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This report describes the specification of a challenge problem and associated challenge milestones for the Waste Integrated Performance and Safety Codes (IPSC) supporting the U.S. Department of Energy (DOE) Office of Nuclear Energy Advanced Modeling and Simulation (NEAMS) Campaign. The NEAMS challenge problems are designed to demonstrate proof of concept and progress towards IPSC goals. The goal of the Waste IPSC is to develop an integrated suite of modeling and simulation capabilities to quantitatively assess the long-term performance of waste forms in the engineered and geologic environments of a radioactive waste storage or disposal system. The Waste IPSC will provide this simulation capability (1) for a range of disposal concepts, waste form types, engineered repository designs, and geologic settings, (2) for a range of time scales and distances, (3) with appropriate consideration of the inherent uncertainties, and (4) in accordance with robust verification, validation, and software quality requirements.To demonstrate proof of concept and progress towards these goals and requirements, a Waste IPSC challenge problem is specified that includes coupled thermal-hydrologic-chemicalmechanical (THCM) processes that describe (1) the degradation of a borosilicate glass waste form and the corresponding mobilization of radionuclides (i.e., the processes that produce the radionuclide source term), (2) the associated near-field physical and chemical environment for waste emplacement within a salt formation, and (3) radionuclide transport in the near field (i.e., through the engineered components -waste form, waste package, and backfill -and the immediately adjacent salt). The initial details of a set of challenge milestones that collectively comprise the full challenge problem are also specified.iii iv CONTENTS

Total System Performance Assessments are an important component in the evaluation of the suitability of Yucca Mountain, Nevada as a potential site for a mined geologic repository for the permanent disposal of high-level radioactive wastes in the United States. The Total System Performance Assessments are conducted iteratively during site characterization to identify issues which should be addressed by the characterization and design activities as well as providing input to regulatoryAicensing and programmatic decisions. During fiscal years 1991 and 1992, the first iteration of Total System Performance Assessment (hereafter referred to as TSPA 1991) was completed by Sandia National Laboratories and Pacific Northwest Laboratory. Beginning in fiscal year 1993, the Civilian Radioactive W a s t e Management System Management and Operating Contractor was assigned the responsibility to plan, coordinate, and contribute to the second iteration of Total System Performance Assessment (hereafter referred to as TSPA 1993). This document presents the objectives, approach, assumptions, input, results, conclusions, and recommendations associated with the Management and Operating Contractor contribution to TSPA 1993. A parallel effort was conducted by Sandia National Laboratories and is reported in Wilson et al. (1994, in press). The principal objectives of the second iteration of Total System Performance Assessment are to (1) enhance the realism/ representativeness of the analyses, (2) incorporate new information and designs that have become available since the completion of TSPA 1991, (3) test the significance (i.e., sensitivity) of various conceptual model and parameter uncertainties on the predicted performance, and (4) evaluate alternate measures of postclosure performance. Additional thermohydrologic analyses are required to evaluate the effect of uncertain and spatially variable thermohydrologic properties, uncertain fracture-matrix conceptual models, and uncertain ambient percolation fluxes on the expected far-field, ncar-field and very-near-field (waste package-scale) thermal and hydrologic regimes as a function of space and time. Considerable uncertainty remains regarding the processes affecting the initiation and rate of aqueous corrosion under the range of possible thennohydrologic environments likely to be encountered under various thermal loading scenarios at Yucca Mountain. Greatcr understanding is required of the cathodic protection of the inner container, the processes affecting the growth of pits, and even the dcfrnition of waste package "failure" in order to provide a more defensible argument for the range of likely waste package lifetimes. The ambient unsaturated zone percolation flux remains a very significant parameter in this iteration of Total System Performance Assessment. Any direct or indircct observations to better quantify the expected flux value and its uncertainty should be employed. It is foreseen that the preliminary site-scale unsaturated-zone model, to be completed by the U.S. Geological Survey ...