The 2011 off the Pacific coast of Tohoku Earthquake occurred on March 11. The earthquake attacked the Fukushima Daiichi nuclear power station with six boiling water reactors (BWRs), three out of which, units 1 through 3 in rated operation except for three reactors of units 4 through 6 in scheduled periodic inspection, automatically shut down in response to the intense seismic motion. Emergency diesel generators started to pump water to cool reactors, and an hour later, the back-up generators lost their all functions by the station blackout resulting from tsunami flooding. In this situation at the unit 1, the isolation condenser system (IC) should have made a critical role to keep the reactor pressure and water level to be safety by removing the decay heat by natural circulation. In fact at the unit 1 during the accident, IC valves were closed by fail-safe and could not have shown the ability of the designed function.

An accident report gave general descriptions of the causes and results of accidents, but not the quantitative data indicative of details; therefore, it seems difficult to identify the specific problems in plant operations. Even in this case, if an appropriate analysis code is available for reproducing events based on the reports, it will be possible to determine individual data quantitatively and identify problems in plant operations.

In our work, we used the nuclear reactor thermal-hydraulic code RETRAN-3D/MOD4, which has been approved and licensed by U.S. Nuclear Regulatory Commission, to model light water reactors (LWRs) and reproduce the circumstances of the 2011 Fukushima Daiichi nuclear accident as the simulation code. Here, we subjected transition analyses of the process on the core-meltdown accident, and put forward the system to prevent the accident, where the accident analysis report was employed to simulate conditions of the accident. It could enable us to suggest adequate operation procedures suitable for LWR to avoid the severe accident, and to propose countermeasures to improve LWR safety level in design and operation.

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