On March 11, 2011, a magnitude 9.0 earthquake off Japan’s northeast coast triggered a tsunami that knocked out power and cooling at the Fukushima Daiichi nuclear plant, and the fuel inside three of its six reactors melted down within days. It’s the worst nuclear accident since Chernobyl, rated the maximum level 7 on the scale built to measure them, and more than a decade later the plant still isn’t fully cleaned up.
The quake, then the wave
The earthquake hit at 2:46 p.m. local time. Fukushima Daiichi’s three operating reactors, units 1 through 3, shut down automatically the moment the shaking started, exactly what a reactor is built to do. Units 4, 5, and 6 were already offline for maintenance. The quake itself didn’t cause a meltdown. What came next did.
About 41 minutes later, a tsunami hit the plant with waves that reached an estimated 14 to 15 meters, several times higher than the roughly 3-meter wave the site had been built to handle. The water went over the seawall, flooded the turbine buildings, and drowned the emergency diesel generators and seawater pumps that cooled them. All six off-site power lines were already dead from the earthquake itself, so with the generators gone too, units 1 through 3 lost every source of power for their cooling systems. That’s a station blackout, the condition that turned an earthquake and a flood into a nuclear accident.
Why a shut-down reactor still needs cooling
A reactor that’s shut down isn’t a reactor that’s cold. The moment the control rods drop and the chain reaction stops, the uranium fuel is still packed with radioactive fission byproducts, cesium and iodine among them, and those keep decaying on their own, giving off heat as they do. Engineers call it decay heat. According to DOE reactor operations training material, it typically runs around 7% of a reactor’s full power the instant it shuts down, falling to roughly 2% within the first hour and about 1% after a day.
One percent still isn’t nothing. Unit 1 was rated at 460 megawatts of electrical output, so even a sliver of its full power is real heat with nowhere to go unless water keeps moving through the core. Cut the pumps, and the water already in the vessel boils away on its own. Once the fuel rods sit exposed to steam instead of water, they keep heating past the point where zirconium cladding reacts with steam and throws off hydrogen gas, and the fuel starts to melt. Anyone trained to run a reactor learns to respect that sequence early. It doesn’t care that the chain reaction already stopped.
How three reactors lost cooling and melted down
With no power for pumps and only battery backups that ran down over hours, units 1, 2, and 3 each melted on its own timeline:
| Reactor | Core damage began | What happened to the fuel |
|---|---|---|
| Unit 1 | About 4 hours after shutdown | Most fuel melted and pooled at the bottom of the pressure vessel by early March 12 |
| Unit 2 | About 77 hours after shutdown | Substantial fuel melting, containment damaged |
| Unit 3 | About 44 hours after shutdown | Fuel melted through the morning of March 13 |
Timing per World Nuclear Association.
Plant operators tried venting steam and pumping in seawater, improvised responses to a situation the plant wasn’t built for. Neither stopped the melting, and the fuel in all three reactors slumped, in places burning through the steel of the vessels themselves.
Three hydrogen explosions in four days
The hydrogen building up inside the damaged reactors had to go somewhere, and in three buildings it found an ignition source. Unit 1’s reactor building exploded on the afternoon of March 12, about 25 hours after shutdown, blowing the roof and upper walls apart. Unit 3’s building went the same way on the morning of March 14. Unit 4 had no fuel loaded in its reactor, but it exploded early on March 15 anyway, after hydrogen vented from Unit 3 backed up through shared ductwork. None of the three blasts breached primary containment, but they scattered debris and made it much harder for workers to get close enough to restore cooling.
Evacuating a quarter million people, and the deaths that followed
Japan widened its evacuation zone in stages as the accident worsened, from 2 kilometers on the evening of March 11 out to 20 kilometers by the next night. At its peak in 2012, roughly 160,000 people were displaced, about 100,000 still inside Fukushima Prefecture and the rest scattered elsewhere in Japan. As of May 2026, more than 22,000 were still registered as evacuees, many from towns where the order has never fully lifted.
The evacuation itself had a body count. A peer-reviewed study tracking the disaster through January 2021 found 2,326 certified “disaster-related deaths” in Fukushima Prefecture, meaning deaths from the physical and medical strain of the evacuation rather than the earthquake, tsunami, or radiation itself. Elderly hospital and nursing-home patients bore the worst of it, pulled off ventilators and moved long distances under chaotic conditions with medication interrupted. That toll exceeded the direct earthquake and tsunami death count in the prefecture itself.
What radiation did to people’s health
Here’s where the record runs against the popular image of the disaster. The United Nations Scientific Committee on the Effects of Atomic Radiation studied Fukushima’s radiation exposures for a decade and published its fullest assessment in 2020 and 2021. Its conclusion: no adverse health effects among Fukushima residents have been documented that can be attributed to radiation from the accident, and future radiation-linked cancers are unlikely to be detectable because the doses were low. The thyroid cancers found in screened children weren’t caused by radiation either, according to the committee. They showed up because Fukushima ran an unusually sensitive ultrasound program that caught abnormalities doctors wouldn’t otherwise have found at that age.
Emergency workers got a heavier dose than the public, as you’d expect from people running hoses inside a damaged plant. They averaged about 13 millisieverts between March 2011 and March 2012, with roughly a third exceeding 10 millisieverts. That’s a meaningful occupational dose, but UNSCEAR still didn’t attribute any deaths to it. The gap between what people feared and what the data shows is the most striking part of the record.
The treated water release that started in 2023
Cooling the melted cores never stopped, and the water used has to go somewhere once it’s contaminated. Tokyo Electric Power Company built an Advanced Liquid Processing System, ALPS, to filter that water and strip out 62 radioactive elements, everything except tritium, which is chemically bound into the water and can’t be filtered out. On August 24, 2023, Japan began releasing the treated water through a pipe extending a kilometer offshore, diluted well below regulatory tritium limits before each release.
The IAEA has kept an on-site presence monitoring the discharge, and its most recent review, published in September 2025, found it still meeting international safety standards. By the end of 2025, TEPCO had discharged roughly 133,000 cubic meters across 17 batches, each one tested below Japan’s operational tritium limit before release. The discharge is expected to run for years as the stored water on site works down.
Fifteen years in, decades still to go
The parts of the plant that can be defueled the normal way mostly have been. All 1,331 spent fuel assemblies came out of Unit 4’s storage pool by December 2014, and Unit 3’s pool was emptied by February 2021. The harder problem is the roughly 880 tons of melted fuel debris fused with structural metal and concrete inside the damaged vessels of units 1 through 3, in ways nobody has fully mapped yet.
Progress there is slow on purpose. The debris is intensely radioactive and the retrieval tools are still being invented as the work goes. Workers pulled the first trial sample from Unit 2 in November 2024, about 0.7 grams with a specially built extendable arm. Full-scale removal, once targeted for the early 2030s, has since slipped to fiscal year 2037 or later, with TEPCO now saying preparation alone needs 12 to 15 years. The government’s broader estimate for full decommissioning still runs 30 to 40 years from the accident, with an earlier cost projection near 22 trillion yen, about $190 billion at the time.
What the nuclear industry changed afterward
Fukushima happened because a plant lost every source of power at once with no way to cool its cores. Regulators everywhere rewrote their rules around that failure mode. In the United States, the NRC issued a set of orders in March 2012 requiring every reactor to keep portable pumps and generators on hand, plus battery packs and chargers, capable of cooling the reactor and its spent fuel pool through an event beyond the plant’s original design. The industry built this out under the name FLEX, backed by two national response centers, in Memphis and Phoenix, that can ship heavier equipment to any U.S. reactor site within 24 hours.
The NRC also ordered better spent fuel pool instrumentation, and for the 31 U.S. reactors sharing Fukushima’s General Electric containment design, hardened vents built to work under severe accident conditions. According to the Nuclear Energy Institute, the industry finished installing all of it at every U.S. plant, at a combined cost topping $4 billion. From the operations side, spending like that is an admission the plants were built to survive the accidents their designers imagined, and Fukushima was a different one.
