Thermal-hydraulic study of a high conversion light water reactor

A high conversion light water reactor (HCLWR) has been developed at JAERI to improve fuel utilization. The fuel assemblies of the HCLWR have a triangular tight rod lattice in order to attain a high conversion ratio by reducing the moderator-to-fuel volume ratio. The tight lattice core leads to some thermal-hydraulic problems peculiar to the HCLWR. Therefore, experiments on steady-state and transient critical heat flux (CHF), turbulent mixing, bundle pressure drop, fuel vibration induced by fluid flow and reflood cooling have been performed to obtain data base and develop evaluation methods for the HCLWR. The evaluation methods were applied to a JAERI-proposed double-flat-core type HCLWR. Under the operational condition, the minimum allowable DNBR criterion is satisfied and the bundle pressure drop and flow-induced vibration and displacement of the fuel rod are within the design limit of a current LWR. Safety analyses under vital conditions such as the large break loss-of-coolant accident (LOCA), small break LOCA, pump trip accident, locked rotor accident, anticipated transient without scram (ATWS) induced by station blackout and control rod cluster ejection accident have been performed with a best-estimate REFLA/TRAC code. The results showed that the present HCLWR meets the current safety criteria for a LWR. The design features such as the large water inventory in the upper plenum, short core length, low axial peaking factor and negative void and coolant temperature reactivity coefficients contribute to the thermal-hydraulic feasibility of the double-flat-core type HCLWR. (author)