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# NRC Sensitivity and Uncertainty Analyses for a Proposed HLW Repository at Yucca Mountain, Nevada Using TPA 3.1:  Conceptual Models and Data (NUREG-1668, Volume 1)

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## Publication Information

**Manuscript Completed:** January 2001**Date Published:** February 2001

**Prepared by:** S. Mohanty, CNWRA T.J. McCartin, NRC

Center for Nuclear Waste Regulatory Analyses Southwest Research Institute 6220 Culebra Road San Antonio, TX 78238-5166

Division of Waste Management Office of Nuclear Material Safety and Safeguards U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

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## Abstract

Total system performance assessment (TSPA) is playing an increasingly vital role in regulatory decision-making. Within the U.S. Nuclear Regulatory Commission (NRC) high-level waste program, TSPA studies are being performed to focus the activities of the NRC key technical issues (KThs); evaluate the hypotheses of the U.S. Department of Energy (DOE) Repository Safety Strategy; and prepare for the NRC review of the DOE viability assessment (VA) for the Yucca Mountain (YM).

Conducting a TSPA for the proposed repository involves application of a total-system model that simulates the processes affecting repository performance, including propagation of the uncertainties associated with model parameters, conceptual models, and future system states. The simulation process, which implements a probabilistic framework, integrates a broad spectrum of site-specific data and information and produces estimates for a set of performance measures. Building on the previously developed NRC assessment methodology, a new Total-system Performance Assessment (TPA) code, designated TPA Version 3.1.4 code, was developed by NRC and the Center for Nuclear Waste Regulatory Analyses to evaluate the relative significance of the NRC-identified KTIs/subissues and to assess the assumptions and models in forthcoming DOE TSPAs, such as the DOE TSPA VA for the YM site.

The TPA Version 3.1.4 code is designed to estimate total-system performance measures of annual individual dose or risk. The TPA Version 3.1.4 code consists of an executive driver, a set of consequence modules, and a library of utility modules. The executive driver controls the probabilistic sampling of input parameters, the calculational sequence and data transfers among consequence modules, and the generation of output files. The various output files are used in parameter sensitivity analyses, post-processing of time-dependent risk curves, and synthesis of statistical distributions (e.g., cumulative distribution functions and complementary cumulative distribution functions) for appropriate performance measures. Consequence models simulate physical processes and events such as unsaturated zone infiltration; evolution of the near-field thermalhydrologic environment; failure of waste packages; dissolution and release of waste; transport of waste in the ground-water system; extraction of ground water; and consumption of ground water which, if contaminated, would expose future populations to radiation. In addition to considering climate change, the TPA Version 3.1.4 code is designed to calculate the effects of disruptive events such as faulting, seismicity, and volcanism. Utility modules ensure the consistency of algorithms and data sets used repeatedly by various consequence modules.

This volume has been prepared to describe the methods and assumptions used in the TPA Version 3.1.4 code. It contains descriptions of
        - Overall TSPA methodology
        - Consequence modules that simulate physical processes and events that affect the release and transport of radionuclides

It should be noted that the TPA Version 3.1.4 code is only an interim version of the code. An improved version will soon supersede it.

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