The mission for the Department of Energy's Office of Civilian Radioactive Waste Management is to safely manage and dispose of the nation's spent nuclear fuel and high level radioactive waste in a geologic repository. A potential site at Yucca Mountain in Nevada is being studied by the DOE. Nuclear waste is to be contained in packages which will be emplaced in the repository for thousands of years. After these manmade packages eventually degrade, the repository should continue to isolate nuclear waste from the environment. The repository is to comply with the 1982 Nuclear Waste Policy Act and its amendments. Also, the health and safety of the workers will not be compromised during the construction and operation of the repository. To investigate important technical issues inherent with the construction, operation, closure, and performance of the repository, a series of in situ experiments have been planned for the Exploratory Studies Facility (ESF) located inside Yucca Mountain. The ESF Thermal Test is an integral part of the Site Characterization Plan developed in 1988 following the Congressional mandate to evaluate only Yucca Mountain as a potential repository. The planning documented in the Site Characterization Program has evolved to include the construction of the ESF to accommodate changing needs and increased understanding of the Yucca Mountain Project. The recently updated ESF thermal testing strategy includes the Drift Scale Test (DST). The DST is more complex, longer duration and larger-scale than its predecessor--the Single Heater Test. The primary purpose of the DST is to acquire a more in-depth understanding of the coupled thermal-mechanical-hydrological-chemical processes anticipated in the rock mass surrounding the proposed repository.