submitted its Compliance Certification Application (CCA) to the EPA to demonstrate compliance with the disposal standards at Subparts B and C of 40 CFR Part 191. This paper summarizes the development of the overall legal framework for radioactive waste disposal at the WIPP, the parallel development of the WIPP performance assessment (PA), and how the EPA disposal standards and implementing criteria formed the basis for the CCA WIPP PA. The CCA resulted in a certification in May 1998 by the EPA of the WIPP's compliance with the EPA's disposal standard, thus enabling the WIPP to begin radioactive waste disposal.     

Computational implementation of sampling-based approaches to the calculation of expected dose in performance assessments for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

A deep geologic repository for high-level radioactive waste is under development by the US Department of Energy (DOE) at Yucca Mountain (YM), Nevada. As mandated in the Energy Policy Act of 1992, the US Environmental Protection has promulgated public health and safety standards (i.e., 40 CFR Part 197) for the YM repository, and the US Nuclear Regulatory Commission has promulgated licensing standards (i.e., 10 CFR Parts 2, 19, 20, etc.) consistent with 40 CFR Part 197 that the DOE must establish are met in order for the YM repository to be licensed for operation. Important requirements in 40 CFR Part 197 and 10 CFR Parts 2, 19, 20, etc. relate to the determination of expected (i.e., mean) dose to a reasonably maximally exposed individual (RMEI) and the incorporation of uncertainty into this determination. This paper is the second part of a two-part presentation on the determination of expected dose to the RMEI in the context of 40 CFR Part 197 and 10 CFR Parts 2, 19, 20, etc. The first part of this presentation is contained in the preceding paper, “Conceptual Basis for the Definition and Calculation of Expected Dose in Performance Assessments for the Proposed High-Level Radioactive Waste Repository at Yucca Mountain, Nevada”, and describes how general and typically nonquantitative statements in 40 CFR Part 197 and 10 CFR Parts 2, 19, 20, etc. can be given a formal mathematical structure that facilitates both the calculation of expected dose to the RMEI and the appropriate separation in this calculation of aleatory uncertainty (i.e., randomness in the properties of future occurrences such as igneous and seismic events) and epistemic uncertainty (i.e., lack of knowledge about quantities that are poorly known but assumed to have constant values in the calculation of expected dose to the RMEI). The present paper describes and illustrates sampling-based procedures for the estimation of expected dose and the determination of the uncertainty in estimates for expected dose.     

Conceptual basis for the definition and calculation of expected dose in performance assessments for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

A deep geologic repository for high-level radioactive waste is under development by the US Department of Energy (DOE) at Yucca Mountain (YM), Nevada. As mandated in the Energy Policy Act of 1992, the US Environmental Protection Agency has promulgated public health and safety standards (i.e., 40 CFR Part 197) for the YM repository, and the US Nuclear Regulatory Commission has promulgated licensing standards (i.e., 10 CFR Parts 2, 19, 20, etc.) consistent with 40 CFR Part 197 that the DOE must establish are met in order for the YM repository to be licensed for operation. Important requirements in 40 CFR Part 197 and 10 CFR Parts 2, 19, 20, etc. relate to the determination of expected (i.e., mean) dose to a reasonably maximally exposed individual (RMEI) and the incorporation of uncertainty into this determination. This paper is the first part of a two-part presentation and describes how general and typically nonquantitative statements in 40 CFR Part 197 and 10 CFR Parts 2, 19, 20, etc. can be given a formal mathematical structure that facilitates both the calculation of expected dose to the RMEI and the appropriate separation in this calculation of aleatory uncertainty (i.e., randomness in the properties of future occurrences such as igneous and seismic events) and epistemic uncertainty (i.e., lack of knowledge about quantities that are imprecisely known but assumed to have constant values in the calculation of expected dose to the RMEI). The second part of this presentation is contained in the following paper, “Computational Implementation of Sampling-Based Approaches to the Calculation of Expected Dose in Performance Assessments for the Proposed High-Level Radioactive Waste Repository at Yucca Mountain, Nevada,” and both describes and illustrates sampling-based procedures for the estimation of expected dose and the determination of the uncertainty in estimates for expected dose.     

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 Agency's (EPA's) 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.     

Construction of complementary cumulative distribution functions for comparison with the EPA release limits for radioactive waste disposal

The US Environmental Protection Agency (EPA) has promulgated a standard for the geologic disposal of radioactive waste (40 CFR 191) that requires the construction of a complementary cumulative distribution function (CCDF) that defines the probability of exceeding radionuclide releases of various sizes. Construction of this CCDF involves three interrelated activities: development of scenarios that describe the various disruptions that could occur at a waste disposal site, determination of probabilities for scenarios, and calculation of radionuclide releases for scenarios. The computational procedures being used in the performance assesement for the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico to bring these three activities together to produce the CCDF in 40 CFR 191 are described and illustrated with results obtained in a recent analysis.     

Uncertainty and sensitivity analysis for two-phase flow in the vicinity of the repository in the 1996 performance assessment for the Waste Isolation Pilot Plant: undisturbed conditions

Uncertainty and sensitivity analysis results obtained in the 1996 performance assessment for the Waste Isolation Pilot Plant are presented for two-phase flow in the vicinity of the repository under undisturbed conditions. Techniques based on Latin hypercube sampling, examination of scatterplots, stepwise regression analysis, partial correlation analysis and rank transformations are used to investigate brine inflow, gas generation, repository pressure, brine saturation, and brine and gas outflow. Of the variables under study, repository pressure is potentially the most important due to its influence on spallings and direct brine releases, with the uncertainty in its value being dominated by the extent to which the microbial degradation of cellulose takes place, the rate at which the corrosion of steel takes place, and the amount of brine that drains from the surrounding disturbed rock zone into the repository.     

Radioactive and nonradioactive waste intended for disposal at the Waste Isolation Pilot Plant

Transuranic (TRU) waste generated by the handling of plutonium during research on or production of U.S. nuclear weapons will be disposed of in the Waste Isolation Pilot Plant (WIPP). This paper describes the physical and radiological properties of the TRU waste that will be deposited in the WIPP. This geologic repository will accommodate up to 175,564 m³ of TRU waste, corresponding to 168,485 m³ of contact-handled (CH-) TRU waste and 7079 m³ of remote-handled (RH-) TRU waste. Approximately 35% of the TRU waste is currently packaged and stored (i.e. legacy) waste, with the remainder of the waste to be packaged or generated and packaged in activities before the year 2033, which is the closure time for the repository. These wastes were produced at 27 U.S. Department of Energy (DOE) sites in the course of generating defense nuclear materials. The radionuclide and nonradionuclide inventories for the TRU wastes described in this paper were used in the 1996 WIPP Compliance Certification Application (CCA) performance assessment calculations by the Sandia National Laboratories/New Mexico (SNL/NM).     

Scenario development for the Waste Isolation Pilot Plant Compliance Certification Application

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 Agency's (EPA's) 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 EPA's 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.     

Drilling intrusion probabilities for use in performance assessment for radioactive waste disposal

Product integral techniques are used to derive computational procedures to determine probabilities for scenarios resulting from drilling intrusions at geologic disposal facilities for radioactive waste. For these derivations, the occurence of individual drilling intrusions is assumed to be random in time and space, although the drilling rate is not assumed to be constant or even continuous through time. The use of product integral techniques allows the probability model for drilling intrusions to be initially cast in a very general form involving interval functions and then specialized to nonhomogeneous Poisson processes and ultimately to homogeneous Poisson processes. The resultant computational procedures are illustrated with results for the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico using the maximum drilling rate specified by the US Environmental Protection Agency (EPA) in its standard for the geologic disposal of radioactive waste (40 CFR 191).     

Latin hypercube sampling and the propagation of uncertainty in analyses of complex systems

The following techniques for uncertainty and sensitivity analysis are briefly summarized: Monte Carlo analysis, differential analysis, response surface methodology, Fourier amplitude sensitivity test, Sobol' variance decomposition, and fast probability integration. Desirable features of Monte Carlo analysis in conjunction with Latin hypercube sampling are described in discussions of the following topics: (i) properties of random, stratified and Latin hypercube sampling, (ii) comparisons of random and Latin hypercube sampling, (iii) operations involving Latin hypercube sampling (i.e. correlation control, reweighting of samples to incorporate changed distributions, replicated sampling to test reproducibility of results), (iv) uncertainty analysis (i.e. cumulative distribution functions, complementary cumulative distribution functions, box plots), (v) sensitivity analysis (i.e. scatterplots, regression analysis, correlation analysis, rank transformations, searches for nonrandom patterns), and (vi) analyses involving stochastic (i.e. aleatory) and subjective (i.e. epistemic) uncertainty.     

Quantification of margins and uncertainties: Example analyses from reactor safety and radioactive waste disposal involving the separation of aleatory and epistemic uncertainty

In 2001, the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy (DOE) in conjunction with the national security laboratories (i.e., Los Alamos National Laboratory, Lawrence Livermore National Laboratory, and Sandia National Laboratories) initiated development of a process designated quantification of margins and uncertainties (QMU) for the use of risk assessment methodologies in the certification of the reliability and safety of the nation's nuclear weapons stockpile. A previous presentation, “Quantification of Margins and Uncertainties: Conceptual and Computational Basis,” describes the basic ideas that underlie QMU and illustrates these ideas with two notional examples. The basic ideas and challenges that underlie NNSA's mandate for QMU are present, and have been successfully addressed, in a number of past analyses for complex systems. To provide perspective on the implementation of a requirement for QMU in the analysis of a complex system, three past analyses are presented as examples: (i) the probabilistic risk assessment carried out for the Surry Nuclear Power Station as part of the U.S. Nuclear Regulatory Commission's (NRC's) reassessment of the risk from commercial nuclear power in the United States (i.e., the NUREG-1150 study), (ii) the performance assessment for the Waste Isolation Pilot Plant carried out by the DOE in support of a successful compliance certification application to the U.S. Environmental Agency, and (iii) the performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, carried out by the DOE in support of a license application to the NRC. Each of the preceding analyses involved a detailed treatment of uncertainty and produced results used to establish compliance with specific numerical requirements on the performance of the system under study. As a result, these studies illustrate the determination of both margins and the uncertainty in margins in real analyses.     

Multiple predictor smoothing methods for sensitivity analysis: Example results

The use of multiple predictor smoothing methods in sampling-based sensitivity analyses of complex models is investigated. Specifically, sensitivity analysis procedures based on smoothing methods employing the stepwise application of the following nonparametric regression techniques are described in the first part of this presentation: (i) locally weighted regression (LOESS), (ii) additive models, (iii) projection pursuit regression, and (iv) recursive partitioning regression. In this, the second and concluding part of the presentation, the indicated procedures are illustrated with both simple test problems and results from a performance assessment for a radioactive waste disposal facility (i.e., the Waste Isolation Pilot Plant). As shown by the example illustrations, the use of smoothing procedures based on nonparametric regression techniques can yield more informative sensitivity analysis results than can be obtained with more traditional sensitivity analysis procedures based on linear regression, rank regression or quadratic regression when nonlinear relationships between model inputs and model predictions are present.