https://github.com/davepinch/pinchy.cc/blob/master/content/topics/chemistry/compounds/potassium-iodide/remm.hhs.gov.md
Nuclear bomb detonation produces a small amount of local I-131 fallout. Most I-131 distributes over large distances, typically with only 10% making its way to the earth’s surface before undergoing spontaneous radioactive decay to stable xenon-131.