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 on a nuclear reactor, radioactive iodine may be released into the environment, impacting significantly 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. An experimental study has been launched at VTT investigating the behaviour of iodine on primary circuit surfaces during a severe nuclear accident. From the caesium iodide, used as precursor material, studies were conducted in order to highlight the effects of carrier gas composition (varying H₂/H₂O molar ratio), especially the behaviour under presence of air, and the presence of boron and/or caesium hydroxide. The study investigated, first, the influence of boron presence on the caesium iodide behaviour at 650°C under three atmospheres: Ar/H₂O, Ar/H₂O/H₂ and Ar/Air. 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 at the surface of primary circuit in these conditions. The amount of steam present in the carrier gas, more than the air, seems to have an influence on the caesium borate formation. Then, experiments were performed with a mixture of CsI, CsOH and B₂O₃, in the aim to have an initial molar Cs/I ratio higher than 1. Preliminary results have shown that the amount of gaseous iodine is higher than the iodine particles. Presence of boron still tends to favour the formation of gaseous iodine however; the competitive reaction with caesium hydroxide may result in the formation of a diffusional barrier that prevents contacts between boric acid and the caesium iodide.