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Ukraine is holding ceremonies to mark the 30th anniversary of the explosion that destroyed a nuclear reactor at Chernobyl. The world's worst nuclear accident sent radioactive clouds of nuclear material across Ukraine's borders, into Russia, Belarus and across large parts of northern Europe. Thousands of people had to be evacuated from the contaminated zone in Ukraine. Three decades on, the US Under Secretary for Arms Control and International Security, Rose Gottemoeller, reflects on the lessons to be learned from Chernobyl. (Caption: Workers walk on the new construction site that is being built to contain any further radioactive leaks from the destroyed nuclear reactor at Chernobyl. Credit: Genya Savilov /AFP/Getty Image)