Alternative Assessments of the Performance of the Yucca Mountain Candidate HLW Repository
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ABSTRACT This paper summarizes some of the most recent total system performance assessment (TSPA) work supported by EPRI for the proposed repository at Yucca Mountain, Nevada. In Part I of the paper, 'standard' TSPA analyses are presented. In Part II examples of two types of analyses that augment standard TSPAs are provided that suggest: (1) many components of the Yucca Mountain system contribute to overall hazard reduction; and (2) the biosphere dose conversions used, based on reasonably maximizing assumptions about future human behavior, provide a reasonable upper bound. These additional analyses should provide further confidence when considering more 'standard' TSPA analyses. PART 1: YUCCA MOUNTAIN TOTAL SYSTEM PERFORMANCE ASSESSMENT USING
IMARC Previous reports [1-3] documented the general model used in the EPRI work and specific inputs to the Yucca Mountain TSPA model. This part of the paper presents results from specific calculational cases of repository performance, updated for the most recent data and conceptual models [4]. The EPRI project uses its Integrated Multiple Assumptions and Release Calculations (IMARC) code to calculate dose vs. time using a logic tree format in which specific critical conceptual models or input parameters are identified and treated as uncertain. The program organizes the uncertainties into a "logic tree" and calculates all possible combinations of these models or parameters (and their weights), and for each combination estimates radionuclide concentration and dose vs. time. The logic tree approach [3] represents uncertain inputs to PA calculations as nodes in a tree, with branches from a node indicating alternative models or parameter values for that input and the weight associated with that model or parameter value. This logic tree results in 108 different combinations of models and parameters, as follows: * Climate Scenarios (2 alternatives) * Infiltration (3 alternatives) * Focused flow factor (2 alternatives) * Solubility/alteration time (3 alternatives) * Retardation (3 alternatives) Input values of climate scenarios, infiltration rates, focused flow factors, solubilities and alterations rates, and retardation values for each radionuclide are documented in references [3] and [4]. They are very briefly summarized here. For future climate conditions, previous work on climate predictions [3] has been abstracted into two assumptions: (1) 'moderate' greenhouse effect, and (3) 'permanent greenhouse' climate. These provide differences among future climates as they affect Performance Assessment (PA) calculations. Within each climate assumption one sequence of climate changes is used. Net infiltration is the net flux of groundwater below the root zone into geologic units. Net infiltrations were calculated from models of precipitation and the effects of soils, vegetation, and hydrologic units. Probability distributions of net infiltration from those calculations were recognized as variable from year to year, and were averaged to generate long-term average flow parameters at depth. L
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