[8] Immediately after the earthquake, the remaining reactors 1–3 shut down automatically, and emergency generators came online to control electronics and coolant systems.

As the pumps stopped, the reactors overheated due to the normal high radioactive decay heat produced in the first few days after nuclear reactor shutdown (smaller amounts of this heat normally continue to be released for years, but are not enough to cause fuel melting).

[10][11] In the intense heat and pressure of the melting reactors, a reaction between the nuclear fuel metal cladding and the remaining water surrounding them produced explosive hydrogen gas.

[15] The Japanese government and TEPCO have been criticized in the foreign press for poor communication with the public and improvised cleanup efforts.

Predicted future cancer deaths due to accumulated radiation exposures in the population living near Fukushima have ranged from none[25] to 100[26] to a non-peer-reviewed "guesstimate"[27] of 1,000.

[28] On 5 July 2012, the parliament appointed The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) submitted its inquiry report to the Japanese parliament,[29] while the government appointed Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company submitted its final report to the Japanese government on 23 July 2012.

[30] Tepco admitted for the first time on 12 October 2012 that it had failed to take stronger measures to prevent disasters for fear of causing lawsuits or protests against its nuclear plants.

Arnold Gundersen, an engineer frequently commissioned by anti-nuclear groups, said that "Fukushima is the biggest industrial catastrophe in the history of mankind".

[42][43] On 24 March, a scientific consultant for anti-nuclear environmental group Greenpeace, working with data from the Austrian ZAMG[44] and French IRSN, prepared an analysis in which he rated the total Fukushima accident at INES level 7.

Assessing "seriousness" as partial or full meltdown at a civilian plant, The New York Times reported on 3 April that based on remote sensing, computer "simulations suggest that the number of serious accidents has suddenly doubled, with three of the reactors at the Fukushima Daiichi complex in some stage of meltdown."

The release is a factor of 2.5 higher than the Chernobyl 133Xe source term", although the "Xenon-133 [main noble gas] does not pose serious health risks because it is not absorbed by the body or the environment.

As occurred in releases at Three Mile Island, radioactive noble gases rapidly vanish upward, and dissipate into space.

Trade minister Banri Kaieda mentioned that TEPCO seriously considered pulling away all staff members from the plant and leaving it abandoned.

In an interview to the Asahi Shimbun newspaper, Kan revealed that he went to the plant itself and visually inspected it from above in a helicopter because: "I felt I had to go there in person and speak to the people in charge or I would never have known what was going on.