Reactor No. 4 Explodes

At 01:23:40 local time, a safety test gone wrong triggers an uncontrolled nuclear chain reaction in Reactor No. 4 of the Chernobyl Nuclear Power Plant. Two successive explosions destroy the reactor's 1,000-tonne roof and rupture the building. The explosions are the result of a steam explosion and possibly a prompt criticality excursion, releasing approximately 400 times more radiation than the Hiroshima atomic bomb.