The purpose of the Single Heater Test (SHT), the design of the test, and the borehole layout were reported by OCRWM M&O (1996a). Briefly, the main purpose of the SHT is to study the thermal- mechanical behavior of the densely welded non-lithophysal Topopah Spring tuff at the Exploratory Studies Facility (ESF). The SHT is also used as a shake-down for the test of the coupled thermal- mechanical- hydrological-chemical (TMHC) processes.