In case of a severe accident in Light Water Reactor (LWR), radioactive iodine may be released into the environment, impacting significantly to the source term. Determination of the amount released, and of the physical state of iodine (gaseous form or solid aerosol form), is thus a major issue, regarding the improvement of the accident management and mitigation measures The experimental EXS1-PC facility has been specifically designed and built to investigate the behaviour of iodine containing fission product deposits on primary circuit surfaces during a severe nuclear accident. Studies were conducted with two mixtures of caesium iodide and molybdenum oxide (Mo/Cs=1.6 and Mo/Cs=5) in order to assess the possible chemical reactions and the effect on the transport of chemical species through the primary circuit. In addition, two carner gas compositions (Ar/H_2O versus Ar/Air) were studied to highlight the effect of oxygen partial pressure. In this work, the influence of molybdenum presence on the caesium iodide behaviour under two atmospheres: Ar/H_2O and Ar/Air (86.7/13.3 vol.%) was studied. The release of gaseous iodine was higher when the oxygen partial pressure was higher (i.e. for Ar/Air atmosphere). In addition, the results showed that an initial Mo/Cs molar ratio of 1.6 produced about 1.5 times higher amount of gaseous iodine than a ratio of 5. The formation of caesium molybdates was identified in the crucible after the experiments, confirming that the reaction between caesium and molybdenum is the reason for the observed formation of gaseous iodine. The experimental results are mostly in accordance with the equilibrium calculations performed with FactSage.
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