Tokaimura criticality accident

The plant was preparing enriched uranium for an experimental fast reactor. The approved process used a tall, narrow column whose *shape* prevents criticality regardless of what is poured in.

An unofficial procedure, adopted to save time, substituted stainless steel buckets and a precipitation tank that was wide and surrounded by a cooling jacket — good moderator geometry. Workers poured in about seven times the safe mass of uranium.

The solution went critical, emitting a blue flash and a sustained fission reaction that continued intermittently for around twenty hours until the cooling jacket was drained.

The two workers who had been pouring received doses far above the lethal threshold and died after 12 and 82 days. 320 residents within 350 metres were evacuated and 667 people in total received measurable doses.

Tokaimura is the clearest case in the record of criticality safety by geometry being defeated by an informal procedure. Nothing failed mechanically; the equipment was substituted by people who did not understand why the original shape mattered.

It also happened at a fuel plant, not a reactor — a reminder that the fuel chain has hazards of its own, and that a country's nuclear safety record is not only about its reactors.