Overview of the Accident of Fukushima Daiichi Nuclear Power Station and the Process for Achieving Stabilization

The earthquake caused loss of all off-site electric power at Fukushima Daiichi Nuclear Power Station, and the following series of tsunamis made all emergency diesel generators except one for Unit 6 and most of DC batteries inoperable. Thus all units resulted in the loss of cooling function and ultimate heat sink. Tokyo Electric Power Co., Inc. (TEPCO) focused on restoration of the instruments and lights in the Main Control Room (MCR), preparation of alternative water injection and venting of Primary Containment Vessel (PCV) in the recovery process. However, the workers faced many difficulties such as total darkness, aftershocks, high radiation, loss of communication means, etc. Massive damages by the tsunamis and lack of necessary equipment and other resources hampered a quick recovery and eventually resulted in the severe core damage of Units 1, 2 and 3 and also the hydrogen explosions in the reactor buildings of Units 1, 3 and 4. By bringing the reactors and spent fuel pools to a stable cooling condition and mitigating the release of radioactive materials, TEPCO made the “Roadmap towards Restoration from the Accident”. Based on the roadmap, the following two steps are set as targets: \"Radiation dose is in steady decline\" (Step1) and \"Release of radioactive materials is under control and radiation dose is being significantly held down\" (Step2). Step1 and 2 were achieved in July and December of 2011, respectively. Through these efforts, the reactors have reached a state of cold shutdown, and it is now possible to maintain an adequately low level of radioactive exposure at the site boundaries, even under unexpected situations in future. This paper outlines the accident, the response made during the accident, and the effort made by all workers involved to achieve the stable state of the reactors.