You mean, something like a RTG (https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectr

You mean, something like a RTG (https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_...)? I vaguely recall some suggestion to use fission products from reprocessing as RTG fuel, or just as a low grade heat source. I'd guess the hazards of FP's would make it very expensive indeed.