Evaluating the Moisture Conditions in the Fractured Rock at Yucca Mountain: The Impact of Natural Convection Processes in Heated Emplacement Drifts

Abstract

The energy output of the high‐level radioactive waste to be emplaced in the proposed geologic repository at Yucca Mountain, NV, will strongly affect the thermal–hydrological (TH) conditions in the near‐drift fractured rock. Heating of rock water to above‐boiling conditions will induce large water saturation changes and flux perturbations close to the waste emplacement tunnels (drifts) that will last several thousand years. Understanding these perturbations is important for the performance of the repository, because they could increase, for example, the amount of formation water seeping into the open drifts and contacting waste packages. Recent computational fluid dynamics analysis has demonstrated that the drifts will act as important conduits for gas flows driven by natural convection. As a result, vapor generated from boiling of formation water near elevated‐temperature sections of the drifts may effectively be transported to cooler end sections (where no waste is emplaced), where it would condense and subsequently drain into underlying rock units. Thus, natural convection processes have great potential for reducing the near‐drift moisture content in heated drift sections, which has positive ramifications for repository performance. To study these processes, we have developed a new simulation method that couples existing tools for simulating TH conditions in the fractured formation with modules that approximate natural convection and evaporation conditions in heated emplacement drifts. The new method is applied to evaluate the impact of in‐drift natural convection on the future TH conditions at Yucca Mountain in a three‐dimensional model domain comprising a representative emplacement drift and the surrounding fractured rock.