It's called radioactive fallout. A large plume of radioactive debris was spread over Europe (in fact, this was how the accident was first detected outside of the USSR, not because the authorities reported it).
The debris contaminated ground water, lakes, rivers, forests, animals, livestock, etc. all over parts of Europe.
I was in the UK at the time and I remember the contamination almost destroyed the lamb and mutton industry there.
Wikipedia has some information on the effects of the disaster: http://en.wikipedia.org/wiki/Chernobyl_disaster
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