An Application of a Variable Overpower Trip Function with Multiple Ceilings into an Advanced Integral Reactor

A series of evaluations for an inadvertent control rod withdrawal event has been performed to identify the appropriateness of the reactor protection system of a 65 MWt advanced integral reactor by using the Transients And Setpoint Simulation/System-integrated Modular Reactor (TASS/SMR) code. Evaluation results show that, if considering only a high core power trip function as the reactor protection system for the limitation of a core power increase, the occurrence of an inadvertent control rod withdrawal event at the lower core power levels under a high speed mode of a main coolant pump (MCP) results in a significant deterioration in the safety margin. Specially, a violation of a safety limit is possible under the MCP middle and low speed modes due to an unsuitable setpoint of the high core power trip function. To enhance the safety margin, a variable overpower trip (VOPT) function having multiple ceilings according to the MCP speed mode has been implemented into the reactor protection system for the 65 MWt advanced integral reactor. Reassessments by considering the VOPT function with multiple ceilings show that a safety enhancement in the case of an inadvertent control rod withdrawal event is obtained by a timely initiation of a reactor trip.