Predictions and observations of the thermal–hydrological conditions in the Single Heater Test

Abstract

The Single Heater Test (SHT) is one of two in-situ thermal tests included in the site characterization program for the potential underground nuclear waste repository at Yucca Mountain. Coupled thermal–hydrological–mechanical–chemical processes in the fractured rock mass around the heater were monitored by numerous sensors emplaced among 30 boreholes. Periodic active testing of cross-hole radar tomography, neutron logging, electrical resistivity tomography, and interference air permeability tests probed the change of moisture content in the rock mass. Thermal–hydrological processes in the SHT have been simulated using a 3-D numerical model and compared to the monitored data. The good agreement between the temperature data and simulated results indicates that the thermal–hydrological responses of the SHT in the 9 months of heating are well-represented by the coupled thermal–hydrological numerical model. The dominant heat transfer process is by conduction and the signature of vapor and liquid counter flow is subtle in the temperature data. The simulated result of a dry-out zone of about 1 m (at the end of the heating phase) around the heater hole, and a condensation zone of increased liquid saturation outside of the dry-out zone, is consistent with the radar tomography and air permeability data. Tomography data and post-test laboratory measurements indicate that the moisture content is larger below than above the heater horizon, suggesting gravity drainage of condensate in the fractures. Model studies show that gravity drainage occurs in simulations using the dual permeability conceptual model, but is absent in the effective-continuum model, where matrix and fractures are required to be in thermodynamic equilibrium at all times.