Plutonium does not become inert of radiation after 88 years, you have got to be kidding.:lol:
Plutonium continues to produce 4000+ degrees C up to 52 Trillion zeros in years, so it is very handy for this purpose. Even 88 years is still very good for a source of heat of that temperature but again it last much, much, much longer than 88 years. Your maths is so wrong lol, plutonium is very, very hot and radioactive!