3 RI - O ' UCID - Is A SCP 5 - 1 765

A primary objective of the Engineered Barrier Design Test (EBDT) at the Yucca Mountain Exploratory Shaft Test Site is to examine how the variably saturated, fractured rock mass surrounding the waste package will respond hydrothermally to heating, cooling, and infiltration. The EBDT will include several partialand/or full-scale electrical heaters to simulate the thermal load generated by the waste packages. An extensive network of geophysical instruments will monitor changes in the temperature, pressure, and saturation distributions in the area around the heaters during the heating and cooling phases of the experiment. The design of the network requires advance knowledge of the spatial and temporal changes in these properties, as well as the ranges in these properties. We model this hydrothermal system as a discrete fracture/matrix system, using the integral finite difference code TOUGH and the best available data on the fracture and matrix properties of Topopah Spring densely welded tuff. These calculations will also be useful for the design of the underground facilities in the vicinity of the EBDT; e.g., in determining the volume of undisturbed rock required for each EBDT heater. After conceptualizing our model to be an infinitely long heater (either horizontal or vertical) which is orthogonally intersected by an infinite set of uniformaly spaced fractures, we justify its applicability to both horizontal and vertical heater emplacement. The calculations show that by the end of the full-power heating stage (t = 6 months), boiling in the rock results in complete desaturation out to a radius of r = 0.8 m from the heater axis, with partial desaturation occuring out to r = 1.8 m. Water vapor which does not leave the system via the borehole (to the drift) moves radially outward to the condensation zone lying 1.8 to 4.0 m from the heater axis. Gas pressures build up considerably within the matrix due to the low matrix permeability, but remain close to ambient within the fracture due to the high fracture permeability. At the end of the cooling stage (t = 24 months), the saturation in the matrix is below ambient for r < 3.0 m. Maximum temperature changes (above ambient) are 252.6 TC at the borehole wall, and 10.2 0C at a radial distance of 10.0 m from the heater axis. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.