The Yucca Mountain Project is currently evaluating the coupled thermal-mechanical-hydrological-chemical ("MHC) response of the potential repository host rock through an in situ thermal testing program. A Drift Scale Test (DST) was constructed during 1997 and heaters were turned on in December 1997. The DST includes nine canister-sized containers with h r t y operating heaters each located within the heated drift (HD) and fifty "wing" heaters located in boreholes in both ribs with a total power output of nominaUy 210 kW. A total of 147 boreholes (combined length of 3.3 km) houses most of the over 3700 TMHC sensors connected with 201 km of cabling to a central data acquisition system. The DST is located in the Exploratory Studies Facility h a 5-m diameter drift approximately 50 m in length. Heating will last up to four years and cooling will last another four years. The rock mass surrounding the DST will experience a harsh thermal environment with rock surface temperatures expected to reach a maximum of about 200°C. I This paper describes the process of designing the DST. The first 38 m of the 50-m long Heated Drift (HD) is dedicated to collection of data that will lead to a better understanding of the complex coupled TMHC processes in the host rock of the proposed repository. The final 12 m is dedicated to evaluating the interactions between the heated rock mass and cast-in-place (CIP) concrete ground support systems at elevated temperatures. Instrumentation includes thermocouples, resistance temperature devices (RTDs), thermistors, multiple-point borehole extensometers (MPBXs) installed both from within and outside the DST, strain gages installed on the CIP concrete section, cross-drift displacement extensometers, humidity sensors, pressure transducers, electrical resistivity tomography, neutron logging, ground penetrating radar, absorbent pads for water collection, gas sampling ports, acoustic emissiodmicroseismic sensors, an infrardvideo camera system, and rapid evaluation of thermal conductivity and diffbsivity probe systems. In addition to a description of the DST design, data from site characterization, and a general description of the analyses and analysis approach used to design the test and make pretest predictions are presented. Test-scoping and pretest numerical predictions of one way thermal-hydrologic, thermal-mechanical, and thermalchemical behaviors have been completed (TRW, 1997a). These analyses suggest that a dry-out zone will be mated around the DST and a 10, OOO m3 volume of rock will experience temperatures above 100°C. The HD will experience large stress increases, particularly in the crown of the drift. Thermoelastic displacements of up to about 16 mm are predicted for some thermomechanical gages. Additional analyses using more complex models will be performed during the conduct of the DST and the results compared with measured data R In situ testing, geomechanics, coupled processes thermal testing, numerical modeling. DISCLAIMER This report was prepared as an account of work sponsored by an...