Simulation of a PWR Reactor Vessel Level Indicating System During Station Blackout with MELCOR 1.8.5

After the Three Mile Island accident, the U.S. Nuclear Regulatory Commission requested that nuclear power plants (NPPs) in the United States provide instruments to record reactor water levels. The instruments will improve reliability in diagnosing the approach of inadequate core cooling. The reactor vessel level indicating system (RVLIS) is a measuring system for determining the water level in the reactor vessel. RVLIS was installed at Maanshan NPP under this requirement. A station blackout (SBO) incident occurred in the Maanshan NPP on March 18, 2001. The SBO incident was simulated with the MELCOR 1.8.5 code. The important parameter, reactor vessel water level, was compared with plant data. The interesting phenomena about RVLIS responses included initial full water level above 100%, reactor pressure vessel (RPV) water shrinkage, and two peaks in upper range train A. The initial full water levels of the upper range were at ~112% because of calibration conditions. The two trains of the upper range dropped at ~1.2 h after SBO because of RPV water saturation. RVLIS upper range train A had two level rises after SBO because of two flows out of the pressurizer into the loop 2 hot leg. The results indicated that MELCOR could reproduce the RVLIS response very well.