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

The earthquake caused the loss of all off-site electric power atthe Fukushima Daiichi Nuclear Power Station, and the followingseries of tsunamis made all emergency diesel generators exceptone for Unit 6 and most of DC batteries inoperable. Thus allunits resulted in the loss of cooling function and ultimate heatsink. Tokyo Electric Power Co., Inc. (TEPCO) focused on restorationof the instruments and lights in the Main Control Room(MCR), preparation of alternative water injection, and venting ofthe 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 damage from the tsunamis and the lack of necessaryequipment and other resources hampered a quick recovery andeventually resulted in the severe core damage of Units 1, 2, and3 and also the hydrogen explosions in the reactor buildings ofUnits 1, 3, and 4. By bringing the reactors and spent fuel pools toa stable cooling condition and mitigating the release of radioactivematerials, TEPCO created the “Roadmap Toward Restorationfrom the Accident.” Based on the roadmap, the followingtwo steps are set as targets: “Radiation dose is in steady decline”(Step 1) and “Release of radioactive materials is under control andradiation dose is being significantly held down” (Step 2). Step 1and 2 were achieved in July and December of 2011, respectively.Through these efforts, the reactors have reached a state of coldshutdown, and it is now possible to maintain an adequately lowlevel of radioactive exposure at the site boundaries, even underunexpected situations in the future. This paper outlines the accident,the response made during the accident, and the effort madeby all workers involved to achieve the stable state of the reactors.