Reducing Scram Frequency by Relaxing Reactor Trip Setpoints at Maanshan Nuclear Power Station

A best-estimate transient analysis performed with the RETRAN-02 code is used to demonstrate that two reactor trip setpoints can be modified to improve plant operating margin and eliminate unnecessary reactor trips in the Maanshan three-loop Westinghouse pressurized water reactor. The trip setpoint relaxation analyses involve the steam generator water level and turbine trip. Analytical results show that for a loss-of-secondary-heat-sink event, the steam generator low-low-level reactor trip signal (SGLLRTS) setpoint can be adjusted down from 17 to 5% narrow-range water level span at low power level (<50%) without opening the primary-side power operated relief valves (PORVs). The effects of the automatic control rod on a loss-of-normalfeedwater transient are discussed. The reactor can be tripped by other signals if the SGLLRTS is inhibited at high power level. The turbine trip reactor trip signal setpoint can be lowered from 10 to 30% power. American National Standard design conditions and the prevention of pressurizer PORV opening to avoid a small loss-of-coolant accident are considered the safety criteria for the relaxation of trip setpoints.