An investigation into the onset of natural circulation during a depressurized conduction cooldown was conducted at the High Temperature Test Facility at Oregon State University. In this set of four tests, the primary loop of the facility was filled with helium and then heated until a temperature difference across the core was: 125°C, 250°C, 375°C, and 500°C. The Reactor Cavity Simulation Tank (RCST) was filled with nitrogen gas. During the heating phase of the test the primary loop and RCST were held at pressures greater than 130 kPa. Once the desired temperature was achieved the primary loop and RCST pressures were reduced to 112 and 110 kPa, respectively. The cold leg break valve was opened and then the hot leg break valve was opened. The hot helium in the primary loop began to flow into the RCST displacing the cold nitrogen, in a lock exchange flow. Once the density differences equalized in the two tanks, a natural circulation will develop as the gas is heated in the core, flows from into the RCST through the upper plenum, upcomer, and cold leg. Once cooled in the RCST the gas then flows through the hot leg and returns into the core. This paper discusses the findings for each of the four tests and compares the time required for the natural circulation to establish as a function of temperature across the core.