Total System Performance Assessment, 1993: An evaluation of the potential Yucca Mountain repository, B00000000-01717-2200-00099, Rev. 01

Abstract

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 ...

Site characterization progress report: Yucca Mountain, Nevada. Number 15, April 1--September 30, 1996

“…For the most part, these analyses as documented in previous total system 6-22 performance assessment iterations in (Barnard et al, 1992Eslinger et al, 1993;Andrews et al, 1994;Wilson et al, 1994;CRWMS M&O, 1995a) have identified the key conceptual models and parameters that significantly affect the predictions of postclosure performance. The results, conclusions and recommendations of these previous total system performance assessment analyses (as well as other performance assessment analyses of individual components of the overall system, such as the geosphere performance as exemplified in the ground-water travel time analyses and the engineered barrier performance as embodied in analyses of releases from the engineered barrier system) have been used to shape the present waste containment and isolation strategy.…”

“…In all instances in which the fracture flow percentage was markedly increased from the nominal values in Total System Performance Assessment -1993 (Andrews et al, 1994;Wilson et al, 1994) or Total System Performance Assessment -1995 (CRWMS M&O, 1995a), the increased fracture flux resulted in reduced travel times through the unsaturated zone and therefore an increased likelihood of obtaining significant releases and doses at the accessible environment boundary in the first 10,000 years after repository closure. These results reemphasize the need to focus the site characterization program on ascertaining the distribution and magnitude of localized fracture flow in the repository block.…”

“…Version 1 .O of WAPDEG computes the failure time of waste packages (defined as having at least one pit penetration) and the subsequent pitting degradation of the waste packages. The model builds on the pitting corrosion model for the Alloy 825 (ASTM B 424 N08825) inner barrier used for Total System Performance Assessment -1993 (Andrews et al, 1994;Wilson et al, 1994), on the waste package degradation modeling approach of Total System Performance Assessment -1995 (CRWMS M&O, 1995a), and on new corrosion models for the carbon steel outer barrier (Lee et al, 1996b). The most recent of these approaches, the Total System Performance Assessment -1995 version of waste 6-67 PROGRESS REPORT #15 package degradation model, captures only a few corrosion degradation modes.…”

“…Performance Assessment -1995 from an expert elicitation on (time-independent) pit growth rate ranges for Alloy 825 (Andrews et al, 1994). For WAPDEG, two additional alternative inner barrier pitting corrosion models were developed from the same expert elicitation and the sensitivity of waste package degradation to the alternative pitting models was studied.…”

“…Event trees that form the basis for scenario development for volcanism and tectonics are now complete. To date, preliminary disruptive scenarios for volcanism, reported in Total System Performance Assessment -1991 (Barnard et al, 1992;Eslinger et al, 1993) and Total System Performance Assessment -1993 (Andrews et al, 1994;Wilson et al, 1994), indicate that volcanic effects are not significant to system performance. Changes that have occurred in the implementation of the reorganized study are summarized below:…”