Windscale Pile 1 fire

The Windscale piles were air-cooled graphite reactors built quickly to make weapons plutonium. Neutron bombardment displaces atoms in graphite and stores energy in the lattice — Wigner energy — which must be released periodically by deliberately heating the core.

During one such anneal the operators, working with very few thermocouples and no good picture of the core, applied a second heating cycle. Fuel ignited. Because the pile was cooled by drawing outside air straight through the core and out of a stack, the fire had an air supply and a direct path to the environment.

It was eventually extinguished with water, a decision taken in the knowledge that it might have caused a hydrogen explosion.

Milk from about 500 square kilometres of surrounding farmland was destroyed for a month because of iodine-131.

The stack filters that captured much of the particulate release had been added late, at the insistence of John Cockcroft, and were widely regarded as an expensive folly beforehand. They are still called Cockcroft's Follies.

The piles were shut permanently. The UK moved to gas-cooled reactors with closed CO₂ circuits rather than once-through air cooling.

The published report was censored by the Prime Minister, reportedly to avoid damaging nuclear cooperation with the United States — one of a pattern in 1957 of serious accidents at military facilities being concealed.