The Single Heater Test (SHT) was a success for the Yucca Mountain Project, not only because it became the first large-scale in situ coupled-process test, but also because of the range and quality of data that it produced. The data and modeling efforts provided critical information for performance assessment and design, to be used in the licensing process. The data have also allowed a realistic evaluation of the predictive codes and conceptual models used to describe the complex coupled processes expected to occur at Yucca Mountain. The data and experience gained from the SHT will allow even greater understanding of the ongoing Drift Scale Test and the proposed Cross-Drift Thermal Test.

Thermomechanical instrumentation was installed within and on the rock mass encompassed by the SHT. This thermomechanical instrumentation included temperature measurements using thermocouples, resistance temperature devices, and thermistors. Mechanical measurements included multiple-point-borehole extensometers (MPBXs), tape extensometers, surface-mounted wire extensometers, load cells on rock bolts, and the NX borehole jack. A series of post-test numerical models of SHT thermal-hydrologic (T-H) and thermal-mechanical (T-M) processes was completed using equivalent continuum and dual permeability T-H models and elastic and compliant joint T-M models. The results of the SHT and comparative analyses have provided the following key insights for repository design and performance assessment.

• Equivalent continuum conceptual models do not adequately capture or describe details of the thermal-hydrological behavior at the scale of the Single Heater Test; dual permeability models fit the data much better. For T-H modeling, the equivalent continuum model washes out the convective effects of the presence of fractures. For T-M modeling, the compliant joint model incorporates temporally and spatially averaged inelastic behavior of fractures, but does not identify discrete joint slippage events and poses mesh scaling problems to the observed joint spacing at the SHT site.
  
• Rock mass modulus, rock mass thermal expansion, and thermal stress are significantly reduced by the presence of fractures.
  
• Intact rock thermomechanical properties are largely unaffected by the thermal pulse of the Single Heater Test. Thermomechanical properties of intact rock should not change over time, based on current laboratory test results.
  

Collaboration with R.E. Finley, N.S. Brodsky, N.D. Francis, J.T. George, and David Bronowski

Animated Temperature Variations
(To start the animation, click the plot in the pop-up window.)