Among the candidates for the next generation of nuclear power plant is the high temperature gas reactor (HTGR). Since the HTGR does not have the same operational history as water cooled reactors do in the United States, investigation into the response of different accident phenomena is still an active area of research. The current work investigates the pressurized conduction cooldown (PCC) accident in an integrally scaled high temperature gas reactor experimental facility. A PCC accident occurs when the primary circulator loses the ability to provide forced convection to the core. During a PCC accident, coolant that is normally forced downward through the core during normal operation loses momentum after the circulator is lost leading to flow stagnation and eventually results in an intracore natural circulation flow path which brings hot coolant into the upper plenum. Critical instrumentation such as the control rod drive shafts are located in the upper plenum. As such, it is important to understand the upper plenum mixing and temperature profile to better support the safety of HTGRs during these accidents. In addition to the upper plenum mixing of the PCC accident, the current work investigates the effects of asymmetric core heating on the upper plenum mixing during a PCC accident. It was hypothesized that core asymmetries could lead to a more developed hot and cold region within the core driving a hotter, more strongly penetrating buoyant plume into the upper plenum during the PCC accident. This was done by performing a series of experiments in the Oregon State University High Temperature Test Facility (HTTF) along with a series of simulations. The outcome of this work will provide reactor designers with a better understanding of how core heating patterns effect the development of buoyant plumes in the upper plenum, the leading mechanisms for these plumes, and insight into performing simulations of the PCC accident.