Chernobyl disaster: Ukraine marks 30th anniversary Ukraine is holding commemorations to mark the 30th anniversary of the nuclear disaster in Chernobyl. Sirens were sounded at the same moment as the first explosion of the reactor, in the early hours of today. The meltdown at the plant remains the worst nuclear disaster in history. An uncontrolled reaction blew the roof off, spewing out a cloud of radioactive material which drifted across Ukraine's borders, into Russia, Belarus and across a swathe of northern Europe. Relatives of those who died attended candle-lit vigils at several churches, including Slavutych, a town built to re-house workers who lived near the nuclear plant. SOURCE: BBC news