Chernobyl disaster

The Chernobyl disaster was the explosion of reactor 4 at the Chernobyl Nuclear Power Plant, near the town of Pripyat in the Ukrainian Soviet Socialist Republic of the Soviet Union, at 01:23 on 26 April 1986. The blast and the fire that followed spread radioactive material across the Soviet Union and much of Europe. It remains the worst nuclear accident in history and one of only two, with Fukushima in 2011, rated at the maximum level of the international scale for nuclear events.

The plant used Soviet RBMK reactors, cooled by water pumped through the core. Engineers wanted to show that, if outside power failed, the spinning-down turbine could keep the cooling pumps running for the gap of about a minute before the backup diesel generators took over. Earlier attempts at this test, in 1982, 1984 and 1985, had not succeeded. A new attempt was planned for 25 April 1986, during a scheduled shutdown of reactor 4 for maintenance, but a request from the Kiev grid controller delayed it into the night shift. After an unexpected drop in power, the operators ran the test with the reactor in an unstable state.

At 01:23:40, as the test was ending, an operator pressed the emergency shutdown button. In the RBMK design, the control rods carried graphite tips which, as the rods began to descend, briefly increased the reaction instead of reducing it. This flaw had been noticed at another plant in 1983, but nothing had been done. The power surged, the fuel channels ruptured and the coolant flashed to steam. Explosions destroyed the reactor and the building around it, and a meltdown and fire in the exposed core sent radioactive contamination into the air. Two workers died at the time of the explosion.

Firefighters and plant staff fought to put out the fires and stabilise the reactor, many of them receiving severe doses of radiation. Pripyat, home to the plant's workers, was not warned at first, and the city was evacuated by bus only from the afternoon of 27 April. On the morning of 28 April, radiation alarms went off at the Forsmark nuclear plant in Sweden, more than 1,000 kilometres away. Faced with a Swedish inquiry, Soviet authorities admitted that an accident had happened, and that evening a twenty-second statement was read on the main television news. An exclusion zone of 10 kilometres, later widened to 30 kilometres, was cleared, from which roughly 117,000 people were evacuated in all. In later years many more were resettled from contaminated areas of Ukraine, Belarus and Russia.

More than half a million people, known as liquidators, took part in the response. Engineers feared that molten material reaching water in the pools beneath the reactor could cause another explosion, though this did not happen. A concrete shelter, the sarcophagus, was completed over the ruined reactor in December 1986. Between 2016 and 2018 a much larger arched structure, the New Safe Confinement, was built over the old one to allow the reactor debris to be removed. Pripyat was abandoned, and a new town, Slavutych, was built for plant staff.

Of 237 workers taken to hospital, 134 developed acute radiation sickness and 28 of them died within three months. By 2005 there had been around 6,000 cases of childhood thyroid cancer among people in affected areas, 15 of them fatal, a large share of them attributed to the accident. Estimates of eventual deaths from long-term radiation exposure remain uncertain and disputed, ranging from about 4,000 in Ukraine, Belarus and Russia, according to the United Nations, to about 16,000 across Europe.

The disaster damaged trust in Soviet officials and in nuclear power worldwide, and many planned reactors were cancelled. The attempt to hide the accident pushed the Soviet government towards greater openness, or glasnost, which helped pave the way for the reforms that ended in the dissolution of the Soviet Union.

Our 1985 map shows the Soviet Union shortly before the accident, including the Ukrainian and Byelorussian republics; the 1994 map shows independent Ukraine and Belarus.