THERMAL-HYDROLOGIC PROCESSES IN THE DRIFT-SCALE TEST AS INDICATED BY MEASURED TEMPERATURE

The Drift-Scale Test (DST) is the largest in-situ thermal test conducted by the Department of Energy to determine the feasibility of Yucca Mountain as a potential repository for high-level nuclear waste. The DST is designed to investigate the coupled thermal-mechanical-hydrologic-chemical (TMHC) processes expected in a geological high-level nuclear waste repository. The heat source in the DST includes nine floor heaters in the 48-m long Heater Drift (HD), and 25 wing heaters that extend about 10 m outward from both sides of the HD. The parameters measured in the DST include heater power, temperature, moisture content, rock mass displacement, air permeability, thermal conductivity and thermal diffusivity, deformation in the concrete liner, and chemistry of gas and water samples. The heating phase of the DST was from December 3, 1997 to January 14, 2002. By October 2002 the DST will be in its ninth month of the planned 4-year natural cooling phase.

The rock mass temperature in the DST is measured with resistance temperature devices (RTD) that are grouted in 28 holes. The measured temperatures provide good indications of the thermal-hydrologic processes during the test. The temperature-time history at each RTD shows that the dominant processes are heat conduction, and evaporation and boiling of pore water. Spatial variation of temperature shows the balancing of energy by thermal-hydrologic processes. The spatial distribution of the temperature-time curves shows that the boiling point of the pore water may vary with distance from the heat source, indicating the effect of the thermal-hydrologic-chemical process. The boiling point of the pore water may have been influenced by the variation of water chemistry caused by rock-water interactions.