ABSTRACT A three-dimensional finite element thermal stress analysis of the RH TRU experiments in WIPP Room T has been performed. This analysis aids in the interpretation of the borehole closure results being obtained from the Room T experiments and helps in assessing potential performance impacts in a typical storage room, during the waste retrieval period. Computed results are presented and compared to available in situ data, and a qualitative agreement between measured and computed closures is seen.
INTRODUCTION The Waste Isolation Pilot Plant (WIPP) is a research and development facility authorized to demonstrate the safe disposal of radioactive wastes arising from the defense activities of the United States. The WIPP is being developed by the U.S. Department of Energy (DOE) and is located in southeastern New Mexico in a bedded salt formation at a depth of about 650 m below the surface. Data are currently being collected from a series of large-scale in situ experiments at the WIPP, among them the simulated Contact Handled and Remote Handled Transuranic (CH TRU and RH TRU, respectively) waste tests in Room T (Molecke 1986). These experiments are being conducted under the auspices of the WIPP Waste Package Performance (WPP) program. The simulated RH TRU waste experiments were emplaced in mid-1986. They have been in heated test operation since September 23, 1986, and are expected to continue for a total of 3 to 5 years (Molecket al. 1988). These experiments involve the emplacement of 8 full-size, 0.66 m diameter by 3.0 m long, simulated (non-radioactive) RH TRU canisters in horizontal unlined boreholes in the rocksalt ribs of test Room T. The canisters are electrically heated at approximately 120 W each, or about 2 to 4 times the expected average thermal output from the actual radioactive RH TRU wastes to be disposed of in the WIPP. Figure 1, taken from the test plan (Molecke 1986), shows some general details of the RH TRU waste experiments in Room T. The test emplacement boreholes are 0.91 m (36 in) in diameter and 4.88 m (16.0 ft) deep. Four of the eight emplacements (TRH03, TRH04, TRH05, and TRH06) are back filled, while the remaining four (TRH01, TRH02, TRH07, and TRH08) are not. There are four holes per side at the southern end of the 3.96 m (13 ft) high by 10.06 m (33 ft) wide room. The centers of the holes are located 1.68 m (5.5 ft) above the drift floor and 2.44 m (8 ft) apart. This spacing coincides with a &reference& spacing which has been inferred by the WIPP project from the specified areal thermal load limitation of approximately 2.5 W/m (or 10 kW/acre). Several of the borehole emplacements are fully instrumented, as shown in Figure 2, with remote-reading thermocouples, pressure gages, and borehole vertical closure gages. These are included to monitor borehole thermal environments as a function of RH TRU thermal output (120 W and, later, 300 W) and to measure the vertical closure of the emplacement boreholes as a function of time and temperature (Molecke et al. 1988). A three-dimensional (3D) geomechanical model was used to simulate the thermal/ structural response of the Room T experiments.