The 26 April 1986 Reactor 4 Explosion at the Chernobyl Nuclear Power Plant That Released About 400 Times the Radiation of the Hiroshima Bomb

The Chernobyl Nuclear Power Plant was a four-reactor RBMK-1000 nuclear facility in northern Ukraine, then part of the Soviet Union. The RBMK design was a graphite-moderated, water-cooled reactor that had been produced in approximately 17 units across the Soviet Union. The design had well-documented stability problems at low power levels — something the standard operating procedures had restricted but the reactor staff had not always followed.

The Reactor Block 4 had been scheduled for a routine safety test on 25-26 April 1986. The test was intended to verify that the turbine generator could provide power to the reactor’s emergency cooling pumps for the 30-second gap between a loss-of-power event and the backup diesel generators starting up.

The test had been delayed across the day shift of 25 April for grid-management reasons. The reactor crew that conducted the test starting at 11:00 p.m. on 25 April was a night shift that had not been specifically briefed for the test.

1:23:40 a.m.

The test conditions required the reactor power to be reduced from the standard 3,200 MW to approximately 700 MW. The deputy chief engineer Anatoly Dyatlov — directing the test from the control room — over-reduced the power to approximately 30 MW. The xenon-poisoning effect that followed required pulling out approximately 200 of the 211 control rods to bring the power back to 700 MW.

The RBMK design’s “positive void coefficient” — the property that steam bubbles in the coolant water increased rather than decreased reactivity — became dangerous at the low power and low control-rod density. The steam-explosion conditions were building.

At 1:23:40 a.m. on 26 April 1986 the reactor power spiked. The operators attempted to shut down the reactor by inserting the control rods (the AZ-5 emergency-shutdown sequence). The graphite tips of the control rods produced a brief local power surge before the neutron-absorbing portion engaged. The coolant flash-boiled. The fuel channels ruptured.

The reactor exploded in a steam explosion at approximately 1:23:43 a.m. The 2,000-ton concrete-and-steel reactor lid was blown off. A second explosion approximately three seconds later scattered the reactor core across the roof and surrounding area. The graphite moderator was exposed to the open air and caught fire.

The graphite fire

The graphite fire burned for approximately ten days — 26 April to 6 May 1986. The fire carried vaporised radioactive fuel into the upper atmosphere, producing a radioactive plume that reached Sweden by 27 April (the Forsmark nuclear power plant’s automated radiation detectors detected the plume that morning — the first outside-Soviet detection of the accident).

Approximately 600 helicopters were used to drop 5,000 tons of sand, lead, boron, and clay onto the burning reactor across 27 April to 5 May. The fire was extinguished by approximately 10 May.

The evacuation

The nearby city of Pripyat — 3 km from the plant, population approximately 49,000 — was evacuated on 27 April 1986. The evacuation was conducted in approximately three hours using approximately 1,200 buses. The inhabitants were told they would be gone for three days. They never returned.

The 30-km Chernobyl Exclusion Zone extended the evacuation across subsequent weeks. Approximately 350,000 people were eventually permanently evacuated from the zone.

The casualties

The direct death toll across the first three months was:

— 2 reactor staff killed by the initial explosion — 28 firefighters and reactor staff killed by acute radiation syndrome over the subsequent weeks — 19 additional deaths from acute radiation effects across 1986-2004

The longer-term cancer death toll has been disputed. The 2005 World Health Organisation estimate projected approximately 4,000 cancer deaths across the cleanup workers, evacuees, and nearest-affected populations. The Greenpeace estimate projected approximately 200,000 cancer deaths across subsequent decades. The true figure lies somewhere between, and is difficult to isolate from background cancer rates.

The sarcophagus

The original concrete sarcophagus enclosing Reactor 4 was constructed across April-November 1986 — seven months of high-radiation construction work by approximately 600,000 Soviet conscripts (“liquidators”). The original sarcophagus was considered structurally inadequate by approximately 2000.