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Abstract: 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. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes the assessment of compliance with ground water protection standards in the 2008 YM PA. The following topics are addressed: (i) regulatory background, (ii) analysis structure including characterization of uncertainty, and (iii) analysis results for each of the ground water protection standards. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. The conceptual structure and organization of the 2008 YM PA is based on decomposing the analysis into the following scenario classes: nominal, early waste package failure, early drip shield failure, igneous intrusive, igneous eruptive, seismic ground motion, and seismic fault displacement. This presentation describes how results obtained for the individual scenario classes are brought together in the determination of expected dose to the reasonably maximally exposed individual (RMEI) specified by the NRC in the regulatory requirements for the YM repository and presents associated uncertainty and sensitivity analysis results. The following topics are addressed: (i) determination of expected dose to the RMEI from all scenario classes, (ii) expected dose and uncertainty in expected dose to the RMEI for 0 to 20,000yr, (iii) expected dose and uncertainty in expected dose to the RMEI from for 0 to 106 yr, (iv) justification for the decomposition procedure used to estimate expected dose to the RMEI from all scenario classes, and (v) effectiveness of individual barrier systems in reducing releases from the repository and thus dose to the RMEI. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the US Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the US Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes uncertainty and sensitivity analysis results for the seismic ground motion scenario class and the seismic fault displacement scenario class obtained in the 2008 YM PA. The following topics are addressed for the seismic ground motion scenario class: (i) engineered barrier system conditions; (ii) release results for the engineered barrier system, unsaturated zone, and saturated zone; (iii) dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository; and (iv) expected dose to the RMEI. In addition, expected dose to the RMEI for the seismic fault displacement scenario class is also considered. The present article is the part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes uncertainty and sensitivity analysis results for the early waste package failure scenario class and the early drip shield failure scenario class obtained in the 2008 YM PA. The following topics are addressed: (i) engineered barrier system conditions, (ii) release results for the engineered barrier system, unsaturated zone, and saturated zone, (iii) dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository, and (iv) expected dose to the RMEI. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes the determination of expected (mean) dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository resulting from an inadvertent drilling intrusion into the repository. The following topics are addressed: (i) assumed properties of an inadvertent drilling intrusion and the determination of the associated dose and expected (mean) dose to the RMEI, (ii) uncertainty and sensitivity analysis results for expected dose to the RMEI, and (iii) the numerical stability of the sampling-based procedure used to estimate expected (mean) dose to the RMEI. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA. [Copyright &y& Elsevier]

Abstract: The representation of aleatory uncertainty associated with the seismic ground motion scenario class in the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, is described. The following topics are considered: (i) occurrence rates for waste package (WP) damage, (ii) conditional distributions for peak ground velocity, (iii) conditional distributions for damaged area on WPs, (iv) distribution of rock fall volume, and (v) probability of WP rupture. Separate results are obtained for commercial spent nuclear fuel and codisposed spent nuclear fuel WPs. [Copyright &y& Elsevier]

Abstract: This paper summarizes the progression of results through four early performance assessments (PAs) conducted to support selection and to evaluate feasibility and three major PAs conducted to evaluate viability, recommend the site, and assess compliance of a repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. The early PAs in 1984, 1991, 1993, and 1995 evaluated cumulative release over 104 yr at a 10-km or 5-km boundary as specified in the draft and final 1985 radiation protection standard, respectively. During the early PAs, the fission products 99Tc, 129I, and activation products 14C, and 36Cl were identified as important radionuclides at the beginning of the regulatory period. The actinide, 237Np, often dominated at the end of the regulatory period. Package and repository design options were evaluated during the early PAs but modeling did not identify strong preferences. In 1992 Congress mandated a change to a dose measure. Dose at a 20-km boundary from the repository was evaluated through 106 yr for the undisturbed scenario class via the groundwater pathway for the Congressionally mandated viability assessment in 1998. For the assessment for the site recommendation in 2000, doses from igneous eruption dominated in the first ~3000yr, doses from igneous intrusion between ~3000yr and ~40,000yr, and doses from the undisturbed scenario class through 106 yr. The 2008 compliance assessment for the license application incorporated the influence of the seismic scenario class on waste package performance. The compliance assessment found that doses from the igneous intrusive scenario class and the combined undisturbed and seismic scenario class were important contributors at the ~18-km boundary. In the compliance PA, 99Tc and 129I fission products and 14C activation product were important in the first 104 yr. Beyond 104 yr, actinides 239Pu, 242Pu, 237Np, and 238U decay product 226Ra were important. In all PAs, parameters of the natural barrier were important, but in the three latter PAs, the slow degradation of the large, in-drift container had an important role in explaining the uncertainty in the peak dose. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes uncertainty and sensitivity analysis results for the nominal scenario class (i.e., for undisturbed conditions) obtained in the 2008 YM PA. The following topics are addressed: (i) uncertainty and sensitivity analysis procedures, (ii) drip shield and waste package failure, (iii) engineered barrier system conditions, (iv) radionuclide release results for the engineered barrier system, unsaturated zone, and saturated zone, and (v) dose to the reasonably maximally exposed individual specified in the NRC regulations for the YM repository. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA. [Copyright &y& Elsevier]

Abstract: A summary is presented for the total system model used to represent physical processes associated with the seven scenario classes (i.e., nominal conditions, early waste package (WP) failure, early drip shield (DS) failure, igneous intrusive events, igneous eruptive events, seismic ground motion events and seismic fault displacement events) considered in the 2008 performance assessment for the proposed repository for high-level radioactive waste at Yucca Mountain, Nevada. The total system model estimates dose to an exposed individual resulting from radionuclide movement through the repository system and biosphere. Components of the total system model described in this presentation include models for (i) climate analysis, (ii) land surface infiltration and associated unsaturated zone flow, (iii) multi-scale thermal hydrology and engineered barrier system (EBS) thermal–hydrologic environment, (iv) EBS physical and chemical environment, (v) WP and DS degradation, (vi) drift seepage and drift wall condensation, (vii) waste form degradation and mobilization, (viii) water and radionuclide movement in the EBS and underlying unsaturated and saturated zones, (ix) radionuclide movement in the biosphere and resultant human exposure, and (x) processes specific to early WP and DS failures, intrusive and eruptive igneous events, and seismic ground motion and fault displacement events. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes the determination of expected (mean) dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository for the nominal scenario class (i.e., under nominal or undisturbed conditions) in the 2008YM PA. The following topics are addressed: (i) properties of the nominal scenario class and the determination of dose and expected (mean) dose to the RMEI, (ii) uncertainty in dose and resultant expected (mean) dose to the RMEI, (iii) expected (mean) dose to the RMEI from individual radionuclides, and (iv) numerical stability of the sampling-based procedure used to estimate the expected (mean) dose to the RMEI. The present paper is part of a special issue of Reliability Engineering and System Safety devoted to the 2008YM PA; additional papers in the issue describe other aspects of the 2008YM PA. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. This presentation describes the overall conceptual structure and computational organization of the 2008 performance assessment (PA) for the proposed YM repository carried out by the DOE in support of a licensing application to the U.S. Nuclear Regulatory Commission (NRC). The following topics are addressed: (i) regulatory background, (ii) the three basic entities underlying a PA, (iii) determination of expected, mean and median dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository, (iv) the relationship between probability, sets and scenario classes, (v) scenario classes and the characterization of aleatory uncertainty, (vi) scenario classes and the determination of expected dose to the RMEI, (vii) analysis decomposition, (viii) disjoint and nondisjoint scenario classes, (ix) scenario classes and the NRC’s YM review plan, (x) characterization of epistemic uncertainty, and (xi) adequacy of Latin hypercube sample size used in the propagation of epistemic uncertainty. This article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA and is intended as an introduction to following articles in the issue that provide additional analysis details and specific analysis results. [Copyright &y& Elsevier]

Abstract: This paper summarizes modeling of radionuclide transport in the unsaturated and saturated zone conducted between 1984 and 2008 to evaluate feasibility, viability, and assess compliance of a repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. One dimensional (1-D) transport for a single porosity media without lateral dispersion was solved in both the saturated zone (SZ) and unsaturated zone (UZ) for the first assessment in 1984 but progressed to a dual-porosity formulation for the UZ in the second assessment in 1991. By the time of the viability assessment, a dual-permeability transport formulation was used in the UZ. With the planned switch to a dose performance measure, individual dose from a drinking water pathway was evaluated for the third assessment in 1993 and from numerous pathways for the viability assessment in 1998 and thereafter. Stream tubes for transport in the SZ were initially developed manually but progressed to particle tracking in 1991. For the viability assessment, particle tracking was used to solve the transport equations in the 3-D UZ and SZ flow fields. To facilitate calculations, the convolution method was also used in the SZ for the viability assessment. For the site recommendation in 2001 and licensing compliance analysis in 2008, the 3-D transport results of the SZ were combined with 1-D transport results, which evaluated decay of radionuclides, in order to evaluate compliance with groundwater protection requirements. Uncertainty in flow within the unsaturated and saturated zone was generally important to explaining the spread in the individual dose performance measure. [Copyright &y& Elsevier]

Abstract: This paper summarizes the evolution of the engineered barrier design for the proposed Yucca Mountain disposal system. Initially, the underground facility used a fairly standard panel and drift layout excavated mostly by drilling and blasting. By 1993, the layout of the underground facility was changed to accommodate construction by a tunnel boring machine. Placement of the repository in unsaturated zone permitted an extended period without backfilling; placement of the waste package in an open drift permitted use of much larger, and thus hotter packages. Hence in 1994, the underground facility design switched from floor emplacement of waste in small, single walled stainless steel or nickel alloy containers to in-drift emplacement of waste in large, double-walled containers. By 2000, the outer layer was a high nickel alloy for corrosion resistance and the inner layer was stainless steel for structural strength. Use of large packages facilitated receipt and disposal of high volumes of spent nuclear fuel. In addition, in-drift package placement saved excavation costs. Options considered for in-drift emplacement included different heat loads and use of backfill. To avoid dripping on the package during the thermal period and the possibility of localized corrosion, titanium drip shields were added for the disposal drifts by 2000. In addition, a handling canister, sealed at the reactor to eliminate further handling of bare fuel assemblies, was evaluated and eventually adopted in 2006. Finally, staged development of the underground layout was adopted to more readily adjust to changes in waste forms and Congressional funding. [Copyright &y& Elsevier]

Abstract: This paper summarizes the investigations conducted to characterize the geologic barrier of the Yucca Mountain disposal system. Site characterization progressed through (1) non-intrusive evaluation and borehole completions to determine stratigraphy for site identification; (2) exploration from the surface through well testing to evaluate the repository feasibility; (3) underground exploration to study coupled processes to evaluate repository suitability; and (4) reporting of experimental conclusions to support the repository compliance phase. Some of the scientific and technical challenges encountered included the evolution from a small preconstruction characterization program with much knowledge to be acquired during construction of the repository to a large characterization program with knowledge acquired prior to submission of the license application for construction authorization in June 2008 (i.e., the evolution from a preconstruction characterization program costing <$0.04×109 as estimated by the Nuclear Regulatory Commission in 1982 to a thorough characterization, design, and analysis program costing $11×109—latter in 2010 constant dollars). Scientific understanding of unsaturated flow in fractures and seepage into an open drift in a thermally perturbed environment was initially lacking, so much site characterization expense was required to develop this knowledge. [Copyright &y& Elsevier]

Abstract: This paper summarizes the progression of modeling efforts of infiltration, percolation, and seepage conducted between 1984 and 2008 to evaluate feasibility, viability, and assess compliance of a repository in the unsaturated zone for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. Scientific understanding of infiltration in a desert environment, unsaturated percolation flux in fractures and matrix of the volcanic tuff, and seepage into an open drift in a thermally perturbed environment was initially lacking in 1984. As understanding of the Yucca Mountain disposal system increased through site characterization and in situ testing, modeling of infiltration, percolation, and seepage evolved from simple assumptions in a single model in 1984 to three modeling modules each based on several detailed process models in 2008. Uncertainty in percolation flux through Yucca Mountain was usually important in explaining the observed uncertainty in performance measures:cumulative release in assessments prior to 1995 and individual dose, thereafter. [Copyright &y& Elsevier]

Abstract: This paper summarizes the evolution of consequence modeling for a repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain in southern Nevada. The discussion includes four early performance assessments (PAs) conducted between 1982 and 1995 to support selection and to evaluate feasibility and three major PAs conducted between 1998 and 2008 to evaluate viability, recommend the site, and assess compliance. Modeling efforts in 1982 estimated dose to individuals 18km from the site caused by volcanic eruption through the repository. Modeling in 1984 estimated releases via the groundwater pathway because of container corrosion. In combination, this early analysis supported the first environmental assessment. Analysts in 1991 evaluated cumulative release, as specified in the 1985 US radiation protection standards, via the groundwater pathway over 104 yr at a 5-km boundary by modeling waste degradation and flow/transport in the saturated and unsaturated zones. By 1992, however, the US Congress mandated a change to a dose measure. Thus, the 1993 and 1995 performance assessments improved modeling of waste container degradation to provide better estimates of radionuclide release rates out to 106 yr. The 1998 viability assessment was a major step in modeling complexity. Dose at a 20-km boundary from the repository was evaluated through 106 yr for undisturbed conditions using more elaborate modeling of flow and the addition of modules for modeling infiltration, drift seepage, the chemical environment, and biosphere transport. The 2000 assessment for the site recommendation refined the analysis. Seepage modeling was greatly improved and waste form degradation modeling included more chemical dependence. The 2008 compliance assessment for the license application incorporated the influence of the seismicity on waste package performance to evaluate dose at an ~18-km boundary. [Copyright &y& Elsevier]

Abstract: This paper summarizes various hazards identified between 1978 when Yucca Mountain, located in arid southern Nevada, was first proposed as a potential site and 2008 when the license application to construct a repository for spent nuclear fuel and high-level radioactive waste was submitted. Although advantages of an arid site are many, hazard identification and scenario development have generally recognized fractures in the tuff as important features; climate change, water infiltration and percolation, and an oxidizing environment as important processes; and igneous activity, seismicity, human intrusion, and criticality as important disruptive events to consider at Yucca Mountain. Some of the scientific and technical challenges encountered included a change in the repository design from in-floor emplacement with small packages to in-drift emplacement with large packages without backfill. This change, in turn, increased the importance of igneous and seismic hazards. [Copyright &y& Elsevier]

Abstract: This paper summarizes modeling of waste degradation and mobilization in performance assessments (PAs) conducted between 1984 and 2008 to evaluate feasibility, viability, and assess compliance of a repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain in southern Nevada. As understanding of the Yucca Mountain disposal system increased, the waste degradation module, or succinctly called the source-term, evolved from initial assumptions in 1984 to results based on process modeling in 2008. In early PAs, waste degradation had significant influence on calculated behavior but as the robustness of the waste container was increased and modeling of the container degradation improved, waste degradation had much less influence in later PAs. The variation of dissolved concentrations of radionuclides progressed from simple probability distributions in early PAs to functions dependent upon water chemistry in later PAs. Also, transport modeling of radionuclides in the waste, container, and invert were added in 1995; and, colloid-facilitated transport of radionuclides was added in 1998. [Copyright &y& Elsevier]

Abstract: This paper summarizes modeling of waste container degradation in performance assessments conducted between 1984 and 2008 to evaluate feasibility, viability, and assess compliance of a repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. As understanding of the Yucca Mountain disposal system increased, modeling of container degradation evolved from a component of the source term in 1984 to a separate module describing both container and drip shield degradation in 2008. A thermal module for evaluating the influence of higher heat loads from more closely packed, large waste packages was also introduced. In addition, a module for evaluating drift chemistry was added in later PAs to evaluate the potential for localized corrosion of the outer barrier of the waste container composed of Alloy 22, a highly corrosion-resistant nickel–chromium–tungsten–molybdenum alloy. The uncertainty of parameters related to container degradation contributed significantly to the estimated uncertainty of performance measures (cumulative release in assessments prior to 1995 and individual dose, thereafter). [Copyright &y& Elsevier]

Abstract: This paper summarizes the historical events from the identification of the Yucca Mountain site in southern Nevada in 1978 to its selection by the US Congress as the sole site to characterize for a repository for spent nuclear fuel and high-level radioactive waste in 1987. Coincident with this selection process and later site characterization, the US spent from 1977 to 2009 establishing long-term, radiation protection standards and a regulatory framework for demonstrating compliance. When first promulgated, the US Environmental Protection Agency's radiation protection standards limited cumulative release of radionuclides at a boundary ≤5km from the edge of a generic repository over a 104-year regulatory period. But in 2001, site-specific standards for a repository at Yucca Mountain were promulgated to limit the dose to an individual at a point ≤∼18km from the repository edge in the predominant direction of groundwater flow over a 106-year period. Also during the 33-year effort, the regulatory framework of the US Nuclear Regulatory Commission, which implemented the radiation protection standards, changed from setting performance criteria on barrier subsystem components in 1983 to the identification and technical justification for barrier performance based on a performance assessment. Also, reasonable expectation as the standard of proof for evaluating compliance was clarified. [Copyright &y& Elsevier]

• Development of a comprehensive five-step infrastructure asset management framework. • Definition of short and long-term investment needs in water and wastewater networks. • Integration of innovative and relevant operational and convenience factors. • Utilization of novel key performance indicators. • Inclusion of adjacency and group replacement considerations. Water supply and sewer networks are critical infrastructures that provide a basic service to society. However, these systems constantly age and degrade over time. In addition, since network infrastructures are so extensive in length, they require a significant investment in maintenance tasks. Hence, within the context of infrastructure asset management (IAM), accurately defining the most efficient investment planning possible is essential to ensure their long-term sustainability. This paper presents an original five-step comprehensive framework to successfully implement an infrastructure asset management strategy and plan long-term investments. Moreover, this methodology integrates innovative and relevant operational and convenience factors that, while provide the problem both with realism and practicality, have not been addressed so far. To illustrate the usefulness and applicability of this methodology, the case study of a large water company in Spain is presented. [ABSTRACT FROM AUTHOR]

Abstract: The estimation of radionuclide migration within a fractured medium is a fundamental task in any performance assessment aimed at verifying the protection offered by radioactive waste repositories. In this paper, we present a novel application of a dual-permeability, Monte Carlo simulation approach based on Kolmogorov–Dmitriev (KD) theory of branching stochastic processes. The Monte Carlo simulation scheme allows to easily handle the high degree of complexity of the problem, accounting for non-homogeneities both in space and time. Particular attention is devoted here to the inclusion of time-dependencies. An illustration is given with reference to a realistic case study of 239Pu migration. [Copyright &y& Elsevier]

Abstract: This paper contributes to the investigation whether sophisticated methods, i.e., the EFAST method are suitable for use in long-term safety assessments of geological repositories for radioactive wastes and also whether they provide deeper insight into a repository system than less sophisticated methods such as rank-based regression or correlation or non-parametric methods, scatterplots or CSM plots. A generic test case for a HLW repository in a clay formation was selected. Despite the skewed and heavily tailed distribution of the model output, the EFAST results were surprisingly good compared to results previously obtained for a repository in rock salt. The important difference between the PA models in clay and salt is that the former produces a less skewed and less scattered distribution of the annual dose than the latter. As a result, the variance of the model output and the sensitivity indices are more stable. For the considered model, the EFAST method yields quantitative information about the parameter importance that cannot be obtained using linear regression, correlation or graphical methods. It is therefore a valuable tool, especially in combination with other sensitivity analysis methods. [Copyright &y& Elsevier]

Abstract: The 2008 performance assessment (PA) for the proposed repository for high-level radioactive waste at Yucca Mountain (YM), Nevada, used a Latin hypercube sample (LHS) of size 300 in the propagation of the epistemic uncertainty present in 392 analysis input variables. To assess the adequacy of this sample size, the 2008 YM PA was repeated with three independently generated (i.e., replicated) LHSs of size 300 from the indicated 392 input variables and their associated distributions. Comparison of the uncertainty and sensitivity analysis results obtained with the three replicated LHSs showed that the three samples lead to similar results and that the use of any one of three samples would have produced the same assessment of the effects and implications of epistemic uncertainty. Uncertainty and sensitivity analysis results obtained with the three LHSs were compared by (i) simple visual inspection, (ii) use of the t-distribution to provide a formal representation of sample-to-sample variability in the determination of expected values over epistemic uncertainty and other distributional quantities, and (iii) use of the top down coefficient of concordance to determine agreement with respect to the importance of individual variables indicated in sensitivity analyses performed with the replicated samples. The presented analyses established that an LHS of size 300 was adequate for the propagation and analysis of the effects and implications of epistemic uncertainty in the 2008 YM PA. [Copyright &y& Elsevier]

Abstract: Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. As part of this development, a detailed performance assessment (PA) for the YM repository was completed in 2008 and supported a license application by the DOE to the U.S. Nuclear Regulatory Commission (NRC) for the construction of the YM repository. The following aspects of the 2008 YM PA are described in this presentation: (i) conceptual structure and computational organization, (ii) uncertainty and sensitivity analysis techniques in use, (iii) uncertainty and sensitivity analysis for physical processes, and (iv) uncertainty and sensitivity analysis for expected dose to the reasonably maximally exposed individual (RMEI) specified the NRC’s regulations for the YM repository. [Copyright &y& Elsevier]

Abstract: In this paper, we show an original application of the Subset Simulation (SS) technique on a model for the performance assessment of a near surface radioactive waste repository. The logic of the protective barriers of the repository is represented by a reliability model. The SS approach is founded on the idea that a small failure probability can be expressed as a product of larger conditional probabilities of some intermediate events; with a proper choice of the conditional events, the conditional probabilities can be sufficiently large to allow accurate estimation with a small number of samples. In the application, the method allows improving the efficiency of the random sampling for estimating the repository containment failure probability. Moreover, the peculiar set-partitioning scheme of the SS method is exploited for performing the analysis of the sensitivity of the failure probability estimate to the uncertain model parameters. [Copyright &y& Elsevier]

Abstract: In 2001, the National Nuclear Security Administration of the U.S. Department of Energy 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. This presentation discusses and illustrates the conceptual and computational basis of QMU in analyses that use computational models to predict the behavior of complex systems. The following topics are considered: (i) the role of aleatory and epistemic uncertainty in QMU, (ii) the representation of uncertainty with probability, (iii) the probabilistic representation of uncertainty in QMU analyses involving only epistemic uncertainty, and (iv) the probabilistic representation of uncertainty in QMU analyses involving aleatory and epistemic uncertainty. [Copyright &y& Elsevier]

Abstract: Key ideas underlying the application of Quantification of Margins and Uncertainties (QMU) to nuclear weapons stockpile lifecycle decisions are described. While QMU is a broad process and methodology for generating critical technical information to be used in U.S. nuclear weapon stockpile management, this paper emphasizes one component, which is information produced by computational modeling and simulation. In particular, the following topics are discussed: (i) the key principles of developing QMU information in the form of Best Estimate Plus Uncertainty, (ii) the need to separate aleatory and epistemic uncertainty in QMU, and (iii) the properties of risk-informed decision making (RIDM) that are best suited for effective application of QMU. The paper is written at a high level, but provides an extensive bibliography of useful papers for interested readers to deepen their understanding of the presented ideas. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: A managed process was used to consistently and traceably develop probability distributions for parameters representing epistemic uncertainty in four preliminary and the final 1996 performance assessment (PA) for the Waste Isolation Pilot Plant. Between 67 probability density functions (PDFs) in the 1989 PA and 236 PDFs in the 1996 PA were assigned by a parameter development team, using a process described in a companion paper. In the five iterative PAs conducted, the most commonly used distributions were the uniform PDF and piecewise-uniform PDF (also referred to as a piecewise-linear cumulative distribution function (CDF)). Other distributions used included the truncated normal, truncated Student-t, and triangular PDFs. In a few instances, a discrete delta (piecewise-uniform CDF), beta, and exponential PDF were also used. The PDFs produced for the 24 most important parameters observed in the five iterative PAs are presented. As background, the list of 194 parameters documented in the first 1989 PA through the 1471 parameters documented in the 1996 compliance PA are also provided. [Copyright &y& Elsevier]

Abstract: A managed process was used to consistently and traceably develop probability distributions for parameters representing epistemic uncertainty in four preliminary and the final 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP). The key to the success of the process was the use of a three-member team consisting of a Parameter Task Leader, PA Analyst, and Subject Matter Expert. This team, in turn, relied upon a series of guidelines for selecting distribution types. The primary function of the guidelines was not to constrain the actual process of developing a parameter distribution but rather to establish a series of well-defined steps where recognized methods would be consistently applied to all parameters. An important guideline was to use a small set of distributions satisfying the maximum entropy formalism. Another important guideline was the consistent use of the log transform for parameters with large ranges (i.e. maximum/minimum>103). A parameter development team assigned 67 probability density functions (PDFs) in the 1989 PA and 236 PDFs in the 1996 PA using these and other guidelines described. [Copyright &y& Elsevier]

• The performance of safety barriers in Natech accidents was investigated. • A reference set of active and passive technical safety barriers was defined. • Qualitative and quantitative data were obtained by expert elicitation. • Safety barriers highly vulnerable to the impact of natural events were identified. • Baseline values for performance modification factors were obtained. The impact of natural hazards on chemical and process facilities may lead to catastrophic technological accidents defined as Natech events. In the present study, a performance assessment of safety barriers during extreme natural events as floods and earthquakes was carried out. Assessing the performance of safety barriers during such complex scenarios is a key issue to identify the final consequences and the evolution of Natech events. Due to the scarcity of available data, an anonymous survey was carried out, involving more than 40 experts of different nationalities and background. Categories of safety barriers highly vulnerable to the impact of natural events were identified. Baseline values for performance modification factors describing how barrier performance may be affected by floods and earthquakes were obtained from the expert elicitation procedure. The results may support the probabilistic analysis of Natech scenarios, in order to achieve a more accurate assessment of final consequences and of possible escalation. [ABSTRACT FROM AUTHOR]