CHERNOBYL, Ukraine (AFP) - The head of the UN warned on a landmark visit to Chernobyl Wednesday that the Ukrainian tragedy and the recent accident in Japan prompted "painful questions" about the future of atomic power. UN chief Ban Ki-moon visited the site of the Chernobyl disaster a day after world donors pledged $800m towards a permanent shelter to secure the ruined reactor, which exploded on April 26, 1986. Speaking in Kiev afterwards, he warned that the recent quake damage to Japan's Fukushima nuclear plant showed that accidents like Chernobyl were likely to occur again in the future. "The unfortunate truth is we are likely to see more such disasters. The world has witnessed an unnerving history of nuclear accidents," he said at a conference, calling for a global debate on the safety of nuclear energy."To many, nuclear energy looks to be a relatively clean and logical choice in an era of increasing resources' scarcity. Yet the record requires us to ask painful questions: have we correctly calculated its risks and costs?" he said. Ban spoke after flying to Chernobyl by helicopter, standing outside the power plant for around 20 minutes with Ukrainian President on a visit that he described as an "extremely moving experience."