Evaluation and Comparative Analysis of Single Heater Test

The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain. This report describes the thermal and thermomechanical data collected for the SHT through the third quarter of heating (through 31 May 1997) and compares the data with numerical predictions in order to gain insight into the coupled Thermal-Mechanical-Hydrologic-Chemical processes. The SHT was energized on 26 August 1996 with the nominal heater power set at 4 kW. The heater was deactivated on 28 May 1997 at 20:31 Universal Coordinated Time, after being on for 275 days, 2 hours. Thermomechanical instrumentation was installed within and on the rock mass encompassed by the SHT. This thermomerchanical instrumentation includes temperature measurements using thermocouples, resistance temperature devices, and thermistors. Mechanical measurements include multiple-pointborehole extensometers (MPBXs), tape extensometers, surface-mounted wire extensometers, load cells on rock bolts, and the NX borehole jack. Data for both thermal and mechanical measurements are presented and discussed. Major findings and recommendations include: * The temperature distribution around the heater is radially symmetric; conduction appears to be the primary mode of heat transfer. * Temperature gages in open boreholes exhibit erratic temperature behavior as their temperatures pass through the boiling point of water. Because gages grouted into sealed boreholes do not exhibit this type of behavior, it is attributed to intra-borehole vapor phase heat transfer effects. * Fifteen of 319 temperature gages (almost 5%) have failed so far. * Four mechanical gages in MPBX-1 have failed for unknown reasons. Two mechanical gages in MPBX-3 have failed. * Several temperature gages on probe TMA-RTD15 exhibit anomalous behavior that cannot be dismissed as attributable to failed gages. The data from these gages may indicate convective heat transport in fractures. * Thermal data indicate the formation of a dry-out zone extending radially outward from the heater to approximately the 1000C isotherm located roughly I m from the heater. * Numerical simulat ,1. of thermal behavior do not accurately predict observed thermal data. This is likely due to limitations imposed by the Equivalent Continuum Model, which is the conceptual model on which the numerical simulations are based. * The expansion of the rock mass is not explained by elastic continuum models; therefore, it is strongly recommended that thermomechanical discrete block modeling be performed. * The calculated rock mass thermal expansion from selected MPBX data is between about 4 and 6xl0O61C. * The rock mass modulus measured in the SHT using a borehole jack is significantly lower than the intact modulus. Measured rock mass moduli between about 3 and 23 GPa have been measured. * Data from a single rock mass modulus test using a borehole jack show thermally induced stiffening of the rock mass in the region near the heater. Rock mass moduli in this region have increased from about 8 GPa on 26 November 1996 to about 23 GPa on 18 March 1997. This modulus increase is likely due to closing of fractures by rock matrix thermal expansion. The data presented in this report should be considered indeterminate until the procurement/calibration issues related to Yucca Mountain Project deviation report YM-97-D-025 have been resolved.