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Berkeley Lab, in collaboration with other national laboratories, has performed the largest thermal test in the world, the Drift Scale Test (DST). The DST investigated the thermal-hydrological-chemical-mechanical coupled processes in a 50 x 60 x 80 m region in the ESF over a temperature range of 25°C to 200°C. In the DST testing methodology, based on experience from the SHT and previous thermal tests, was expanded and improved to meet the demands of the large-scale test. Nine full-scale heaters simulate waste packages emplaced in the potential repository. Two panels of wing heaters simulate the thermal environments of multiple drifts. Heating was initiated at 187 kW in December 1997, with a heating period of 4 years, and a cooling period of 4 years.
Monitoring in the nearly 100 boreholes drilled for thermal, hydrological, mechanical, and chemical measurements detected significant water movement and water-vapor condensation effects. A prominent heat-pipe signature in the temperature measurements provided evidence of thermal-hydrological coupling. Geophysical imaging techniques and permeability monitoring indicated significant redistribution of water caused by thermal effects. Substantial increases in gas-phase CO2 concentration occurred in large regions around the heaters. Chemical analysis of the water collected in boreholes indicated calcite and silicate mineral interaction with water.
Investigators tested conceptual models of coupled processes by comparing simulations of the DST with measured data on saturation distributions, CO2 concentrations, and water chemistry. These models also provide input to coupled-process seepage simulations.