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 . 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 .