Simulation analysis on accident at Fukushima Daiichi Nuclear Power Plant unit 2 by sampson code

The accident occurred at t he Fukushima Daiichi Nuclear Power Plant Unit 2 has been investigated by the severe accident analysis code, SAMPSON with more realistic boundary conditions and newly introduced models. In Unit 2, the Reactor Core Isolation Cooling system (RCIC) is thought to have worked for unexpectedly long ti me (about 70 hours) without batteries. It is tho ught to be due to b alance between injected water from the RCIC pump and supplied mixture of steam and water to the RCIC turbine. To confirm the RCIC working condition and reproduce the measured plant properties, such as pressure and water level i n the reactor pressure vessel (RPV), we introduced two-phase turbine driven p ump model into SAMPSON. In the model, mass flow rate of water injected by RCIC was calculated through mass flow rate of steam included in extracted two-phase flow, steam generated from flashing of water included in extracted two-phase flow, and turbine efficiency degradation originated by the mixture of steam and water flowing to the RCIC turbine. To reproduce the dry well (DW) pressure, we assumed that torus room was flooded by the tsunami and heat was removed from the su ppression chamber to the sea wat er. Simulation results b y SAMPSON basically agree with the measured values such as pr essure in the RPV and in the DW until several days after the scr am. However, some contradictions between the si mulation results and the measured values, such as that inversion of the RPV pressure at 10 hours after scram in the measurement happened at 14 hours in the simulation and t hat the DW pressure sho wed different behavior between simulation and measurement when SRV started periodic operation at 71 ho urs, are sti ll remain and are under consideration. In the current calculation, model for falling core to the lower plenum was modified so that debris is not retained at the core plate based on observation of the XR2-1 experiment. Additionally, model of the RPV failure by melting of the penetrating pipe was newly introduced. Though actual mass flow rate of water injection by fire pump, which affects the core and the RPV damage, is still unclear, simulation result indicates that a part of the core fell to the lower plenum but the RPV did not failed.