US-A

Between 1967 and 1988 the Soviet Union operated a constellation of nuclear-powered active-radar reconnaissance spacecraft to monitor oceanwide NATO and merchant vessel traffic. This is the most widespread use to date of nuclear reactors in space, and as could be expected given the Soviet Union's environmental track record, it had consequences, most of which were not good. You can read more about the program here:
US-A

Officially the program was known as Upravlyaemy Sputnik Aktivnyj. The internet claims that this roughly means "Managed Active Satellite," but I suspect some nuance is lost on Google's automation. In the west the program was known by the almost-equally uninspiring name "Radar Ocean Reconnaissance Satellite," or RORSAT. Whatever you call it, to this day the program is responsible for a significant slice of the total orbital debris pie. The reason for this has to do with the satellites' mission and powerplant.

The inverse square law is not kind to interplanetary watchmen. Each doubling of the distance a radar pulse travels quarters its intensity on a one-way trip, so the intensity of the echo of an active radar ping drops with the inverse-fourth power of distance. Double the distance between ship and satellite, and the ship's radar signature dims by a factor of 16. Looking at Earth's oceans from space, the physics shows that it's more reasonable to fly as close to the atmosphere's top as possible rather than jacking up the dish's power. The US-A constellation was nuclear-powered to avoid the drag penalty of hauling large sail-like solar arrays in a low orbit where drag is significant. Each spacecraft (33 in all) was powered by a liquid-metal-cooled reactor producing between two and six kilowatts of electrical power routed into a large radar antenna.

Even the environmentally-challenged Soviet government knew it wouldn't do to let a nuclear reactor fall indiscriminately on some unsuspecting part of the Earth once its mothership's mission was complete. Except for the time that totally happened. Aside from one launch failure and two on-orbit failures, each US-A successfully launched its reactor into a higher disposal orbit at the end of its mission. These reactors will continue to orbit for centuries to come as their fuel gradually decays away from lethality. Unfortunately thousands of pellets of solidified sodium-potassium collant were released in the disposal process, adding significantly to the debris hazards of low Earth orbit, and the derelict reactors continue to shine like signal flares in the gamma-ray spectrum, disturbing the astronomy that studies exotic physics in space.

Citing the many technical, reliability, environmental, and political problems of occasionally dropping unshielded nuclear reactors on NATO countries and often dumping swarms of metal pellets into the paths of unsuspecting satellites, the final Soviet premier shut down the program as the Cold War came to a close. I presume the Russians have since found other ways to keep close tabs on American shipping, but I won't be surprised if nuclear reactors have their day in space once again, this time for the exploration of the cosmos.