When we say a spacecraft is nuclear-powered, we're not actually referring to a reactor (with rare exceptions[1]), but instead to something called a Radioisotope Thermal Generator[2], or RTG for short.
Basically, an RTG contains radioactive material (material that undergoes constant, passive radioactive decay). This constant decay produces heat, which is then used to generate electricity via thermocouples.
There's no feedback loop or active control needed, at its simplest an RTG is a solid hunk of metal with no moving parts.
There isn't physically enough of the radioactive material, let alone the precise materials, pressures, and other things needed for a proper criticality / runaway fission effect like an atomic bomb. It's actually much harder to make one than you think ;)
