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Cosmos 954 was a Soviet Radar Ocean Reconnaissance Satellite (RORSAT) with an onboard nuclear reactor. The satellite's reactor core failed to separate and boost into a nuclear-safe orbit, and instead remained onboard in an orbit that decayed until the satellite reentered Earth's atmosphere on January 24, 1978. The satellite crashed in the Northwest Territories, Canada, spreading its radioactive fuel on a 600-kilometre path from the Great Slave Lake to Baker Lake. In an attempt to recover radioactive material, a search was conducted covering a total of 124,000 km² (48,000 miles)
One has to wonder how many reactors are floating around in orbit......with rapidly decaying orbits.