( Vladimir Repik / Associated Press )

This May 1986 file photo shows the Chernobyl nuclear power plant, the site of the world's worst nuclear accident. Twenty-five years ago, power plant exploded in Ukraine, spreading radioactive material across much of the northern hemisphere. The April 26, 1986, explosion affected about 3.3 million Ukrainians, including 1.5 million children, according to Ukraine's Chernobyl Union report. The plant was closed for good in 2000.