Fukushima I Nuclear Power Plant

Unit Fukushima I – 1 goes critical with a generation capacity of 460 MW.

Unit Fukushima I – 2 goes critical with a generation capacity of 784 MW.

Unit Fukushima I – 3 goes critical with a generation capacity of 784 MW.

Unit Fukushima I – 5 goes critical with a generation capacity of 784 MW.

Unit Fukushima I – 4 goes critical with a generation capacity of 784 MW.

Unit Fukushima I – 6 goes critical with a generation capacity of 1 100 MW.

During the 2011 Sendai earthquake, reactors 1, 2 and 3 are in operation. The remaining three reactors (4, 5, and 6) are shut down for maintenance. The operating are automatically shut down when the earthquake is detected. As the facility and the surrounding electriciy grid are also shut down or damaged and no electricity is available, emergency cooling is powered by diesel generators.

The earthquake is followed by a tsunami which floods the emergency generators which abruptly cease functioning. Article 10 of the Japanese law on Special Measures Concerning Nuclear Emergency Preparedness, requires authorities to be informed of such an incident: TEPCO does so immediately and also issues a press release declaring a "First Level Emergency". Once the diesel generators fail, power is supplied by emergency batteries with a capacity for roughly eight hours. Additional batteries and mobile generator units are dispatched to the site within hours but cannot be connected until the following morning because of incompatibles cables and/or inaccessibility due to roads being blocked by tsunami debris.

A "nuclear emergency situation" (Article 15 of the Japanese law on Special Measures Concerning Nuclear Emergency Preparedness) is declared when the status of reactor water coolant injection can no longer be confirmed for the emergency cooling systems of reactor Units 1 and 2. The alert is cleared when the reactor water level monitoring function is restored for Unit 1 but reinstated again at 17:07. Potentially radioactive steam is released from the primary circuit into the secondary containment area to reduce mounting pressure. The declaration of the emergency is followed by an evacuation order to people residing within a 3 kilometer radius of the site. The radius is later enlarged to 10, then 20 kilometers.

An explosion occurs at Unit 1. Four workers are injured and the upper portion of the concrete outer reactor building collapses. The building is not part of the containment, however, as in the Fukushima I reactors the containment consists of "drywell" and "wetwell" concrete structures immediately surrounding the reactor pressure vessel. The likely cause is a hydrogen explosion which is highly likely to have formed inside the reactor building due to falling water levels. Safety systems normally igniting the hydrogen before explosive concentrations are reached seem to have failed.

According to the nuclear regulation act and to the directives by the Prime Minister, the Japanese government orders the injection of sea water into Unit 1. Seawater is injected at 20:20 along with boric acid acting as a neutron absorber to prevent a criticality accident. The process is expected to take between 5 and 10 hours to fill the reactor core. At 22:15 the process is interrupted by an aftershock and resumed shortly thereafter.