Chernobyl Nuclear Power Plant, Reactor 4

A safety test that destroyed a reactor

The Chernobyl nuclear accident occurred at 01:23:40 local time on Saturday, 26 April 1986, during a planned safety test of the emergency core cooling system at Reactor 4 of the V.I. Lenin Nuclear Power Plant near Pripyat in northern Ukraine, then Ukrainian SSR.

The test required the reactor to be run at low power — a power- level regime in which the RBMK-1000 graphite-moderated reactor had known instabilities.

Operating errors, combined with a design flaw in the control rods that briefly increased reactor power as they were inserted (the positive scram effect), produced an exponential power excursion.

Within four seconds the reactor's thermal output reached 30 000 megawatts. Ten times its rated maximum.

A steam explosion at 01:23:45 blew the 2 000-tonne upper biological shield off the reactor and opened the core to the atmosphere. A second explosion seconds later — possibly hydrogen, possibly nuclear — completed the destruction.

Thirty-one official deaths

The official death toll from the Chernobyl accident, as recognised by the Soviet government and confirmed by subsequent reports, is 31 people.

Two plant workers killed in the initial explosion. One plant worker dying overnight of injuries. And 28 emergency responders — firefighters and plant staff — who died over the following weeks from acute radiation syndrome.

The total number of deaths attributable to Chernobyl over the long term — primarily from thyroid cancers in people who were children at the time, and from solid cancers and leukaemias in the Russian, Belarusian, and Ukrainian populations exposed to the plume — is contested.

The WHO's 2005 estimate was 4 000 long-term deaths. Greenpeace's 2006 estimate was 200 000.

Both figures involve heavy methodological choices about how to attribute individual cancers to a specific radiation source.

A city evacuated 36 hours later

The city of Pripyat — population 49 000, built in 1970 to house the plant's workers — was 3 kilometres from Reactor 4.

The evacuation was not ordered until 14:00 on 27 April 1986. 36 hours after the explosion.

Soviet authorities initially withheld information about the accident. The Swedes detected the radiation plume on 28 April, when increased radiation triggered alarms at the Forsmark nuclear plant 1 100 kilometres from Chernobyl, and international pressure forced a Soviet acknowledgement that evening.

Pripyat residents had been exposed to a substantial dose over the day-and-a-half before evacuation.

The town has been sealed off ever since. The buildings stand, the rooms have been left as they were on the morning of 27 April 1986. School books on desks. Washing on lines. Food in fridges. The exclusion zone around the plant is 2 600 square kilometres and is still uninhabited.

The sarcophagus, then the arch

The destroyed Reactor 4 was sealed by Soviet workers — the liquidators — under desperate conditions in the months following the accident.

A concrete sarcophagus was built over the reactor between May and November 1986, designed to contain the radioactive material for 30 years. By the early 2000s the sarcophagus was beginning to fail.

Between 2010 and 2016, an international consortium built the New Safe Confinement — a 36 000-tonne arch of steel and concrete, the largest moveable land-based structure ever built — and rolled it on rails over the sarcophagus in November 2016.

The arch is designed to contain the reactor for 100 years, by which time some technology is presumably to be developed for actually dismantling the reactor and removing the solidified core — the elephant's foot of radioactive corium that formed in the reactor basement.

No such technology yet exists.