Accidents Involving U.S. Space Nuclear Power Sources
The United States has launched 22 missions with RTG power sources. Three accidents have occurred, though only one has resulted in release of
radioactive materials. The U.S. has launched only one experimental space reactor, the SNAP 10-A in 1965. This reactor is currently in a nuclear-safe
storage orbit with an estimated life of three-thousand years. The eventual re-entry of SNAP-10A will not occur until the level of radioactivity has
decayed to a very low level.
Accidents Involving Soviet Nuclear Power Systems
There have been two accidents involving Soviet RTG's, and at least three incidents involving Soviet space nuclear reactors (Ref. 1).
In January of 1969, the launch failure of COSMOS 305 lunar mission with a lunar rover presumably powered by RTGs created detectable amounts of
radioactivity in the upper atmosphere (Ref. 2, Ref. 3). In the fall of that year, another lunar probe failed to make a translunar injection from Earth
orbit. The atmospheric burnup of this RTG also created detectable amounts of radioactivity in the upper atmosphere. Any surviving debris from these
incidents is presumed to be on the floor of the ocean (Ref. 3).
More : Link