S. David Sevougian

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.

Towards a Defensible Safety Case for Deep Geologic Disposal of DOE HLW and DOE SNF in Bedded Salt

The primary objective of this study is to investigate the feasibility and utility of developing a defensible safety case for disposal of United States Department of Energy (U.S. DOE) high-level waste (HLW) and DOE spent nuclear fuel (SNF) in a conceptual deep geologic repository that is assumed to be located in a bedded salt formation of the Delaware Basin. A safety case is a formal compilation of evidence, analyses, and arguments that substantiate and demonstrate the safety of a proposed or conceptual repository. A safety case also provides the necessary structure for organizing and synthesizing existing knowledge in order to help DOE prioritize its future research and development (R&D) activities. We conclude that a defensible initial safety case for potential licensing could be readily compiled by capitalizing on the extensive technical basis that exists from prior work on the Waste Isolation Pilot Plant (WIPP), work on other repository development programs, and the work published through international efforts in salt repository programs such as in Germany. It should be emphasized that the DOE has not made any decisions regarding the disposition of DOE HLW and DOE SNF. This study provides additional information that could be used to inform DOE‘s decision making regarding management of this waste. Furthermore, the safety case discussed herein is not intended to either site a repository in the Delaware Basin or preclude siting in other media at other locations. Rather, this study simply presents an approach for accelerated development of a safety case for a potential DOE HLW and DOE SNF repository using the currently available technical basis for bedded salt. This approach includes a summary of the regulatory environment relevant to disposal of DOE HLW and DOE SNF in a deep geologic repository, the key elements of a safety case, the evolution of the safety case through the successive phases of repository development and licensing, and the existing technical basis that could be used to substantiate the safety of a geologic repository if it were to be sited in the Delaware Basin. We also discuss the potential role of an underground research laboratory (URL). Towards a Defensible Safety Case for Deep Geologic Disposal of DOE HLW and DOE SNF in Bedded Salt

Application of Generic Disposal System Models

This report describes specific GDSA activities in fiscal year 2015 (FY2015) toward the development of the enhanced disposal system modeling and analysis capability for geologic disposal of nuclear waste. The GDSA framework employs the PFLOTRAN thermal-hydrologic-chemical multi-physics code (Hammond et al., 2011) and the Dakota uncertainty sampling and propagation code (Adams et al., 2013). Each code is designed for massively-parallel processing in a high-performance computing (HPC) environment. Multi-physics representations in PFLOTRAN are used to simulate various coupled processes including heat flow, fluid flow, waste dissolution, radionuclide release, radionuclide decay and ingrowth, precipitation and dissolution of secondary phases, and radionuclide transport through the engineered barriers and natural geologic barriers to a well location in an overlying or underlying aquifer. Dakota is used to generate sets of representative realizations and to analyze parameter sensitivity.

Performance Assessment Modeling and Sensitivity Analyses of Generic Disposal System Concepts.

directly, rather than through simplified abstractions. It also a llows for complex representations of the source term, e.g., the explicit representation of many individual waste packages (i.e., meter - scale detail of an entire waste emplacement drift). This report fulfills the Generic Disposal System Analysis Work Packa ge Level 3 Milestone - Performance Assessment Modeling and Sensitivity Analyses of Generic Disposal System Concepts (M 3 FT - 1 4 SN08080 3 2 ).

Implementation of Localized Corrosion in the Performance Assessment Model for Yucca Mountain

Abstract A total system performance assessment (TSPA) model has been developed to analyze the ability of the natural and engineered barriers of the Yucca Mountain repository to isolate nuclear waste over the period following repository closure. The principal features of the engineered barrier system are emplacement tunnels (or “drifts”) containing a two-layer waste package (WP) for waste containment and a titanium drip shield to protect the WP from seeping water and falling rock. The 25-mm-thick outer shell of the WP is composed of Alloy 22, a highly corrosion-resistant nickel-based alloy. There are five nominal degradation modes of the Alloy 22: general corrosion, microbially influenced corrosion, stress corrosion cracking, early failure due to manufacturing defects, and localized corrosion (LC). This paper specifically examines the incorporation of the Alloy 22 LC model into the Yucca Mountain TSPA model, particularly the abstraction and modeling methodology, as well as issues dealing with scaling, spatial variability, uncertainty, and coupling to other submodels that are part of the total system model, such as the submodel for seepage water chemistry.

Description of Performance Allocation: A Case Study at Yucca Mountain

A methodology was developed to determine the effectiveness of different components/barriers in isolating waste from the accessible environment for the potential nuclear waste repository at Yucca Mountain, Nevada [1]. Quantifying the performance offered by different barriers aids in prioritizing the testing and analysis activities aimed at resolving uncertainty in processes and parameters of greatest significance to long-term performance. If a substantial fraction of the overall performance of a repository system may be attributed to the performance of a particular barrier, then it is prudent to devote the necessary resources toward enhancing the confidence in our predictive ability regarding that barrier (through more in-depth characterization and testing).