Analysis Of Thermal Hydraulic Behaviors And Event Tree At Mid-loop Operation In Pwr Npp

The risk probability that the plant is under low power and shutdown is similar to the one that the plant is at full power operation in the pressurized water reactor (PWR). Especially, loss of residual heat removal (RHR) system at mid-loop operation is very significant. Once, RHR system is unavailable, core decay heat will gather in the core, and lead to core damage. Therefore, it's necessary to pay more attention and concern on mid-loop operation.Thermal hydraulic behavior of nuclear power plant (NPP) is analyzed for loss of RHR system during mid-loop operation at 300 MWe pressurized water reactor in this study. And, according to the results obtained in the simulation and abnormal procedure, event trees during mid-loop operation are built.Five possible accident plant configurations are investigated to research the thermal hydraulic behaviors for loss of RHR system during mid-loop operation in the plant, which are no vent in primary system, pressurizer relief valve (PROZ) open, reactor pressure vessel (RPV) top vent valve open, reactor coolant pump (RCP) seal open, and steam generator (SG) manway open. The calculated results show that, core temperature will increase due to core decay heat gathered after loss of RHR system during mid-loop operation, which may even lead to core damage.If the plant is loss of RHR system during mid-loop operation, three available mitigate measures can be adopted to develop accident management, which is safety injection system, gravity feed from refueling water storage tank (RWST) and SG reflux-condensation. The analysis results show that available mitigate measures can recover primary system water level back to mid-line of hot leg and restore RHR system, and avoid the risk of core damage effectively due to the increasing of core temperature. The plant operational states of mid-loop operation are defined through dividing plant operational states during low power and shutdown in the plant. Then, the initial events during mid-loop operation are confirmed. By the success criterion of reflux-condensation, time windows can be analyzed in the event trees. Finally, according to abnormal procedure and the results obtained in the simulation, then event trees for loss of RHR system during mid-loop operation are built during different initial events.