Modeling of the decomposition of iodine oxides aerosols (IOx) in the containment - Consequences on the understanding of volatile iodine behaviour in the containment

Iodine oxides aerosols (IOx) are fine aerosol particles that are formed in a reactor containment in case of severe accident. Their stability was studied under irradiation in representative conditions of the containment. Under the influence of the temperature, humidity and irradiation they are found to be unstable. Their decomposition kinetics and stability are studied in EPICUR facility thanks to the STEM, STEM2 and MIRE projects. An IOx decomposition model is set up in ASTEC-SOPHAEROS code (version 3.1). It considers the influence of the humidity, the temperature, and dose rate. Its application to the PHEBUS FPT-0/1/2/3 tests lead to (1) an improved modeling and understanding of the inorganic iodine volatility in the FPT-3 containment all along the transient and to (2) an improved modeling after the washing when the gaseous temperature increases (FPT-0 and FPT-1) and (3) before the washing when the humidity decreases (FPT-2) as long as a relevant molar gaseous iodine fraction coming (xI2_RCS) from the reactor coolant system (RCS) is set up for FPT-0/1/2. A sensitivity analysis indicates that values of xI2_RCS up to ≈ 30% would be needed to improve the modeling of iodine volatility before the washing for FPT-0/2. A fraction of 10% would be more appropriate for FPT-1. A complementary approach is on-going to quantify the influence of the uncertainties of each chemical model developed in ASTEC-SOPHAEROS code within the objective to identify which chemical phenomena are the most influent on iodine volatility in the PHEBUS containment. [Display omitted] •An IOx aerosol decomposition model has been set up in ASTEC severe accident code•It leads to a better modeling of the PHEBUS-FPT iodine volatility tests over time•The effect of the elliptic floor washing on iodine volatility is better modeled and understood•Compatible values of xI2_RCS reaching PHEBUS FPT-1,2,3 containment can be up to 30%•IOx decomposition can contribute significantly to the iodine source term