A Separate Effect Study of the Influence of Molybdenum on Iodine and Caesium Transport in the Primary Circuit in Nuclear Severe Accident Conditions

In case of a severe accident on a Light Water Reactor (LWR), iodine may be released into the environment, impacting significantly the source term. The understanding of the iodine release from the damaged reactor core and its transport in the different parts of the reactor up to the reactor containment, is thus a major issue. The analysis of the Phébus-FP tests brought the hypothesis that iodine keeps a gaseous form up to the containment due to some processes that limit the formation of caesium iodide in the reactor coolant system (RCS). The formation of caesium iodide, which is assumed to be the dominant form of iodine in the RCS, would be notably limited due to the presence of other elements reacting with caesium. An experimental study has been launched at VTT investigating the behaviour of iodine on primary circuit surfaces during a severe nuclear accident. Caesium iodide and molybdenum trioxide were used as a nonradioactive precursor materials in order to highlight the effects of carrier gas composition and oxygen partial pressure on the chemistry and transport. Aerosols and gaseous species released from the reaction crucible were sampled at 150°C on filters and liquid scrubbers and analysed with High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS). Aerosol number size distributions were measured with Scanning Mobility Particle Sizer (SMPS), with series 3080 platform, series 3081 Differential Mobility Analyzer (DMA) and series 3775 Condensation Particle Counter (CPC) as well as with Electric Low Pressure Impactor (ELPI). The study investigated, first, the influence of molybdenum presence on the caesium iodide behaviour under two atmospheres: Ar/H2O and Ar/Air. Experiments showed that the transport of gaseous iodine was enhanced by the presence of molybdenum, explained by the formation of a caesium molybdates in the crucible. This suggests that a reaction between condensed caesium iodide and molybdenum is possible at the surface of primary circuit in these conditions. In addition, the oxygen partial pressure prevailing in the studied conditions was determined as an influential parameter in the reaction. The effect of molybdenum on the release of gaseous iodine from CsI-MoO3 precursor mixtures with different Mo/Cs molar ratios and atmosphere compositions was investigated.