Multi-Scale Near-Field Thermohydrologic Analysis of Alternative Designs for the Potential Repository at Yucca Mountain
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Table I. TH performance measures estimated by the multi-scale TH modeling methodology Measures calculated for TSPA-VA are indicated by bold, uppercase X; other measures also calculated by the approach are indicated by lowercase x. The measures are temperature (T), relative humidity (RH), gas-phase air-mass fraction (Xar,,), liquid-phase saturation (SIiq), and liquid-phase flux (qliq). qi SlIq Xairgas RH T Location X x x x x Rock above drift wall x x x X X Rock at drift wall X Drift above WP x X X WP surface x X x x x Invert NUMERICAL MODELS AND ASSUMPTIONS Model calculations were conducted with the NUFT code [3], which was developed at
Lawrence Livermore National Laboratory to simulate the coupled transport of water, vapor, air.
and heat in fractured porous media. Calculations were carried out for the range of hydrologic properties and percolation fluxes considered by TSPA-VA [4], including repository-areaaveraged percolation fluxes from 2.6 to 126 mm/yr. Calculations were carried out for the VA design, which uses point-load WlP spacing, a concrete invert, and no backfill; for alternative designs, such as line-load WP spacing, which places WPs nearly end to end in widely spaced drifts; and for cases which are backfilled at 100 years with either crushed tuff or quartz sand. Backfill is assumed to fill 85% of the drift, with the upper 15% of the drift an open cavity [4]. For the backfill cases, the invert is assumed to be filled with the same material as the backfill. For line-load cases, the backfill is assumed not to fill the gap between WPs. For all cases, the areal mass loading (AML, in metric tons of uranium per acre, MTtJ/acre) is 85 MTU/acre. A mix of WP types was modeled (Table 1I), ranging from those containing hot 10-year-old, highburnup CSNF to cold defense high-level waste [4]. The multi-scale TtI model (Figure 1) integrates the results from four classes of models: * 2-D line-averaged-heat-source drift-scale TH (LDTH) models, which use the dual-permeability method to represent fracture-matrix interaction, are spaced uniformly throughout the repository at 35 locations in a 5 x 7 grid [1]. The LDTII models are run in parallel with the I-D smearedheat-source drift-scale thermal (SDT) models, a total of 140 LDTH-model runs and 140 SDTmodel runs per scenario. LDTH models are used to obtain the functional relations between the drift-wall Tand these variables: drift-wall RI! and Sliq, in-driftXair. as, and invert Sliq. * 1-D SDT (conduction-only) models are run in parallel with the LDTH models at 35 locations for AMLs of 21.25. 42.5. 56.67, and 85 MTlU/acre to obtain functional relations between the SDT-model-predicted host-rock T and the LDTH-model-predicted drift-wall T. * 3-D discrete-heat-source drift-scale thermal (DDT) models account for WP-specific heat output and for thermal radiation between all WP and drift surfaces to determine WP-specific deviations (relative to line-averaged-heat-source conditions) in drift-wall and WP Ts. For backfill cases, heat conduction in the backfill controls temperature de
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