Original: Protactinium-231 arises from the decay of uranium-235 formed in nuclear reactors, and by the reaction 232Th + n â '231Th + 2n and subsequent beta decay. It may support a nuclear chain reaction, which could in principle be used to build nuclear weapons.
Simplified: Protactinium-231 (which is made by the alpha decay of Uranium-235 followed by beta decay of Thorium-231) could possibly sustain a nuclear chain reaction and might be used to build a nuclear weapon.