The accident at the Fukushima Dai-ichi Nuclear Power Station. Accident progression behavior of the nuclear reactor and lessons learned

On March 11, 2011, the Great East Japan Earthquake -rated a magnitude 9.0- occurred and precipitated a large tsunami that is estimated to have exceeded 14 meters in height at the Fukushima Dai-ichi Nuclear Power Station site. As a result of the earthquake, all of the operating units experienced a normal reactor shutdown within the capability of the safety design of the reactors. However the large tsunami resulted in extensive damage to site facilities and a complete loss of AC electrical power at Units 1 through 5, a condition known as station blackout (SBO). Also the DC electrical power lost its function due to the tsunami effect or exhaustion of batteries, thereby leading to loss of most instrumentation and control systems. Consequently the cooling was lost to the fuel in the reactor cores and spent fuel pools, and eventually the core meltdown occurred at Units 1 through 3. In the course of accident progression the confinement function of reactor pressure vessels and containment vessels was seriously impaired and the significantly large amount of radioactive materials was released to the environment. This paper gives an explanation on the basic philosophy of nuclear reactor safety, the event sequence of the Fukushima accident and the implication of the accident, as well as the lessons learned. (author)