These aren't nuclear reactors. This isn't a miniaturized Chernobyl in your pocket, a delicate balance between control rods and radioactive material maintaining the right proximity to critical mass. This is a rock that's (metaphorically) kinda warm and takes some decades to become noticably "cooler" and we can use that temperature differential to power tiny devices (and we have used these devices continuously since the 70s)

The thing that nuclear reactors (and weapons) use is the chain reaction of certain radioactive materials whereby the decay of atoms can be triggered by the decay of a previous atom. That is not the mechanism nuclear batteries use and they in fact are incapable of that chain reaction; the radioactive materials that nuclear batteries are made of don't work the same way as the special isotopes of uranium or plutonium.