At 1:24 am on April 26th 1986, Reactor 4 at the Chernobyl nuclear power plant exploded and then caught fire. Over the following 2 weeks, 3 × 1018 Bq radioactivity were released to the environment, and were deposited over the surrounding countryside and the rest of Europe. The reactor was a Soviet designed pressurised water reactor called an RBMK. At the time of the accident, tests were being conducted on the reactor, in the course of which, operators deliberately overrode safety systems. Investigations revealed that the accident was the result of a flawed reactor design that was operated with inadequately trained personnel and without proper regard for safety. Under the test conditions, it was impossible to operate the reactor safely due to inbuilt design faults. Coolant water turned to steam during the test, which led to a rapid power surge. It was not possible to drop the control rods quickly enough to suppress the surge, which generated more steam. A vicious cycle had started, which ended with the explosion. The resulting steam explosion and fire released at least five percent of the radioactive reactor core into the atmosphere and downwind. 31 people were killed, and there have since been around ten deaths from thyroid cancer apparently due to the accident. The explosions ejected 8 tonnes of fuel from the core, and more volatile radionuclides were vaporised. These were mostly fission products. A second explosion threw out fragments of burning fuel and graphite from the core and allowed air to rush in, causing the graphite moderator to burst into flames. The graphite burned for nine days, causing the main release of radioactivity into the environment. The fuel particles were deposited locally, whereas the fission products were transported further afield by weather systems..

The Impact

It is estimated that all of the xenon gas, about half of the iodine and caesium, and at least 5% of the remaining radioactive material in the Chernobyl-4 reactor core was released in the accident. Most of the released material was deposited close by as dust and debris , but the lighter material was carried by wind over the Ukraine, Belarus, Russia and to some extent over Scandinavia and Europe. The main casualties were among the firefighters, including those who attended the initial small fires on the roof of the turbine building.

Many children in the surrounding areas were exposed to radiation doses sufficient to lead to thyroid cancers (usually not fatal if diagnosed and treated early). Initial radiation exposure in contaminated areas was due to short-lived iodine-131, later caesium-137 was the main hazard (both are fission products dispersed from the reactor core). On 2-3 May, some 45,000 residents were evacuated from within a 10 km radius of the plant, notably from the plant operators' town of Pripyat. On 4 May, all those living within a 30 kilometre radius - a further 116000 people - were evacuated.

The Lessons Learnt from the Disaster

The Chernobyl accident was a radiation event unique in human history. In terms of human losses, it was a minor event as compared with many other man-made catastrophes. But, in political, economic, social and psychological terms, its impact was enormous. This was the worst possible catastrophe of a badly constructed nuclear reactor; with a complete meltdown of reactor core; followed by ten days of completely free emission of radionuclides into the atmosphere. Nothing worse could happen. The most tangible practical benefits that have resulted from the Chernobyl accident concern reactor safety and crisis management of the nuclear industry. The occupational death toll may not have been severe, but the catastrophe also changed the view of people engaged in radiological protection on the paradigm on which the present safety regulations are based.