The Immediate

Helicopters and trucks abandoned after taking part in the clean-up operation

WITH the core now fully exposed, a plume of smoke, radioactive fission products and debris rose up to about 1km into the air. The material was carried northwest by the wind - mainly to Belarus, though other areas were affected, including Ukraine.

Fires broke out all over the plant. About 250 firemen were called, many of whom were not equipped with measuring instruments to monitor the radiation dosages they were receiving. The operators and rescue workers are to be commended - many stayed on call in the area after having been relieved of their duties and many risked their lives to save others and bring the situation under control. Most of the fires had been extinguished by 05:00, but the graphite fire continued for another nine days. The main release of radioactivity into the environment was caused by the burning graphite.

A group of liquidators

Once the order was given, on 27 April, the town of Pripyat was evacuated completely within 2.5 hours. The evacuees were never to return, and the town remains how it was left. While the fire raged, about 5,000 tonnes of different materials were dropped by helicopter onto the exposed core. Some were intended to smother the fire, some to absorb neutrons to prevent renewed chain reaction, some to act as a heat sink, lead to act as a radiation shield, and sand and clay to prevent further contaminants escaping. 1800 helicopter flights were made, their task severely hampered by not being able to hover over the core due to high radiation levels. These efforts were relatively successful, although there was a further major release of contaminants when (it is thought) the core melted. On 9 May, work began on the digging of a tunnel underneath the core to install a huge concrete slab and cooling system. The slab was intended to cool the core if necessary, and also act as a barrier to prevent radioactive material leaching into the groundwater. Finally, the core was entombed in a 300,000-tonne concrete and steel sarcophagus, and the surrounding land and buildings decontaminated.

Monument in Bragin to the firefighters who died

It is estimated that about 3.5%, or 6 tonnes, of the uranium dioxide fuel and fission products escaped as well as many other radionuclides - principally xenon, krypton, iodine, tellurium and caesium. A total of about 12 x 10 18 Bequerels of radioactivity was released, around 200 times that of the Hiroshima and Nagasaki bombs according to the World Health Organisation. The highest levels of contamination were within a 30km radius of the site; levels of Caesium-137 exceeded 1500kBq/m 2 . Caesium-137 is used as it is easily measurable, and posed the greatest health risk after the Iodine-131 had decayed. I-131 has a half-life of eight days and Cs-137 30 years . Levels of 40kBq/m 2 covered large parts of Northern Ukraine and Southern Belarus, with a number of extreme 'hot-spots' occurring where it happened to be raining as the cloud passed over.

Predictably, the Soviet government tried to deny that anything had happened, so the first time the plume was detected outside of the USSR by workers at a Swedish nuclear plant, they suspected another Swedish facility. It was only then that the Soviets, under extreme international pressure, owned up. The cloud was tracked thereafter and passed over Scandinavia, Holland, Belgium and the UK, carried by the northwesterly wind. It then went south, covering much of the rest of Europe after the wind changed. Contamination was detected in nearly every country in the northern hemisphere, as far as North America and Japan, although the southern hemisphere seems to have escaped. National authorities were surprised at the scale of the problem, and none had effective mitigation plans in place, particularly for a disaster that transcended national borders. Outside of the Soviet Union, countermeasures were pretty much limited to restrictions and prohibitions on the marketing, consumption and importation of certain foods thought to be contaminated or from contaminated areas.