Radioactive iodine1 is a byproduct of the fission of uranium atoms. Two processes that lead to the creation of radioiodine are the fission of uranium as fuel in nuclear reactors and its use as an explosive material in atomic bombs. Under normal circumstances, minimal radioiodine is released to the environment from operating nuclear reactors, whether such reactors are operated for power production, for production of radioisotopes for use in medical diagnosis or treatment, or in materials testing or teaching. In the case of nuclear power plants (NPPs), the uranium fuel is contained in sealed metal tubes (fuel rods), the fuel rods are placed inside an eight inch thick steel reactor vessel, the reactor vessel is contained inside a thick (several feet), reinforced-concrete reactor building. In addition, all aspects of the reactor operation are carefully monitored with sensitive instruments, and highly effective filtration systems are used to remove radioiodine from air or water released from the reactor facility. For the smaller, teaching-type, reactors, many of these same safety features are used. The quantities of radioactive iodine normally found at locations related to medical use are small enough to be unlikely to be used in radioactive dispersal devices (so-called dirty bombs).
Before radioactive iodine from the nuclear fuel of a NPP can reach the environment, extensive damage must occur to the fuel elements in the reactor core with additional damage to the containment structure enclosing the reactor. Radioactive iodine (131I) has a short half-life (8 days) and is of concern only with respect to the fuel in an operating NPP or with fuel from a reactor core that has recently been shut down. Spent nuclear fuel stored at NPPs is free of significant radioiodine within weeks after shutdown of the reactor and cessation of the fissioning process that produces the radioiodine. Spent nuclear fuel that is stored outside a nuclear-reactor building is old fuel that has been kept so long that it contains minimal quantities of short lived radioiodines. Spent nuclear fuel in transportation does