Experimental study of the cadmium effects on iodine transport in the primary circuit during severe nuclear accident

In case of a severe accident in a light water reactor, iodine is one of the most important fission products in the context of reactor safety because of its significant total fuel inventory, high specific activity and radiotoxicity. Consequently, understanding its behavior under severe accident transient conditions is a major point in the optimization of the accident management and mitigation. An experimental study has been launched at VTT investigating the behavior of iodine on primary circuit surfaces during a severe nuclear accident. The paper presents results obtained from the heating of metallic cadmium and caesium iodide in a crucible at temperature below 400°C under three different atmospheres. Aerosols and gaseous species released from the reaction crucible were sampled at 150°C on filters and liquid scrubbers and analyzed with HR-ICP-MS. At first, experiments were conducted with one precursor (cadmium or caesium iodide). It was observed that cadmium is predominantly present as aerosol in all atmospheres but represents less than 1% of mass what has been recovered from the facility leaching. To the contrary, caesium iodide experiment showed that sampled iodine exists mainly as gaseous species. By comparison to one-precursor studies, a change in the behavior of element was noticed when cadmium was added to caesium iodide. Different observations let to suggest a reaction between caesium iodide and cadmium in the crucible. The reaction would lead to the formation of compound made of cadmium and iodine in condensed phase below 150°C. More gaseous iodine was found from the sampling line with Ar/H2O/H2 atmosphere than in Ar/H2O atmosphere, suggesting that the cadmium is more reactive toward iodine in hydrogen-free atmosphere.