Research on the transport and chemistry of fission products in primary circuit and containment conditions at VTT

The behaviour of fission products (FPs) during a severe nuclear power plant accident has been studied at VTT for the past 30 years. The focus has been on the transport and chemistry of gaseous compounds and aerosols in the primary circuit and the containment. In the past containment aerosols have been studied e.g. with intermediate scale facilities AHMED and VICTORIA. In primary circuit the studies have included resuspension and revaporisation phenomena as well as retention of FPs in steam generators. In recent years the main interest has been on ruthenium and iodine, because of their high radiotoxicity and possibility to form gaseous compounds. FPs deposited on primary circuit surfaces may be released back into the gas phase, even after a long time from the beginning of a severe accident. Therefore high temperature chemistry of iodine has been studied with EXSI-PC facility. It was found out that when using CsI as a precursor with Ag, B2O3 or MoO3 the release of gaseous iodine even at 400 °C was significant. Boron seemed to react with caesium forming a solid, glassy compound and thus the release of gaseous iodine was enhanced. The effect of chemical reactions on primary circuit surfaces on the transport of FPs is not well-considered in current severe accident codes. Gaseous iodine reacts with air radiolysis products, such as ozone, in the gas phase of containment. The radiolytical oxidation of elemental and organic iodine was investigated using EXSI-CONT facility. The formation of iodine oxide aerosol particles in air by UV(c) radiation was detected on-line in the experiments. The main gaseous reaction products from the radiolytical oxidation of CH3I were methanol and formaldehyde. Further studies on the oxidation by beta radiation with BESSEL facility verified the formation of iodine containing particles. During a severe accident, a part of the nucleated iodine oxide particles in the atmosphere will deposit on the various surfaces of containment. It was found that the desorption of iodine from the particles deposited on painted surface was enhanced by gamma radiation. The filtration of gaseous and particulate iodine has been investigated using a wet electrostatic precipitator (WESP) technique. The filtration efficiency of a modern WESP can be higher than 99.9 % for the particles. To trap gaseous iodine in this study, additional ozone is fed to the gas flow in order to oxidize all gaseous iodine to iodine oxide particles. The newly formed particles are mixed with a spray of water droplets. Inside the filtration unit of WESP, large droplets and particles are charged and driven to the collection electrode. The droplets are very efficient in trapping small particles, for which charging efficiency may otherwise be too low for effective filtration. As a result of the first filtration experiments, 95 % of iodine was filtered with the WESP. The remaining 5 % of iodine was transported through the filter in a gaseous form.