A DFT study of Iodine interaction with nuclear reactor cooling system surfaces under severe accident conditions
Résumé
A severe nuclear accident can lead to releases of radiotoxic iodine compounds either under aerosol form (e.g. metal iodides or iodine oxides) or under gas form (e.g. organic iodide as CH3I or inorganic as I2) species. 131I is particularly dangerous due to its possible absorption in human body and more particularly on thyroid. Gaseous iodine, mainly formed in the nuclear containment building, is dispersed in case of outside releases and may contribute, at short term, at near-distance contamination. Metallic iodide species are mainly formed at high temperature and a part is condensed on reactor coolant system (RCS) walls, the remaining forms aerosols which are either deposited on RCS or transported to the containment building. In our work, we study theoretically the adsorption metallic iodides on the surface of the primary circuit that is composed mainly by Fe and Cr oxides in severe accident conditions. At high coverage, AgI and CdI2 form networks on the surfaces of the RCS while at low coverage the molecules are isolated. Starting from the stable adsorbates, the chemical mechanisms leading to the iodine re-vaporization were investigated. The formation of I2(g) is thermodynamically and kinetically possible, from over-oxidized chromium oxide surfaces from either adsorbed AgI or CdI2. On the other alternative surfaces, the co-adsorption of an oxidant, OH● coming from the radiolysis of steam, is mandatory to form I2(g). These works tends to show that possible delayed releases of gaseous iodine can happen from aerosols iodide deposited.
Domaines
Chimie
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