Design flaws in the reactor, and breaches of safety procedures caused the accident.

Chernobyl put far more radioactive material into the atmosphere that the Hiroshima atomic bomb.

Nuclear weapons tests in 1950s and 60s put more radioactive material into the atmosphere than Chernobyl.

What happened at Chernobyl?
The world’s most devastating nuclear accident happened at Chernobyl, in the Ukraine, in 1986. The nuclear reactor blew up releasing about 7 tonnes of radioactive material, into the atmosphere over the course of a number of days. The contamination of the surrounding areas was widespread and the associated problems could persist for a long time. More than thirty died in the clean-up from burns and radiation. Many others have become ill gradually over time.

What went wrong?
The Chernobyl reactor was a graphite-moderated water-cooled
reactor. The fuel used was uranium dioxide enriched to 2% with uranium-235. It was decided to test certain aspects of the production system. Similar tests had been done successfully on two previous occasions but some electrical changes had been made since the last test. As part of the test the emergency cooling
system was switched off breaching safety regulations.
When the test on the generator started insufficient power was delivered to the pumps that circulate the coolant. Without the backup of the emergency cooling system, the reactor quickly overheated.

What happened then?
The heat output soared from 6% to 50% in about 10 seconds. The control rods could not be replaced in time. The fuel rods burst and the fuel mixed with the water which, turned to steam and could therefore no longer cool anything. At this stage the whole reactor building blew up and dispersed hot radioactive material through the surrounding area causing more than 30 fires, most of which were brought under control in a matter of hours. However it took 10 days to get the fire in the core of the reactor under control. This was achieved by dropping thousans of tonnes of lead and dolomite on the reactor by military helicopter.

What was the human and material impact?
The discharge included many short-lived radioisotopes including iodine and caesium. Radioactive material from the plant was subsequently detected over practically the entire Northern Hemisphere. When the fires died down the reactor was encased in concrete. An ‘exclusion zone’, initially some 30 kilometres in radius was established around the site and about 116 000 people within it were evacuated to less contaminated areas in the months following the accident. The exclusion zone was later extended to cover an area of 4300 square kilometres. In the eight years following the accident, more than 300 people suffered radiation sickness, and there has been an increase in the number of thyroid cancers in neighbouring regions, which may be related.

The future
Experts from all over the world have examined the Chernobyl incident in detail so as to ensure that an accident like it will never happen again. The reactor’s remains are now enclosed within a large concrete structure.

More Information

Chernobyl notes
Chernobyl photographs