ICONE23-1073 LPSD CONDITION MBLOCA SUCCESS CRITERION ANALYSIS FOR TYPICAL THREE LOOP NPP

Abstract

PSA (Probabilistic Safety Assessment) method has been widely used in nuclear power plant safety designs. Thermal hydraulic analysis is a basic and important issue in PSA. During the process of thermal hydraulic analysis, the response of reactor coolant system and performance of the reactor core can be simulated, so that the safety functions can be assured. Also, safety equipment demands can be defined, including the success criterion of useful equipment and time window for operator to action. The damage state of reactor core will be judged, too. Nuclear power plant operates under certain conditions, such as full power condition, hot standby, hot shutdown, cold shutdown. LPSD condition includes nuclear power plant conditions when NPP operate under low power and shutdown conditions. LPSD is divided to several plant operational state (POS), based on important parameters of nuclear power plant, such as, core power and decay heat, water level of primary circus, temperature and pressure of reactor coolant system, state of the containment, and etc. Under POSC condition, the plant is subcritical; the primary pressure is between 2.4MPa and 13.9MPa; the average coolant temperature is between 160℃ and 284℃. MBLOCA (middle break loss-of-coolant accident) under a certain LPSD condition of POSC for typical three loop nuclear power plant is analyzed in this article. The break equivalent diameter of a MBLOCA may be between 50.8mm and 152.4mm. Thus, the spectrum of MBLOCA is calculated firstly and a limiting break area is selected to envelope all break sizes. Then success criterions of relevant systems and time windows for operators were calculated and selected. The results show that, after MBLOCA under POSC condition takes place, the operator needs to open one train of HHSI (high pressure safety injection system) or open one GCT-a (turbine bypass to atmosphere) valve on intact loop to decrease reactor coolant system's pressure and temperature within 1800s, so that the reactor core will be safe. The results could be used for Level 1 PSA.