A DFT study of iodine interaction with nuclear reactor cooling system surfaces under severe accident conditions

Abstract

A severe nuclear accident can lead to the release of radiotoxic iodine compounds in either aerosol form (e.g. metal iodides or iodine oxides) or gaseous form (e.g. organic iodide as CH3I or inorganic as I2) species. 131I is particularly dangerous because of its possible absorption by the human body especially by the thyroid. Gaseous iodine is mainly formed in the nuclear containment building, is dispersed in the case of outside releases and may contribute in short term to long-distance contamination. Metallic iodide species are mainly formed at high temperature and partly condensed on the walls of the reactor coolant system (RCS), the rest being either deposited on the RCS or transported to the containment building. In this paper, we study theoretically, in severe accidental conditions, the adsorption of the metallic iodides on the surface of the primary circuit which is composed of Fe or Cr oxides. At high coverage, AgI and CdI2 form networks on the surfaces of the RCS whereas at low coverage the molecules are isolated. This study, setting out from the stable adsorbates, investigates the chemical mechanisms leading to the iodine re-vaporization. The formation of I2(g) from adsorbed AgI or CdI2 is thermodynamically and kinetically possible on over-oxidized chromium surfaces. On alternative surfaces, the co-adsorption of an oxidant, OH● issued from the steam radiolysis, is necessary to form I2(g). This study tends to show that delayed releases of gaseous iodine are likely to happen from the deposited iodide.