The Yucca Mountain Project

The Drift Scale Test

Berkeley Lab, in collaboration with other national laboratories, has performed the largest thermal test in the world, the Drift Scale Test (DST). The DST investigates 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, has been 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 has detected significant water movement and water-vapor condensation effects. A prominent heat-pipe signature in the temperature measurements provides evidence of thermal-hydrological coupling. Geophysical imaging techniques and permeability monitoring indicate significant redistribution of water caused by thermal effects. Substantial increases in gas-phase CO₂ concentration occur in large regions around the heaters. Chemical analysis of the water collected in boreholes indicates calcite and silicate mineral interaction with water.

Investigators can test conceptual models of coupled processes by comparing simulations of the DST with measured data on saturation distributions, CO₂ concentrations, and water chemistry. The models also provide input to coupled-process seepage simulations.

Results:

• Drift wall temperature reached ~190°?C after 2 years of heating (since December 1997 at 187 kW).
• Most of the condensate accumulated below the wing heaters at early times.
• Wetting and drying zones were identified by periodic air-injection tests and geophysical methods.
• Gas-phase CO₂ concentration increased strongly in the large region around the heaters.
• Interactions of calcite and silicate minerals were indicated by chemical analyses of collected water.

Reference:
Birkholzer, J.T., and Y.W. Tsang, "Modeling the Thermal-Hydrologic Processes in a Large-Scale Underground Heater Test in Partially Saturated Fractured Tuff," Water Resources Research, 36 (6), 1431-1447, 2000.

For more information, please contact:
Jens Birkholzer
Ph: 510-486-7134

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