Fukushima Daiichi nuclear power plant accident and analysis evaluation

On March 11, 2011, Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station (NPS) was struck by a tsunami caused by the Great East Japan Earthquake, resulting nuclear severe accidents in Units 1 through 4. The author has been involved in investigating the causes of accidents and developing countermeasures for other nuclear power plants (NPPs) in Japan as a member of the Committee of JSME, Atomic Energy Society of Japan (AESJ), advisory meeting members of the Nuclear and Industrial Safety Agency (NISA), and the Nuclear Regulation Authority (NRA) with regard to technical lessons learned from the accident. Based on these activities, this chapter summarized the outline of accident, event progress, and analysis evaluation at each unit. Though the Fukushima Daini NPS struck by a tsunami, they avoided severe accidents by emergency response. From the lessons learned, protection measures such as seawall and waterproof doors to protect Tsunami, a filtered containment venting system to protect contamination, water injection into core and containment, and special emergency heat removal system are introduced to the new nuclear regulatory requirements for restart PWR and BWR NPPs. Activities toward decommissioning of Fukushima Daiichi: current status of Units 1 to 4 reactors, minimization of leakage paths of contaminated water from PCVs, isolation of groundwater flow from contaminated water especially by frozen wall. Preparations are underway for and tritium treatment water to release diluted by seawater up to 1/7 of the WHO drinking water standard, and to remove fuel debris using robotic technology. In addition, at the Fukushima Daini Power Station, they quickly grasped the impact of the tsunami, established an incident command system, and delivered seawater pump motors to the power station by airlift of the Self-Defense Forces. Four 1100 MWe nuclear power plants achieved cold shutdown with the restoration of seawater pumps and 200 new cabling actions. These incident responses were reported to the IAEA and incorporated into emergency mitigation responses for nuclear power plants in Europe and the United States. Important lessons learned from the Fukushima Daiichi accident and the success of the Fukushima Daini emergency response teams will help improve the safety of many types of nuclear power plants around the world.