Problem then is the allowing of that horrible corrosive element Oxygen into the sealed enclosure. Fuel cells all degrade with time. Best would be around 3g of Americium and a few thermocouples ( peltier junctions are cheap these days) to convert the low grade heat to electric power. That at least will last the century or more required, and will fit in a small package and is relatively easy to shield with only a small amount of lead and Boron.