SITE CHARACTERIZATION FOR THE DEEP BOREHOLE FIELD TEST

The US Department of Energy Office of Nuclear Energy is beginning a 5-year Deep Borehole Field Test to investigate the feasibility of constructing and characterizing two large-diameter boreholes in crystalline basement rock to a depth of 5 km. The concept of deep borehole disposal for radioactive waste has some possible advantages over mined repositories, including incremental construction and loading, and the enhanced natural barriers provided by deep continental crystalline basement. Site characterization efforts at a future disposal site would be centered around determining the following characteristics: presence and nature of vertical pore fluid potential gradients, presence of economically exploitable natural resources, and presence of high permeability fault or fracture zone connections to the shallow subsurface. Characterization activities at the field test site will include geomechanical (i.e., rock in situ stress state, and fluid pressure), geological (i.e., rock and fracture infill lithology), hydrological (i.e., quantity of fluid, fluid convection properties, and solute transport mechanisms), and chemical (i.e., rock-water interaction and natural tracers) aspects. We discuss both direct (i.e., sampling and in situ testing) and indirect (i.e., borehole geophysical) methods for efficient and effective characterization of these site features or physical processes in large diameter boreholes with possibly elevated temperature and extensive borehole breakouts. Borehole-based characterization will be demonstrated at the field test site to determine the variability of system state (i.e., stress, pressure, temperature, and chemistry) with depth and interpretation of material and system parameters relevant to numerical site simulation.