The Chernobyl disaster of 1986 was the worst nuclear power accident in history. During a systems test of Reactor 4, a surge in power caused two explosions that released radiation into the atmosphere. Over 100,000 people had to be evacuated from the surrounding area in Ukraine and Belarus. The incident was caused by an unstable reactor design combined with human error and safety violations during the test. It resulted in many deaths and long-term health and environmental effects.