Preliminary scoping calculations of hydrothermal flow

The Grouse Canyon member of the Belted Range tuff found in G-Tunnel consists of a highly fractured, densely welded tuff. These fractures have been observed to be major conduits for fluid flow and must therefore be accounted for in the near-field hydrothermal model. Because of the low surface recharge rate, the rock at the Prototype Engineered Barrier System Field Tests (PEBSFT) horizon is only partially saturated, and multiphase flow of air, water vapor, and liquid water is therefore possible. Heat generated by the waste package can lead to water vaporization and vapor flow in the matrix and fractures. Condensation of this vapor outside the boiling zone and capillary-driven flow of liquid water can result in the development of a heat-pipe (Pruess et al., 1984). To account for these conductive, convective, and latent heat transport mechanisms, the authors use the integral finite difference code TOUGH. They have conceptualized the physical medium as a discrete fracture/matrix system. As such, their model utilizes what they consider to be the most appropriate available data for the fracture and matrix properties of Grouse Canyon tuff. Due to the lack of some of the matrix data for Grouse Canyon tuff (and due to its similarity to Topopah Springs tuff), they decided to apply some of the Topopah Springs tuff matrix data to their model. This approach is well suited to small-scale systems in which the fracture spacing is large in comparison to the scale of the simulation domain. Because their calculations are to be used in assuring that the instruments surrounding the heater are adequately sensitive to the range of pressures, temperatures, and saturations to be encountered, their model must be sufficiently resolved to indicate small-scale variations