The isotope of plutonium that's used in radiothermal generators for space missions, Pu-238, is something you produce specially. Usually you get Np-237 from spen

The isotope of plutonium that's used in radiothermal generators for space missions, Pu-238, is something you produce specially. Usually you get Np-237 from spent reactor fuel, and then bombard it with neutrons in a reactor designed for producing useful isotopes.