The Chernobyl accident in 1986 was the result of a flawed reactor design that was operated with poorly trained workers. The resulting condensation explosion and fires released at least 5% of the radioactive reactor core into the atmosphere and downwind.
Two Chernobyl plant workers died on the night of the accident, and a further 28 people died in a few weeks as a result of acute radiation poisoning. UNSCEAR says that apart from increased thyroid cancers, there are no facts of a major public health impact attributable to radiation revelation 20 years after the accident.
The Chernobyl disaster is widely considered to have been the worst nuclear power plant accident in history, and is one of only two classified as a level 7 event on the worldwide Nuclear Event Scale.