Assessment of the countermeasure improvement of ABWR using RETRAN-3D

The Great East Japan Earthquake occurred on March 11, 2011 fatally damaged the Fukushima Daiichi Nuclear Power Plant (NPP), caused prolonged station blackout (SBO). Following the SBO, the reactor water level gradually dropped due to the increase in steam discharge from safety relief valves and eventually led to nuclear fuel melt down. Almost four years have passed since the accident, and official reports by the Japanese regulatory have given the general description of causes and progressions of the fatal accident. Even after the Fukushima accident, more than 430 nuclear power plants are currently operating and over 80 units are under construction worldwide. From this fact alone, it is extremely important to learn from the Fukushima accident and enhance the safety culture of the reactor operation to completely eliminate the possibilities of catastrophic accidents seen in 2011. In this study, the best estimate transient thermal-hydraulics code, RETRAN-03/MOD04 was utilized to focus on the effectiveness of the Isolation Condenser (IC) installed on Advanced Boiling Water Reactors (ABWR). The ICs turned out to be one of the very few operable safety systems during the Fukushima accident, and this simple yet reliable safety system should be utilized to secure ABWR from possible reactor core damages. In the present paper, several case studies conducted utilizing the ICs are presented and methods of countermeasures to improve light water reactor safety level in design and operation features are proposed.