Fukushima Daiichi

The reactors did what they were supposed to when the earthquake struck: all operating units scrammed and shut down. But a reactor that has just shut down still produces several percent of full power as decay heat, and it must be cooled for days.

Grid power was gone, so emergency diesel generators started — and then, 41 minutes later, a tsunami of about 15 metres arrived at a plant whose seawall was built for 5.7 metres. The water flooded the generators and the switchgear, both sited low and, in several cases, in basements.

Without power there was no way to remove decay heat. Cores in Units 1, 2 and 3 overheated and melted. Zirconium fuel cladding reacted with steam at high temperature to produce hydrogen, which accumulated in the reactor buildings and exploded — including in Unit 4, which had been defuelled, via shared ducting.

Over 100,000 people were evacuated. Units 1 to 4 were written off, 2,719 MWe of capacity.

Japan shut its entire nuclear fleet afterwards and has restarted it only slowly; that is why Japanese units still appear in "suspended operation" in international statistics more than a decade later. Germany brought forward its phase-out. Global reactor construction slowed sharply.

The most consequential finding was about the evacuation. Moving frail and elderly people out of hospitals and care homes at speed killed 2,313 of them. Radiation killed none. Emergency planning has been rewritten in several countries as a direct result.

The technical lessons were about common-cause failure: siting every emergency generator where a single flood could reach them, and assuming a design-basis tsunami far smaller than the historical record for that coast.

Regulators worldwide required portable backup power and water injection independent of the site's own systems, filtered containment venting, and hardened spent fuel pool instrumentation. Japan replaced its regulator entirely, having concluded the previous one was too close to the industry it oversaw.