Depends entirely on the amount of activity released, so there is no single answer that can be quoted.
The Chernobyl nuclear accident.
12 miles is the minimum distance of evacuation, peope living 12 to 19 miles from reactors have been told to stay indoors.
evacuate the area immediately
There are many ways a nuclear accident may occur, however most nuclear power plant accident occur due to problem in an reactor.
Nuclear power accidents are a disaster. The 1986 nuclear accident at the Chernobyl Nuclear Power Plant in Ukraine was a catastrophic accident.
The tsunami on 11 March 2011 caused damage and ultimately shutdown of three nuclear reactors in the Fukushima Daiichi Nuclear Power Plant complex in Japan. Residents within a 20 km (12 mi) radius of the Fukushima Nuclear Power Plant were requested to evacuate during the accident.
the reactor accident at the chernobyl nuclear power plant.
Chernobyl is famous for the worst nuclear power plant accident in history.
Three Mile Island.
Russia, the nuclear plant was in the place called chernobyl :)
The Chernobyl Nuclear Power Plant underwent a level 7 event - the worst accident so far. The plant, located in the Soviet Union near Pripyat in Ukraine lost its number four reactor on 26 April 1986. A link to the Wikipedia article on the accident is provided.
The 1986 accident at the Chernobyl nuclear power plant in Ukraine is the only accident in the history of commercial nuclear power to cause fatalities from radiation. It was the product of a severely flawed Soviet-era reactor design and human error.