The sequences of events in the reactor accident were as follows:

1. Fukushima was shaken by an earthquake measuring magnitude 9.0 on the Richter scale; however, the six NPPs were designed on the basis of an earthquake equivalent to magnitude 8.2.

2. The three units in operation, units 1, 2 and 3, automatically went into SCRAM (sudden shutting down of a nuclear reactor, usually by rapid insertion of control rods), which was triggered by detecting the high earthquake acceleration. Following the total loss of off-site power, emer­gency power generators automatically started to supply electricity.

3. The standard post-shutdown cooling modes started up to remove the decay heat. This residual heat must be removed to prevent the nuclear fuel, mainly UO2 , cladding metal, and supporting structural elements from melting in the core of the reactor. The melting point of UO2 is approximately 2,900°C, while those of cladding and supporting parts are in the range of 1,300-1800°C.

4. About 45 minutes after the earthquake, tsunami waves variously hit the units, destroying seawater pumps for the residual heat removal system and many of the emergency generators. Eventually, this lead to the total loss of the electricity that powered the water pumps used to maintain cooling water circulation around the reactor cores. The spent fuel (SF) storage pools suffered the same problem.

5. In spite of the performance of various emergency core cooling systems, as well as trials to vent the reactor vessel enabling water injection from outside, the core eventually became uncovered by cooling water. Along with the increase in temperature of the uncovered fuel, the reaction of cladding material with steam to generate hydrogen proceeded rapidly, and the fuel started melting leading to core destruction through meltdown.

6. According to the results of the simulation calculation conducted even with insufficient records of the instrumentation, most of the core is believed to have melted in unit 1. In units 2 and 3, much of the fuel apparently melted but to a lesser extent than in unit 1 and dropped to the bottom of the pressure vessel. It is considered that a certain part of the fused fuels and structural materials flowed out from the reactor pressure vessels (RPVs) into the primary containment vessels (PCVs).

7. During the severe accident process, appreciable amounts of volatile radionuclides (typically these are noble gases, cesium and iodine) are considered to have evaporated. They must have escaped from the RPV into the PCVs, and finally escaped via cracks or openings made under the severe conditions.

8. Hydrogen explosions occurred in units 1, 3 and 4, and these seriously damaged their operation floors at the top of the reactor building, and also the upper side walls of unit 4.