Iodine behaviour during a severe accident in a nuclear power plant

Iodine is a main fission product of the fuel generated during power operation in nuclear power plants (NPPs). A severe NPP accident can cause fuel, containing fission products (FPs), control rod assemblies and core structures to melt and then release into the containment. The key signatures of iodine behaviour during a NPP severe accident are reviewed, i.e. its release from the molten core in the reactor pressure vessel and transfer via the primary coolant system into the containment. Containment or filter failure would disperse radioactive iodine into the environment. The iodine radioisotopes, 131I (T1/2 ca. 8 d) in particular, in sufficient concentrations can pose a health hazard. The overview focuses on the current state of knowledge on iodine behaviour in containment. The formation of volatile iodine species (molecular iodine and organic iodides) by FP-radiation and recent efforts at PSI to suppress their release by reduction to non-volatile iodide for engineered systems are described. Despite extensively available data, further experimental studies are needed to reduce the significant uncertainties in model development. In particular, additional studies on organic iodide formation are required, since these species form the major volatile fraction under certain conditions. Gas phase reactions and the potential effects of many other sump constituents are also important. (author)