During a hypothetical severe accident on a Pressurized Water Reactor (PWR), iodine is one of the most hazardous fission products that can be released from the nuclear fuel into the reactor containment and to some extent to the environment. The strong radiological impact of iodine on human health and environment, mostly through its 131I isotope, has made it a key point for the study of potential fission product release from a Nuclear Power Plants (NPP) in case of a severe accident.
The evolution of the volatile iodine concentration in the containment is determined by a balance between formation and deposition or destruction processes. Released from the fuel as a gaseous form and being cooled down in the Reactor Coolant System (RCS), iodine enters the containment in gaseous or particulate form and undergoes deposition processes that eventually trap it to the containment surfaces (mostly painted surfaces) and sump. In balance with these deposition processes, volatile iodine formation processes are involved in the medium and long term: organic iodides compounds can be produced from iodine (under molecular or aerosol forms) deposited on painted surfaces present in the containment atmosphere or in the sump and volatile inorganic iodine can be produced under radiation either from soluble iodine species dissolved in the sump  or from iodine deposited on painted surfaces.
The main formation and destruction routes were already identified ; however, uncertainties in quantifying some of these processes remain too large to make reliable iodine source term evaluations for various accident scenarios, in particular for organic iodide production and destruction from painted surfaces in contact with the containment atmosphere or in the sump. To complete the existing database, the extensive experimental program EPICUR has been performed by the French “Institut de Radioprotection et de Sûreté Nucléaire” in collaboration with many international partners to obtain data concerning iodine volatilization and particularly to provide on-line measurements of iodine volatilization during irradiation. Results obtained in EPICUR concerning molecular iodine volatilization from the sump have been presented in an earlier paper .
The present paper describes the study carried out to investigate the formation of organic iodides from painted surfaces of the containment under different conditions. The paper presents the EPICUR facility and gives an overview of the experiments performed, simulating severe accident conditions. Experiments were performed using painted coupon, representative of the NPP painted surfaces, loaded with labeled molecular iodine (131I). Gamma spectrometers were used to measure on-line and after the irradiation phase the quantity of molecular iodine and organic iodine volatilized from the painted coupon under radiation. The impact of the initial concentration of iodine fixed on the painted coupon, of the irradiation temperature and of the relative humidity (RH) of the irradiation atmosphere was investigated. The results are presented and discussed. These experimental results will be used to validate the IODE module of the ASTEC code developed by IRSN in collaboration with GRS and used in water reactor safety analysis.
1. Wren, J.C., J.M. Ball, and G.A. Glowa, Chemistry of iodine in containment. Nuclear Technology, 129(3), pp. 297-325, 2000.
2. Guilbert S., et al. Radiolytic oxidation of iodine in the containment at high temperature and dose rate. Proceedings of the Nuclear Energy for New Europe, Portoroz, Slovenia, Sept 10-13 2007.