Iodine behaviour under LWR accident conditions: Lessons learnt from analyses of the first two Phebus FP tests

Thermochromatographic behavior of iodine in 316L stainless steel columns when evaporated from lead–bismuth eutectic

Iodine evaporated from lead–bismuth eutectic (LBE) has been examined with respect to its adsorption behavior on stainless steel in various gases to establish a base for safety evaluations on LBE based nuclear reactors. In inert conditions the iodine forms a single species with an adsorption enthalpy between − 97 and − 106 kJ/mol. The adsorbed species is tentatively identified as bismuth monoiodide, BiI. Addition of moisture to the inert gas has no substantial influence on the adsorption behaviour. For the reducing hydrogen carrier gas depositions with adsorption enthalpies ranging from − 87 to − 134 kJ/mol were observed in dry and water saturated conditions. The larger variation of adsorption enthalpies compared to analogous experiments in helium likely result from surface effects induced by the reactive gas. Formation of highly volatile species such as hydrogen iodide HI was not observed. In oxidizing conditions multiple iodine species with adsorption enthalpies ranging from − 67 to − 83 kJ/mol were observed, with the exception of one experiment where only a lower limit of –ΔHads < 64 kJ/mol could be determined due to high volatility. The species occurring in oxidizing atmosphere are most likely monatomic iodine, iodine oxides and hydroxides. While oxygen as a carrier gas changes the speciation of iodine to more volatile compounds, it also introduces a retentive effect on the evaporation of iodine from the LBE sample. These results provide important information that establish a better understanding of safety related aspects pertaining to iodine transport in an LBE reactor. The determined thermodynamic data can be used for safety assessments of LBE-based nuclear facilities in normal operation conditions as well as for accident scenarios.