SNAP-10A was the first and so far only known launch of a U.S. nuclear reactor into space (although the US has launched many radioisotope thermoelectric generators). This is in comparison with the Soviet-launched reactors in the RORSAT satellites. The Systems Nuclear Auxiliary Power Program (SNAP) reactor was developed under the SNAPSHOT program overseen by the U.S. Atomic Energy Commission.
Read more about SNAP-10A: Spacecraft History, Construction and Operation, Safety, Development, Destructive Testing