Chernobyl

The test required running Unit 4 at low power. The RBMK-1000 is unstable there: it has a positive void coefficient, meaning that when cooling water boils into steam, reactivity goes up rather than down. Steam voids therefore increase power, which makes more steam, which increases power again.

Compounding it, the control rods were tipped with graphite. Inserting them displaced water with a *better* moderator for the first few seconds — so pressing the emergency shutdown button briefly added reactivity instead of removing it.

At 1:23 a.m. the operators triggered the shutdown. Power surged, the fuel fragmented, and steam pressure blew the 1,000-tonne upper biological shield off the reactor. There was no containment building of the kind Western reactors have. The exposed graphite core burned for ten days, lofting fission products high into the atmosphere.

Firefighters and plant staff received enormous doses fighting the graphite fire, many without knowing what they were dealing with. 116,000 people within 30 km were evacuated by 14 May, and 220,000 more were resettled later.

The contamination crossed Europe, first detected outside the USSR at a Swedish plant whose workers set off radiation alarms arriving for their shift. Restrictions on sheep grazing in upland Britain lasted until 2012.

Surviving RBMKs were retrofitted with higher fuel enrichment, faster-acting shutdown systems and modified rod designs to reduce the void coefficient. No RBMK has been built since.

The accident also produced the international safety architecture that did not previously exist: the Convention on Early Notification, the Convention on Nuclear Safety, and the IAEA's operational safety review programme. WANO, through which operators inspect each other's plants, exists because of Chernobyl.