What happened at the Chernobyl nuclear plant in 1986?

On 26 April 1986, the Unit 4 RBMK reactor at the Chornobyl plant in Ukraine, then part of the Soviet Union, went out of control during a planned low-power test. The result was an explosion and fire that demolished the reactor building and released large amounts of radiation into the atmosphere. It was rated level 7, the highest on the International Nuclear and Radiological Event Scale. According to Reuters, the blasts came at 1:24 a.m. and blew off the reactor's heavy lid. [1] [2] [3]

The test was meant to show that, after a loss of coolant and an outside power failure, the slowing turbine could keep the main feed water pumps running until emergency diesel generators started, which took roughly 40 to 50 seconds. German regulator BASE says the test was wrongly treated as a purely electrical experiment with no expected effect on the nuclear side of the plant. The automatic emergency core cooling system was deactivated beforehand. [2]

During the power reduction, the reactor's output fluctuated and fell below 1 percent of operating power, a state in which the reaction is unstable. The test was not stopped. Control rods were raised almost completely to prevent a further drop, and an emergency shutdown signal was ignored so the test could be repeated if needed. When the pumps were switched off to begin the test, reduced coolant flow raised power. Operators started a manual shutdown, but a peculiarity of the RBMK control rods caused a dramatic power surge as they were inserted. Water flashed to steam, pressure rose, and the reactor exploded. The plant's own chronology puts the start of the test at 01:23:04 and the rapid power rise at 01:23:43. [2] [4]

The IAEA names several factors: the design of the reactor and its shutdown system, failure to follow procedures, and the absence of a safety culture as understood today. BASE adds that early RBMK reactors have a self-amplifying link between reactivity and temperature, and that control rods briefly boost the reaction before suppressing it. Some of these traits were not considered by the operating staff, and the test programme did not account for effects on the chain reaction. [1] [2]

RBMK reactors also lacked a pressure-retaining containment structure able to limit releases during severe core damage, so radioactive material escaped directly. The graphite moderator ignited after air entered the damaged core, driving emissions for several days. BASE reports major releases continued for 10 days, until about 5,000 tonnes of sand, clay, lead and boron were dropped from helicopters and nitrogen was blown in to cool the melted core. [1] [2]

The initial explosion killed two workers, and 28 firemen and emergency workers died of acute radiation sickness within four months. The town of Prypyat, with 49,360 people, was evacuated 36 hours after the accident. About 67,000 more people were moved in following weeks and months, and roughly 200,000 are believed to have been relocated in total. [1]

The IAEA estimates about 3.5 percent of Unit 4's fuel was dispersed outside the reactor building. Some 150,000 square kilometres of Belarus, Russia and Ukraine were contaminated, reaching as far as 500 kilometres north of the plant, and a 30-kilometre exclusion zone remains essentially uninhabited. Fallout also spread across much of the northern hemisphere, though often in insignificant amounts. The search results here do not cover long-term health effects. [1]