The Chernobyl disaster was a nuclear accident that occurred on April 26, 1986 at the Chernobyl Nuclear Power Plant in Pripyat, Ukraine (then Soviet Union). During a safety test on Reactor 4, a sudden power surge caused a series of explosions that destroyed the reactor and released massive amounts of radioactive material into the atmosphere. Over 30 emergency responders died from acute radiation exposure in the initial aftermath. The radioactive fallout spread over much of Europe and long-term health effects are still being studied. It remains one of the worst nuclear disasters in history based on cost and radioactive contamination.