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.

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.