There have been over a dozen serious incidents involving Nuclear Power in space. Despite aiming to provide a source of energy for various satellites and exploration probes in orbit and beyond, many have resulted in dispersals of highly radioactive materials throughout the Earth’s atmosphere.
United States
The United States has launched the most missions (at least 22 that we know about) with nuclear power sources, the most serious have been complied by Greens Against Nuclear Power below. At least three accidents resulted in the release of radioactive materials. One experimental space reactor (aboard SNAP 9-A) was launched in 1965, what remains of the stricken satellite is now in a 3,000-year orbit.
SNAP 9-A, April 1964: Launched aboard a Department of Defense weather satellite that failed to reach orbit. Reactor, as designed, released radioactive contents in upper atmosphere during re-entry and then burned. Remnants struck the Indian Ocean. Total of 2.1 pounds of plutonium-238 vaporized in atmosphere and spread worldwide.
SNAP 19, May 1968: Meteorological satellite. Nuclear fuel, 4.2 pounds of uranium-238, stayed intact and was recovered off Southern California coast and reused.
Apollo 13, 1970: Nuclear material, 8.3 pounds of plutonium-238, inside lunar module when it was jettisoned before return to Earth. Now at bottom of South Pacific Ocean near New Zealand. Sampling so far shows no radiation leak.
USA-193, February 21, 2008: The military spy satellite malfunctioned shortly after launch, then intentionally destroyed 14 months later. Speculation suggests “hazardous materials” and perhaps a nuclear-isotope powered reactor core were on board, and that the craft was destroyed to prevent technology falling into enemy hands.
Russia / Soviet Union
The former Soviet Union (CCCP) and Russia have both been responsible for a number of significant radiological releases into Earth’s atmosphere as a result of their space programme. Like the United States it is believe many more missions than have been revealed to the public may have launched over the years, a summary of the accidents known are as follows.
COSMOS 305, January 1969: Soviet unmanned lunar rover lost rocket power and stayed in orbit, dispersing radiation in upper atmosphere.
Soviet lunar probe, Fall 1969: Unmanned lunar probe burned up and created detectable amounts of radioactivity in the upper atmosphere. Any surviving debris from incident presumed to be on the ocean floor.
RORSAT, April 1973: Soviet satellite launch failed; reactor fell into Pacific Ocean north of Japan. Radiation detected.
COSMOS 954, January 1978: Launch failed; 68 pounds of uranium-235 survived fall through the atmosphere and spread over a wide area of Canada’s Northwest Territories. Canadian-U.S. teams cleaned up; no detectable contamination found.
COSMOS 1402, 1982: Failed launch; reactor core separated from spacecraft and fell to Earth separately in February 1983, leaving radioactive trail in atmosphere and landing in South Atlantic Ocean. Not known if any radioactive debris reached Earth surface or ocean.
COSMOS 1900, April 1988: Soviet radar reconnaissance satellite failed to separate and boost the reactor core into a storage orbit, but backup system managed to push it into orbit some 50 miles below its intended altitude.
COSMOS 1402, February 1993: Crashed into the South Atlantic carrying 68 pounds of uranium-235.
MARS96, November 1996: Disintegrated over Chile or Bolivia, possibly spreading its payload of nearly a half pound of plutonium.
Kosmos 1818, 4 July 2008: An object hit and cracked a coolant tube which formed part of the 1987 launched military satellite’s (highly-enriched) uranium oxide-powered TOPAZ reactor. The craft shattered into pieces, some are still being tracked.
Other Countries
Its not known if any other of nations that may have utilised nuclear technology as part of their space programmes, however if it hasn’t already happened it may be a real possibility in future. With the emergence of new global superpowers, who have both nuclear weapons and nuclear energy resources such as China and India currently expanding their expansion of their own military and scientific missions. Iran and North Korea also have the ability to put nuclear technology into space although it’s doubtful if they would any-time soon.
Sources: WikiPedia, NASA, Christian Science Monitor, International Atomic Agency, Greenpeace
See also: List of Nuclear Accidents: at Sea (coming soon)
Greens Against Nuclear Power 
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