Nuclear reactors can't generally be scaled down too far. The shielding requirements are about 4 ft of concrete on all sides, basically as a minimum. You can use

Nuclear reactors can't generally be scaled down too far. The shielding requirements are about 4 ft of concrete on all sides, basically as a minimum. You can use pure-alpha decay radioactive isotopes as long lasting heat sources with a thermionic to make a 100W battery that last ~100 years. These are used in space probes like Voyager 1/2, Cassini, and Mars rovers like Curiosity and (soon) Perseverance. It used to also be used in pacemakers and soviet arctic light-houses. The material is expensive (though it could be made much cheaper if a big market showed up), but more seriously it's a radiological hazard. Someone could disassemble such a thing and blow it up on a busy street with conventional explosives and cause a massive radiophobia-induced panic. You don't want to inhale a strong alpha-emitter.