The Chernobyl accident: What happened

Chernobyl in 1999: Human error was a major factor

Reactor Four at the Chernobyl nuclear power plant began to fail in the early hours of 26 April, 1986.

Seven seconds after the operators activated the 20-second shut down system, there was a power surge. The chemical explosions that followed were so powerful that they blew the 1,000 ton cover off the top of the reactor.

Design flaws in the power plant's cooling system probably caused the uncontrollable power surge that led to Chernobyl's destruction.

An accident has occurred at Chernobyl nuclear power station. One of the atomic reactors has been damaged. Measures are being taken to eliminate the consequences of the accident. Aid is being given to the victims. A government commission has been set up

Moscow Radio's first report of the accident, 28 April 1986

Serious mistakes had also been made by the plant operators, who had disengaged several safety and cooling systems and taken other unauthorised actions during tests of electrical equipment.

With procedures intended to ensure safe operation of the plant operating less than effectively, the Chernobyl unit was even more vulnerable to unforeseen power discharges.

The Chernobyl plant did not have an effective containment structure, and without that protection, radioactive material escaped into the wider environment.

The crippled reactor is still encased in a hurriedly constructed concrete sarcophagus, which is growing weaker over time.

Contamination

The accident that destroyed the reactor in Unit Four killed 31 people almost immediately.

The plant is sealed now by a concrete and steel sarcophagus

Soviet scientists estimate that about 4% of the 190 tons of uranium dioxide products escaped and began to spread unevenly across the surrounding countryside, affected by the vagaries of the weather.

Both Soviet and Western scientists agree that most of the contaminating material affected Belarus, but that 40% spread to other nearby areas, including Ukraine.

Immediately after the accident, the main health concern involved levels of radio-iodine radiation.

Although the 600,000 workers involved in the recovery and clean-up after the accident were exposed to high doses of radiation, the exact amount cannot be accurately measured.

The workers, many of them volunteers, were not equipped with measuring instruments to monitor the dosages they were receiving, but estimates of about 165 millisieverts have been made.

Doses of radiation above 10 millisieverts pose significant threats to the human body.

The first Russian media report of the accident came two days after it occurred, and was the fourth item in Moscow Radio's evening news bulletin.

The initial media cover-up of the scale of the accident, which happened a year after Mikhail Gorbachev came to power, helped give rise to the programme of Glasnost (Openness) and Perestroika (Reconstruction) which led ultimately to the collapse of the Soviet Union.

Evacuation

Soviet authorities started evacuating people from the area around Chernobyl within 36 hours of the accident.

It is estimated that five million people were exposed to radiation

A month later, all those living within a 30 kilometre (18 mile) radius of the plant - about 116,000 people - had been relocated.

Several international organisations have studied the environmental and health impacts of the Chernobyl accident.

The World Health Organisation says that, so far, there has been a large increase in thyroid cancer among children in the affected areas.

It is estimated that five million people were exposed to radiation in Ukraine, Belarus and Russia.