Robert J. MacKinnon

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: Disturbed conditions

Uncertainty and sensitivity analysis results obtained in the 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) are presented for two-phase flow in the vicinity of the repository under disturbed conditions resulting from drilling intrusions. 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 and brine flow from the repository to the Culebra Dolomite are potentially the most important in PA for the WIPP. Subsequent to a drilling intrusion repository pressure was dominated by borehole permeability and generally below the level (i.e., 8 MPa) that could potentially produce spallings and direct brine releases. Brine flow from the repository to the Culebra Dolomite tended to be small or nonexistent with its occurrence and size also dominated by borehole permeability.

Representation of two-phase flow in the vicinity of the repository in the 1996 performance assessment for the Waste Isolation Pilot Plant

Abstract The following topics related to the representation of two-phase (i.e. gas and brine) flow in the vicinity of the repository in the 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) are discussed: (i) system of nonlinear partial differential equations used to model two-phase flow; (ii) incorporation of repository shafts into model; (iii) creep closure of repository; (iv) interbed fracturing; (v) gas generation; (vi) capillary action in waste; (vii) borehole model; (viii) numerical solution; and (ix) gas and brine flow across specified boundaries. Two-phase flow calculations are a central part of the 1996 WIPP PA and supply results that are subsequently used in the calculation of releases to the surface at the time of a drilling intrusion (i.e. spallings, direct brine releases) and long-term releases due to radionuclide transport by flowing groundwater.

Nodal Superconvergence and Solution Enhancement for a Class of Finite-Element and Finite-Difference Methods

A class of finite-element methods for elliptic problems is shown to exhibit nodal superconvergence in the approximate solution, and some equivalence properties to familiar finite-difference operators are demonstrated. The superconvergence property is exploited in a Taylor series analysis to demonstrate Gauss-point superconvergence for the derivatives of the approximation. A post-processing formula for the derivative at the nodes is constructed and shown to exhibit superconvergence. The nodal superconvergence property can be exploited recursively to further enhance the finite-element or finite-difference solution. Supporting numerical studies are given.

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

Overview of the United States Department of Energy’s Used Fuel Disposition Research and Development Campaign

The United States Department of Energy (US DOE) is conducting research and development (R&D) activities within the Used Fuel Disposition Campaign (UFDC) to support storage, transportation, and disposal of used nuclear fuel (UNF) and wastes generated by existing and future nuclear fuel cycles. R&D activities are ongoing at nine national laboratories, and are divided into two major topical areas: (1) storage and transportation research, and (2) disposal research. Storage R&D focuses on closing technical gaps related to extended storage of UNF. For example, uncertainties remain regarding high-burnup nuclear fuel cladding performance following possible hydride reorientation and creep deformation, and also regarding long-term canister integrity. Transportation R&D focuses on ensuring transportability of UNF following extended storage, addressing data gaps regarding nuclear fuel integrity, retrievability, and demonstration of subcriticality. Disposal R&D focuses on identifying multiple viable geologic disposal options and addressing technical challenges for generic disposal concepts in various host media (e.g., mined repositories in salt, clay/shale, and granitic rocks, and deep borehole disposal in crystalline rock). R&D will transition to site-specific challenges as national policy advances. R&D goals at this stage are to increase confidence in the robustness of generic disposal concepts, to reduce generic sources of uncertainty that may impact the viability of disposal concepts, and to develop science and engineering tools that will support the selection, characterization, and ultimately licensing of a repository. The US DOE has also initiated activities that can be conducted within the constraints of the Nuclear Waste Policy Act to facilitate the development of an interim storage facility and supporting transportation infrastructure.Copyright

A Control Variate Method for Probabilistic Performance Assessment. Improved Estimates for Mean Performance Quantities of Interest

We present a method of control variates for calculating improved estimates for mean performance quantities of interest, E(PQI) , computed from Monte Carlo probabilistic simulations. An example of a PQI is the concentration of a contaminant at a particular location in a problem domain computed from simulations of transport in porous media. To simplify the presentation, the method is described in the setting of a one- dimensional elliptical model problem involving a single uncertain parameter represented by a probability distribution. The approach can be easily implemented for more complex problems involving multiple uncertain parameters and in particular for application to probabilistic performance assessment of deep geologic nuclear waste repository systems. Numerical results indicate the method can produce estimates of E(PQI)having superior accuracy on coarser meshes and reduce the required number of simulations needed to achieve an acceptable estimate.

Update of the used fuel disposition Campaign Implementation Plan

This Campaign Implementation Plan provides summary level detail describing how the Used Fuel Disposition Campaign (UFDC) supports achievement of the overarching mission and objectives of the Department of Energy Office of Nuclear Energy Fuel Cycle Technologies Program The implementation plan begins with the assumption of target dates that are set out in the January 2013 DOE Strategy for the Management and Disposal of Used Nuclear Fuel and High-Level Radioactive Waste (http://energy.gov/downloads/strategy-management-and-disposal-used-nuclear-fuel-and-high-level-radioactive-waste). These target dates and goals are summarized in section III. This implementation plan will be maintained as a living document and will be updated as needed in response to progress in the Used Fuel Disposition Campaign and the Fuel Cycle Technologies Program.

HYDROLOGIC, CHEMICAL, AND THERMAL CONSTRAINTS ON WATER AVAILABILITY INSIDE BREACHED WASTE PACKAGES IN THE YUCCA MOUNTAIN REPOSITORY

Abstract Water is the most important reacting agent that directly controls radionuclide release from a nuclear waste repository to a human-accessible environment. In this paper, we present a water balance model to calculate the amount of water that can accumulate inside or percolate through a breached waste package in Yucca Mountain repository environments as a function of the temperature and relative humidity in the surrounding waste emplacement drift, the rate of water dripping from seepage, the area of breaches on the waste package, and the extent of waste degradation. The model accounts for sheet flows created as water drips fall onto the waste package surface, water vapor diffusion across waste package breaches, and water vapor equilibrium with unsaturated porous corrosion products. Preliminary model simulation results indicate that a breached waste package may maintain a large part of its barrier capability, and probably <1% of the total seepage flux impinging on the waste package surface can enter the package. Vapor diffusion of water through the breaches can be as important as liquid water flow into the waste package. Waste degradation reactions can consume a significant fraction of water entering the waste package. The water saturation inside waste packages will be low (<0.5), and the advective water flux out of a waste package will be small (with the mean value <0.5 ℓ/yr per package) over a wide range of seepage rates considered (1 to 1000 ℓ/yr). Furthermore, the ionic strength of in-package water will remain relatively high for the first 10000 yr, which will likely destabilize colloid suspensions and limit colloid releases.

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.