On the afternoon of 11 March 2011, Japan's north-east coast was hit by a magnitude 9.0 earthquake — the largest in the country's recorded history. Seismic safeguards worked as designed: at the Fukushima Daiichi plant, three of six reactors were operating and shut down automatically, control rods dropping into the cores; once the shaking stopped, the plant disconnected from the grid and switched to backup diesel generators. So far, everything was according to plan. What the design had not accounted for arrived 50 minutes later: a tsunami of 13 to 15 metres swept over the plant's 10-metre seawall and flooded the diesel generators in the basements.

With power gone, the cooling pumps stopped. A nuclear reactor that has stopped fissioning still has to be cooled indefinitely: the decay heat of fission products produces a furnace lasting hours and days. Over the next four days, fuel in reactors 1, 2 and 3 melted; the zirconium cladding reacted with water and produced hydrogen; on 12, 14 and 15 March the accumulated hydrogen exploded in the upper buildings — reactor 4 wasn't even operating, but hydrogen that drifted in from reactor 3 blew it apart as well. The images went out live to the world: stripped concrete skeletons with their guts exposed.

The human toll is the sum of seismic and nuclear catastrophe. The earthquake and tsunami themselves killed or left missing some 18,500 people, most of them by drowning. Deaths directly attributed to radiation at the plant officially stand at 1 (the Japanese government in 2018 recognised a TEPCO worker who died of lung cancer); but the stress, interrupted care and displacement from the forced evacuation of 154,000 people has been linked to over 2,200 deaths, mostly of elderly residents. The leaking and accumulating contaminated water — and the 2023 release of treated water into the Pacific — remains contested.

Fukushima's real legacy lies in energy policy. Within days Germany shut down eight reactors and by law committed to a full nuclear exit (Atomausstieg) by 2022. Japan temporarily idled all 54 of its reactors; only a handful had restarted a decade later. Switzerland, Belgium and Italy abandoned new programmes. The vacuum was filled in the short term by coal and gas — which is why Fukushima is a paradox for global climate targets: backing away in fear from the lowest-carbon base-load source in the wake of a feared accident actually increased fossil-fuel use. By the mid-2020s the nuclear-versus-renewables debate had reopened, with small modular reactors (SMRs) and life extensions for existing plants back on the table. The seawall was 10 metres; science woke up late to the 15-metre wave.