Funny you should use the phrase "black helicopter." During the Cold War, the military used tiny nuclear reactors to power automated remote sensing stations in the Arctic. We called them "Power Pigs" and they were delivered by helicopter to very remote places.
They were fascinating little devices, and because of where they were placed, used the outside air/ice/snow/rocks for cooling.
Don't know if they are still being used, or what happened to the old ones, but they were very clever technology.
I guess these are the Strontium-90 RTGs made by the Soviets. There's about 1000 out there still - some have been vandalised, some washed out to sea, some sitting rusting in a lighthouse outhouse etc.
http://bellona.org/news/nuclear-issues/radioactive-waste-and...
I am guessing that you are talking about betavoltaics [1]. They were used to power pacemakers and are very useful for remote devices. They can't power much, but they are smaller than PV and don't have to be exposed to the sun (reflective).
Though there are larger devices that were used too.
