The proximate cause of the accident was the 2011 Tōhoku earthquake and tsunami, which resulted in electrical grid failure and damaged nearly all of the power plant's backup energy sources.

The expense of cleaning up the radioactive contamination and compensation for the victims of the Fukushima nuclear accident was estimated by Japan's trade ministry in November 2016 to be 20 trillion yen (equivalent to 180 billion US dollars).

The waves then flooded all turbine and reactor buildings, damaging EDGs and other electrical components and connections located on the ground or basement levels[31][8]: 31–32 [33] at approximately 15:41.

[8]: 31–32  In response, the operators assumed a loss of coolant in units 1 and 2 and developed a plan in which they would vent the primary containment and inject water into the reactor vessels with firefighting equipment.

[48] The isolation condenser (IC) was functioning prior to the tsunami, but the DC-operated control valve outside of the primary containment had been in the closed position at the time to prevent thermal stresses on the reactor components.

[8]: 37–42 [50][51] Subsequent analysis in November 2011 suggested that this extended period without cooling resulted in the melting of the fuel in unit 1, most of which would have escaped the reactor pressure vessel (RPV) and embedded itself into the concrete at the base of the PCV.

[52] Computer simulations, from 2013, suggest "the melted fuel in Unit 1, whose core damage was the most extensive, had breached the bottom of the primary containment vessel and had even partially eaten into its concrete foundation, coming within about 30 cm (1 ft) of leaking into the ground".

The safety relief valves (SRVs) would intermittently release steam directly into the PCV suppression torus at its design pressure and the RCIC properly replenished lost coolant.

[8]: 29–31  However, the water within the RPV proved sufficient to cool the fuel, with the SRVs venting into the PCV, until AC power was restored on 13 March using the unit 6 interconnection, allowing the use of the low-pressure pumps of the residual heat removal (RHR) system.

[8]: 107 According to the French Institute for Radiological Protection and Nuclear Safety, the release from the accident represents the most important individual oceanic emissions of artificial radioactivity ever observed.

Significant pollution along the coast near the plant might persist, because of the continuing arrival of radioactive material transported to the sea by surface water crossing contaminated soil.

[65] In response to the station blackout during the initial hours of the accident and the ongoing uncertainty regarding the cooling status of units 1 and 2, a 2 km radius evacuation of 1,900 residents was ordered at 20:50.

[92][93] A 2014 metareview of 48 articles indexed by PubMed, PsycINFO, and EMBASE, highlighted several psychophysical consequences among the residents in Miyagi, Iwate, Ibaraki, Tochigi and Tokyo.

[106][107] A survey on local mayors by the Yomiuri Shimbun newspaper in 2013 found that most of them from cities hosting nuclear plants would agree to restarting the reactors, provided the government could guarantee their safety.

[110] In May 2011, UK chief inspector of nuclear installations Mike Weightman traveled to Japan as the lead of an International Atomic Energy Agency (IAEA) expert mission.

The main finding of this mission, as reported to the IAEA ministerial conference that month, was that risks associated with tsunamis in several sites in Japan had been underestimated.

[14][135][142][13]: 93 Outside the geographical areas most affected by radiation, even in locations within Fukushima prefecture, the predicted risks remain low, and no observable increases in cancer above natural variation in baseline rates are anticipated.Estimated effective doses outside Japan are considered to be below (or far below) the levels regarded as very small by the international radiological protection community.

[160] In June 2016, the political advocacy group "International Physicians for the Prevention of Nuclear War", asserted that 174,000 people have been unable to return to their homes and ecological diversity has decreased and malformations have been found in trees, birds, and mammals.

[161] Although physiological abnormalities have been reported within the vicinity of the accident zone,[162] the scientific community has largely rejected any such findings of genetic or mutagenic damage caused by radiation, instead showing it can be attributed either to experimental error or other toxic effects.

[164] A group campaigning to help prevent global warming has demanded the Food and Drug Administration disclose the name of the importer of fish from Fukushima and of the Japanese restaurants in Bangkok serving it.

Srisuwan Janya, chairman of the Stop Global Warming Association, said the FDA must protect the rights of consumers by ordering restaurants serving Fukushima fish to make that information available to their customers, so they could decide whether to eat it or not.

[178] The Commission recognized that the affected residents were still struggling and facing grave concerns, including the "health effects of radiation exposure, displacement, the dissolution of families, disruption of their lives and lifestyles and the contamination of vast areas of the environment".

[198][199] On 10 September 2015, floodwaters driven by Typhoon Etau prompted mass evacuations in Japan and overwhelmed the drainage pumps at the stricken power plant.

[205] Japan said the water is safe, many scientists agreed, and the decision came weeks after the UN's nuclear watchdog approved the plan; but critics say more studies need to be done and the release should be halted.

[21] In April 2023, fishers and activists held protests in front of the Japanese embassy in the Philippines in opposition to the planned release of 1.3 million tons of treated water into the Pacific Ocean.

[214] On 30 September 2020, the Sendai High Court ruled that the Japanese government and TEPCO are responsible for the accident, ordering them to pay $9.5 million in damages to residents for their lost livelihoods.

[215] In March 2022, Japan's Supreme Court rejected an appeal from TEPCO and upheld the order for it to pay damages of ¥1.4 billion ($12 million) to about 3,700 people whose lives were harmed by the accident.

[228] For Generation II reactors located in flood or tsunami prone areas, a 3+ day supply of back-up batteries has become an informal industry standard.

[225] Upon a station blackout, similar to the one that occurred after the back-up battery supply was exhausted,[231] many constructed Generation III reactors adopt the principle of passive nuclear safety.

[237] On 5 July 2012, the NAIIC found that the causes of the accident had been foreseeable, and that TEPCO had failed to meet basic safety requirements such as risk assessment, preparing for containing collateral damage, and developing evacuation plans.