The Thermal-Hydrological Impact on Increased Spent-Fuel Storage Capacity in Yucca Mountain Repository

This paper describes the recent work to evaluate the technical storage capacity for spent fuel in the Yucca Mountain repository. To increase the capacity from the current statutory limit of 63000 tonnes HM commercial spent nuclear fuel (CSNF), two alternative repository designs are proposed and analyzed, which add two additional emplacement drifts adjacent to each current-design drift. All designs assume the same waste package inventory, or heat generation rate, and drift ventilation as the current design. As both alternative designs would fit the well-characterized repository footprint, no additional site characterization at Yucca Mountain would be necessary. The work also examines extended ventilation and phased waste-loading assumptions in anticipation of an expanded role for nuclear power in electricity generation. The key parameter to the storage capacity in the Yucca Mountain site is water movement. To study the thermal and hydrological responses to increased storage capacity, series of two-dimensional models were used to simulate coupled heat and mass (water and air) transfer within the repository system and the near-field subsurface environment, including all geological formations above and below the repository horizon from the surface to the water table. A three-dimensional model was applied to investigate the effect of axial heat transfer and fluid flow. The results show that the current repository footprint can accommodate three times the currently legislated 63000 tonnes HM of CSNF without compromising repository performance.