I know that this is a joke, but reactors cannot scale down that far. Unlike an electrical source powered by radioactive decay (like certain pacemakers [1]), the minimum mass for a nuclear reactor includes several kilograms of material. It's possible to shed neutron moderator mass by using high-purity fuel operating on fast fission, but these require more fuel to go critical so they still have multi-kilogram masses.

[1] "Plutonium powered pacemakers (1974) https://www.orau.org/health-physics-museum/collection/miscel...