Experimental study on the reactions between cadmium and caesium iodide on the surface of primary circuit piping during a severe nuclear Accident

In case of a severe accident in a pressurized water reactor (PWR), iodine is one of the most important fission products in the context of reactor safety because of its significant total fuel inventory, short half-life and radiotoxicity. Consequently, understanding its behaviour under severe accident transient conditions is a major point in the improvement of the accident management and mitigation measures. An experimental study has been launched at VTT Technical Research Centre of Finland to investigate the behaviour of iodine on primary circuit surfaces during a severe nuclear accident. The paper presents results obtained from the heating of metallic cadmium pellets and caesium iodide powder in a crucible 650°C under two different atmospheres (Ar/H2O and Ar/H2O/H2). Aerosols and gaseous species released from the reaction crucible were collected on filters and trapped in liquid scrubbers respectively and analysed with XRD and ICP-MS. The first experiments were conducted as blank tests to have a sort of baseline for the cadmium and caesium/iodine behaviour in the same conditions. In the following experiments, caesium iodide and cadmium were both added in the crucible. While it was expected that cadmium reacts with iodine forming cadmium iodide, this compound was not detected in the post-test analyses. However, it was shown that the transport of cadmium was enhanced by the presence of caesium iodide, by condensing on caesium iodide particles, particularly in Ar/H2O/H2 atmosphere.