(ANSA) - BELGRADE, 25 APR - "The Chernobyl accident was a watershed moment for nuclear safety. It resulted in legally binding agreements, increased transparency, and international cooperation," the IAEA, the United Nations' International Atomic Energy Agency, stated on its X account. Ukraine commemorates the 40th anniversary of the nuclear power plant explosion, which was the worst civilian nuclear disaster in history. This anniversary comes four years after the Russian invasion, which once again threatened the plant and raised the prospect of another nuclear disaster. The explosion, at 1:23 a.m. on April 26, 1986, was due to human error during a safety test that triggered an explosion in reactor number four of the plant, now in northern Ukraine but then part of the Soviet Union. In the following days, a radioactive cloud contaminated Ukraine, Belarus, and Russia before spreading throughout Europe. According to a 2005 UN report, the number of confirmed and expected deaths in the three most affected countries: Ukraine, Belarus, and Russia, is 4,000. In 2006, Greenpeace estimated that the disaster killed 100,000 people. (ANSA).