STUDY OF ABNORMAL HEAT TRANSFER DURING FORCED AND NATURAL CONVECTION SCENARIOS IN A PRISMATIC CORE OF A VHTR: NUMERICAL AND EXPERIMENTAL RESULTS

摘要

Very High Temperature Reactors (VHTRs) are one of the Generation IV gas-cooled reactor models proposed for implementation in next generation nuclear power plants. A high temperature/pressure test facility for forced and natural circulation experiments has been constructed. This test facility consists of a single flow channel in a 2.7 m (9') long graphite column equipped with four 2.3kW heaters. Extensive 3D numerical modeling provides a detailed analysis of the thermalhydraulic behavior under steady-state, transient, and accident scenarios. In addition, forced convection experiments with air and nitrogen were conducted for inlet Reynolds numbers from 1,500 to 70,000. Numerical results, using commercial finite element package, COMSOL Multiphysics, were validated with forced convection data displaying maximum percentage errors under 15%. Based on this agreement, simulations are also extended to study natural circulation of nitrogen and helium between two connected vessels, resembling the behavior between inner and outer cooling channels in a VHTR reactor core. Results were obtained for Rayleigh numbers of ~106. This work also examines flow laminarization for a full range of Reynolds numbers including laminar, transition and turbulent flow under forced convection and natural circulation and its impact on heat transfer under various scenarios to examine the thermal-hydraulic phenomena that could occur during both normal operation and accident conditions.