A military helicopter sprays a decontaminating substance over the region surrounding the Chernobyl nuclear power station a few days after its No. 4 reactor's blast, the worst nuclear accident of the 20th century. #

An aerial view of the Chernobyl nuclear power plant, the site of the world's worst nuclear accident, is shown in this May 1986 photo made a few days after the April 26 explosion in Chernobyl, Ukraine. In front of the chimney is the destroyed 4th reactor. Behind the chimney and very close to the 4th reactor is the 3rd reactor which was stopped on December 6, 2000. #

Repairs are carried out on the Chernobyl nuclear plant in Ukraine on October 1st, 1986, following a major explosion in April 1986 which, according to official statistics, affected 3,235,984 Ukrainians and sent radioactive clouds all over Europe. #

Part of the collapsed roof at the Chernobyl nuclear plant, damaged in a fire, is pictured in this photo taken, Friday, October 13, 1991 in Chernobyl, Ukraine during a media tour of the facility. #

Lt. Colonel Leonid Telyatnikov, Head of the Pripyat Fire Brigade which fought the Chernobyl blaze, points at a photograph of the power station's damaged fourth reactor following the April 26, 1986 nuclear accident. The reactor has since been entombed in concrete. Telyatnikov, 36, was hospitalized for two months with acute radiation sickness and was twice decorated for bravery. #

A Kurchatov Nuclear Institute worker walks in the light streaming into the cement-entombed room of the Chernobyl nuclear power plant's exploded reactor on Friday, Sept. 15, 1989, three years after the nuclear disaster. #

A graveyard for vehicles highly contaminated by radiation, near the Chernobyl nuclear power plant, seen on November 10, 2000. Some 1,350 Soviet military helicopters, buses, bulldozers, tankers, transporters, fire engines and ambulances were used while fighting against the April 26, 1986 nuclear accident at Chernobyl. All were irradiated during the clean-up operation. #

A Kurchatov Nuclear Institute worker stands in the operators room of block number four Chernobyl's nuclear power plant inside the sarcophagus on Friday, Sept. 15, 1989, three years after the nuclear disaster. #

A nurse at a children's health clinic in Warsaw administers an iodine solution to a three-year-old girl held in her mother's arms in Poland, May 1986. Protective measures were taken for possible radiation poisoning from the Soviet nuclear accident in Chernobyl. #

Trucks filled with concrete wait for unloading at the construction site of the concrete sarcophagus at the Chernobyl nuclear power plant's fourth reactor in this October 1986 file photo. #

Ukrainian Academy of Sciences member Vyacheslav Konovalov holds a preserved mutated colt in Zhytomyr, Ukraine, on Monday, March 11, 1996. Konovalov had been studying biological mutations appearing after Chernobyl nuclear plant explosion. The colt was dubbed "Gorbachev colt" after Konovalov brought a lifesize photo of it to the Supreme Soviet in 1988 to show the Soviet President Mikhail Gorbachev what Chernobyl was doing the country's wildlife. #

A statue of Vladimir Lenin stands in the middle of a small park in the port of Chernobyl near the frozen river of Pripyat on January 29, 2006 in Chernobyl, Ukraine. The Chernobyl Port was abandoned soon after the 1986 Catastrophe. #

A monitor screen of the first power block's control assembly of Chernobyl nuclear power plant displays the unloading of the last atomic fuel from the reactor on November 30th, 2006. #

A raven stretches its wings as it sits on a post inside the 30 km (18 miles) exclusion zone around the Chernobyl nuclear reactor near the village of Babchin, Belarus on December 23, 2009. The sign reads: "Radiation hazard". #

Baby cradles are seen in a hospital in the abandoned town of Pripyat, inside the exclusion zone around the closed Chernobyl nuclear power plant Sunday, April 2, 2006. Pripyat, a town of 47,000-people near Chernobyl nuclear power plant, was completely evacuated within days after the accident. #

A guide holds a Geiger counter showing radiation levels 37 times higher than normal as a woman takes a picture in front of the sarcophagus of the destroyed fourth block of the Chernobyl nuclear power plant on September 16, 2010. Thousands of people each year visit the Chernobyl nuclear power plant, where the world's worst nuclear disaster took place in April 1986, and the 30-km zone around it that remains uninhabited. #

Nastasya Vasilyeva, 67, cries at her home in the devastated village Rudnya in an isolated zone some 45km (28 miles) from Ukraine's Chernobyl nuclear power plant, Monday, April 3, 2006. Dozens of villages in the contaminated zone stand empty, their residents having been evacuated following the world's worst nuclear disaster. However, despite radiation warnings many residents have returned to their homes, saying they had difficulties starting their lives anew in other parts of the country. #

A wolf stands in a field inside the exclusion zone around the Chernobyl nuclear reactor near the village of Babchin, Belarus on February 1, 2008. Wildlife in the exclusion zone has been teeming despite radiation, since people left the area around Chernobyl after the 1986 nuclear disaster, keepers of the ecological reserve said. #

A man lights a candle at the Chernobyl victims' monument in Slavutich, some 50 kilometers (30 miles) away from the accident's site, and where many of the power station's personnel used to live, during a memorial ceremony on the night of April 25-26, 2009. #

Pictures of workers who worked at Ukraine's Chernobyl nuclear power plant immediately after the explosion in 1986 are exhibited in the Chernobyl museum in Kiev April 18, 2006. #

In April of 1996, a worker operates a drilling machine as he makes tests under the sarcophagus, built over the Chernobyl nuclear power plant's fourth reactor which exploded on April 26, 1986 #

In this November 10, 2000 photo, the control room with its damaged machinery, is seen inside reactor No. 4 in the Chernobyl nuclear power plant. Geiger counters registered about 80,000 microroentgens an hour, 16,000 times the safe limit. #

An employee of the Chernobyl Nuclear Power Plant walks in the control room of the destroyed 4th block of the plant on February 24, 2011 ahead of the 25th anniversary of the meltdown of reactor number four due to be marked on April 26, 2011. #

A man visits his ruined house inside the exclusion zone around the Chernobyl nuclear reactor in the abandoned village of Lomysh, southeast of Minsk, Belarus on March 18, 2011. #

Anya Savenok, 9, who was born physically affected due to high radiation according to doctors, plays in her home in the village of Strakholissya, just outside the exclusion zone around the closed Chernobyl nuclear power plant April 1, 2006. #

A woman passes by a sign on a fire station, displaying the local time, temperature and radioactivity level data, in Russia's far eastern city of Vladivostok on March 16, 2011. #

Vika Chervinska, an eight-year-old Ukrainian girl suffering from cancer waits to receive treatment with her mother at the children's hospital in Kiev Tuesday, April 18, 2006. Greenpeace stated in a 2006 report that more than 90,000 people were likely to die of cancers caused by radiation from the Chernobyl nuclear disaster, countering an earlier United Nations report that predicted the death toll would be around 4,000. The differing conclusions underline the contentious uncertainty that remains about the health effects of the world's worst nuclear accident as its 25th anniversary approaches. #