Experimental study of the boron and air effects on iodine transport in the primary circuit during a severe nuclear accident

In case of a severe accident in a nuclear reactor, radioactive iodine may be released into the environment, impacting significantly on the source term. Determination of the amount released, and of the physical state of iodine (gaseous or solid aerosol form), is thus a major issue, regarding the improvement of the accident management and mitigation measures An experimental study has been launched at VTT to investigate the behaviour of iodine containing deposits on primary circuit surfaces during a severe nuclear accident. Caesium iodide was used as a precursor material in order to study the effects of carrier gas composition (varying H₂/H₂O molar ratio) on the release of gaseous iodine from the precursor. A special interest was on the behaviour under presence of air, and the effect of CsI mixed with boron and/or caesium hydroxide was also examined At first, the influence of boron on the caesium iodide behaviour at 650°C under three atmospheres: Ar/H₂O, Ar/H₂O/H₂ and Ar/Air was investigated. The experiments showed that the transport of gaseous iodine is enhanced by the presence of boron, because of the formation of a glassy caesium borate compound in the crucible, suggesting that a reaction between condensed caesium iodide and boron is possible on the surface of primary circuit in these conditions. The amount of steam present in the carrier gas, more than air, seems to have an influence on the caesium borate formation Secondly, experiments were performed with a precursor mixture of CsI, CsOH and B₂O₃. The aim was to have an initial Cs/I molar ratio higher than one, which is more representative to the conditions in case of the severe nuclear accident. Preliminary results have shown that the amount of gaseous iodine is higher than the iodine containing particles. Presence of boron still tends to favour the formation of gaseous iodine.