The Chernobyl Nuclear Plant in 1986 was part of the Soviet Union, but is now in the Ukraine. On April 26, 1986 the power plant suffered a catastrophic nuclear accident, when a reactor exploded and spilled radiation across most of western Russia and into Europe.
Wikipedia says this much better than I can, because I would get emotional. But the facts alone are stark enough:
The Chernobyl disaster was the worst nuclear power plant accident in history in terms of cost and casualties. It is one of only two classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale, the other being the Fukushima Daiichi nuclear disaster in 2011. The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles. During the accident itself, 31 people died, and long-term effects such as cancers are still being investigated.