Analysis of thermohydrologic behavior for above-boiling and below-boiling thermal-operating modes for a repository at Yucca Mountain

We report results from a multi-scale thermohydrologic modeling study for two alternative thermal-operating modes for the potential repository system recently analyzed by the Yucca Mountain Project. These include a Higher-Temperature Operating Mode (HTOM), which results in a localized boiling zone around each emplacement drift, and a Lower-Temperature Operating Mode (LTOM), which always maintains sub-boiling temperatures throughout the repository. The HTOM places all waste packages nearly end to end, making the lineal power density greater than in the LTOM. The lower lineal power density in the LTOM was achieved by placing some waste packages farther apart (which results in a larger repository footprint), and through an increased reliance on pre-closure ventilation to remove the waste-package-generated heat. We focus on temperature T and relative humidity RH at the waste-package and drift-wall surfaces, and on in-drift evaporation. In general, HTOM temperatures are greater than corresponding LTOM temperatures, exhibit similar spatial variability and have a stronger dependence on infiltration flux. The duration of RH reduction on waste packages is similar for the LTOM and HTOM. A major difference between the LTOM and HTOM is the lower waste-package temperature at any given value of waste-package RH for the LTOM. Waste-package temperatures in the LTOM, by design, remain below ∼85 °C; the absence of RH reduction arising from host-rock dryout causes waste-package RH to remain above about 40%. The HTOM waste packages experience higher temperatures and correspondingly lower RH conditions as a result of RH reduction arising from host-rock dryout. For most of the repository area, the HTOM delays the potential onset of gravity-driven seepage compared to the LTOM (as indicated by the duration of boiling at the drift wall). Boiling conditions in the HTOM also delays the onset of capillary-driven seepage into the granular invert, causing the HTOM to have less evaporation in the invert during the first 800–1500 years than the LTOM; subsequent evaporation rates are higher in the HTOM, due to the higher power density.