We do use it - in nuclear weapons and certain kinds of solid-state power devices. Why not in reactors? Because they rely on a certain kind of subatomic process

We do use it - in nuclear weapons and certain kinds of solid-state power devices. Why not in reactors? Because they rely on a certain kind of subatomic process (nuclear fission) to produce energy that can only happen once in the controlled environment of a nuclear reactor. But you idea that "being more radioactive" makes a better nuclear fuel is wrong. A good nuclear fuel is one that produces lots of energy for little input energy, is easy to mine and process, can be controlled easily, and can be disposed of safely.