An aerial view of the Chernobyl Nuclear Power Plant in Ukraine with an arrow pointing to the location of the explosion. On the morning of April 26, 1986, no one could yet tell that a meltdown in reactor 4 of the nuclear plant in then-Soviet Ukraine was poisoning the air with so much deadly radioactivity that it would become the world's worst nuclear accident. REUTERS/File

A helicopter sprays a decontaminating substance over the region surrounding the Chernobyl nuclear power plant. The accident killed 31 right away and forced tens of thousands to flee. The final death toll of those killed by radiation-related illnesses such as cancer is subject to debate. REUTERS/Tass

A traffic policeman checks vehicles entering the restricted zone surrounding the Chernobyl nuclear station. Mikhail Gorbachev has since said he considered Chernobyl one of the main nails in the coffin of the Soviet Union which eventually collapsed in 1991. REUTERS/File

A child drinks an anti-radiation iodine solution in a Warsaw clinic following the Chernobyl disaster. A Greenpeace report ahead of the 30th anniversary cites a Belarusian study estimating the total cancer deaths from the disaster at 115,000, in contrast to the World Health Organisation's estimate of 9,000. REUTERS/File

An aerial view of the Chernobyl nuclear power plant still smoking shortly after the explosion of its fourth reactor. In particular, "the 30 km exclusion zone around the Chernobyl reactor remains highly contaminated and unsuitable to live in," it said. REUTERS/File

A Ukrainian policeman decontaminates a bus used to carry workers who built a sarcophagus around the fourth reactor of the Chernobyl nuclear station. REUTERS/File

The Chernobyl nuclear power plant after the explosion of its fourth reactor. REUTERS/Vladimir Repik

An aerial view of the fourth reactor of the Chernobyl nuclear power plant after its explosion. REUTERS/File

A helicopter drops concrete onto the fourth reactor of the Chernobyl nuclear power after its explosion. REUTERS/File

A wedding party crosses a street weeks after the Chernobyl nuclear disaster in the settlement of Polesskoe, near Chernobyl. REUTERS/Vladimir Repik

Major Leonid Telyatnikov, one of the first firefighters at the Chernobyl nuclear power plant disaster and head of the Chernobyl fire brigade, hugs his wife Larisa in the hospital grounds where he is being treated for exposure to radiation. REUTERS/File

Representatives measure the level of radioactivity on the Koenigsplatz in Munich to combat radiation fears following the Chernobyl disaster. REUTERS/Claus Hampel

A worker at the Chernobyl nuclear power plant checks the radiation level in the engine room of the first and second power units. REUTERS/File

The building of the sarcophagus around the fourth reactor of the Chernobyl nuclear power plant after its explosion. REUTERS/File

The number four reactor at the Chernobyl nuclear plant after completion of the work to entomb it in concrete. REUTERS/File

Workers enter the Chernobyl nuclear power plant weeks after the 1986 explosion in its fourth reactor. The sign reads: 'Comrades, we guarantee the launch of the first and second bloc by October 1986', referring to the first and second reactor that were switched off after the accident. REUTERS/File

A technician monitors nuclear reactor number 3 in a control room of the Chernobyl power plant. REUTERS/File

Evacuees from the Chernobyl region's collective farms walk through the streets of a newly built village in Makarovsky district near Kiev. REUTERS/Tass

Alexander Kovalenko, the former Information Chief of the Chernobyl clean up, holds a radiation meter showing a level of 2.3 milliRoentgens (hundreds of thousands of times less than in the first days after the accident). In the background are the third and the adjoining damaged fourth reactor now entombed in concrete. Photo is undated. REUTERS/Meg Bortin

A view of the Chernobyl nuclear plant, with reactor no. 4 in the foreground. REUTERS/Dominique Dubouble