Re: How do the manage the fuel
It's a* thermocouple. A* thermocouple stuffed into a lump of something fiercely radioactive. Lumps of fiercely radioactive stuff are warmer than their surroundings as they're heated by their own decay. Stuff one end of a* thermocouple into a lump of fiercely radioactive stuff and you have simple and potentially** long lasting but feeble power source. Verbose quasitechnical crap like "plutonium-powered radioisotope thermoelectric generators" is just "gosh, aren't I clever" for "a* thermocouple stuffed into a lump of radioactive stuff"
* Actually a great wodge of thermocouples. May as well make as much hay as possible while the plutonium shines.
** Life expectancy depends on the half-life of the source and its products and how quickly the various emissions from all that decay destroy your thermocouple(s)