It's only fissile material that behaves this way, not all radioactive material. The supercritical state must be reached very quickly (with explosives) to get c

It's only fissile material that behaves this way, not all radioactive material. The supercritical state must be reached very quickly (with explosives) to get city-destroying power levels out of a few kilograms of plutonium. But getting enough power out of a few kg of supercritical plutonium to kill someone standing nearby is easy to do even accidentally.