If something is Nuclear enough it can generate heat

That's an extreme oversimplification. RTGs don't use nuclear waste. Spent reactor fuel still emits a large amount of gamma and neutron radiation, but not with enough intensity to be useful in a reactor. The amount of shielding required makes any kind of non-terrestrial application impossible.

The most common RTG fuel is plutonium (^238^Pu, usually as PuO~2~), which emits mostly alpha and beta particles, and can be used with minimal shielding. It can't be produced by reprocessing spent reactor fuel. In 2024, only Russia is manufacturing it. Polonium (^210^Po) is also an excellent fuel with a very high energy density, but it has a prohibitively short half-life of just over a hundred days. It also has to be manufactured and can't be extracted.

^90^Sr (strontium) can be extracted from nuclear fuel, and was used by early Soviet RTGs, but only terrestrially because the gamma emission requires heavy shielding. Strontium is also a very reactive alkaline metal. It isn't used as RTG fuel today.