[Buckeye McFrog]

The US Air Force Museum in Dayton once hosted an exhibit of Soviet space items. Fascinating stuff. But I seem to recall they used onboard reactors more often than not.

[Army Air Corps]

But I seem to recall they used onboard reactors more often than not.

Not so much for their planetary probes. On the other hand, their RORSAT programme featured nuclear-powered spy satellites. This programme lanched 33 nuke-powered satellites. Thirty-one of those satellites used the BES-5 reactor, while two of them used the larger liquid-cooled TOPAZ reactor. Out of those 33 launches, there were five failures - three of these failures resulted in re-entry of the reactor core.

[Army Air Corps]

Also, bear in mind, that the remaining cores are just parked in higher orbits that will decay.