SUMMARY

High temperature gas cooled nuclear reactors are being considered for near term deployment over other next generation reactor designs for their enhanced safety features and ability to produce high grade thermal energy for process heat applications. For such reactor designs, under station blackout conditions or loss of offsite power accidents, it is estimated that the reactor core can safety shutdown without forced circulation. This safety claim will be evaluated through the use of a newly developed transient thermal hydraulic method designed specifically for prismatic hexagonal block graphite cores. Simulation of such accidents requires detailed core modeling of the prismatic fuel block geometry. Typical safety analysis codes used for standard light water reactors are not well equipped to model this geometry. Considerable adaptation and extrapolation and creative modelling of the fuel blocks is required to capture all the relevant heat transfer and fluid flow phenomena to enable the use of these existing safety analysis codes. In particular, bypass flow heat transfer has not been accurately accounted for in transient or safety analyses. Results from the newly developed method will be compared to similar RELAP5 simulations with enhanced hexagonal block core modelling to capture as much physical detail as possible. The results of the proposed study will provide a clear and reliable foundation for future safety basis and licensing efforts.