Study of the stability of iodine oxides (IxOy) aerosols in severe accident conditions

Iodine oxide aerosol are formed during a nuclear severe accident in the containment due to oxidation of volatile iodine either molecular iodine or organic iodide. These oxides may exist under different chemical forms more or less stable (I2O4, I4O9 and I2O5). Decomposition rate has to be experimentally studied to better estimate the iodine cycle and thus the source term. Iodine oxide aerosols decompose under radiation, whatever the surface deposition and lead to mainly form again gaseous molecular iodine. Decomposition rates under irradiation were determined at 80 °C and 120 °C in presence of humidity and kinetics is rapid. It also appears that IxOy aerosols are also not thermally stable and temperature effect promotes decomposition. The aerosols speciation seems to evolve in humid environment under irradiation towards less stable species. Such phenomena have to be considered in simulation severe accident software to model outside radioactive iodine releases and predict in the course of the accident the partition aerosol/gaseous iodine forms.