Formation of particles by radiolytical oxidation of organic iodine

During a severe nuclear power plant (NPP) accident, iodine is transported from damaged fuel rods through a primary circuit into the gas phase of the containment building. There iodine is exposed to air radiolysis products as well as to the ionizing radiation (such as alpha, beta, gamma and X-ray) itself. This may lead to oxidation of gaseous iodine and subsequent formation of iodine containing aerosol particles. Currently, the effect of radiation on the speciation of gaseous inorganic and organic iodine compounds is not well known. In the largescale Phébus FP tests one of the most surprising finding for iodine behaviour in containment was a steady state iodine concentration in the containment gas phase, which was reached in every test. While the processes leading to this are not known, several hypotheses have been formulated. One of the hypotheses was that gaseous iodine species released from the containment surfaces would be radiolytically destroyed to form fine particulate iodine oxides or iodine nitrogen oxides. The aim in this study was to find out experimentally, whether UV or beta radiation would be efficient in oxidizing gaseous methyl iodide (CH3I). In experiments gaseous methyl iodide was fed into the EXSI CONT facility in an air mixture [Kärkelä et al., 2010]. In some experiments the flow contained also humidity. The reactions took place in a quartz tube heated either to 50 °C, 90 °C or 120 °C. UV-light (including 185 nm wavelength) was used as a source of radiation to produce ozone from oxygen. A separate generator was also applied to reach higher ozone concentrations. Further studies on the radiolytical oxidation of CH3I in oxygen by beta radiation at 20 °C were carried with the BESSEL facility [Kärkelä et al., 2015]. As a result of CH3I experiments with EXSICONT, there was a clear trend in the formation of gaseous reaction product species. The main gaseous reaction products were methanol and formaldehyde. Especially at elevated temperature other reaction products, such as formic acid and methyl formiate, became important as well. Increasing amount of reaction product species were detected while the concentration of ozone was increased. Similarly, the mass concentration of aerosols increased as well, thus aerosol nucleation was enhanced. Increase in temperature seemed to increase also the aerosol mass concentration. This is probably partly due to more efficient decomposition of gaseous CH3I and subsequent aerosol formation by ozone. In the beta irradiation experiments with the BESSEL facility, it was found out that the concentration of formed particles (ca. 10 nm to 50 nm in diameter) decreased very slowly with increasing irradiation time, see Figure 1. It seemed that an equilibrium was reached between gas phase iodine compounds and iodine species deposited on wall surfaces. At that equilibrium the rate of new particle formation was low. When the facility was purged with oxygen, a new formation of particles was observed in every CH3I experiment. It suggested that the radiolysis reaction products were limiting the particle formation. Oxygen, a precursor of ozone when irradiated, was also needed for the nucleation to take place, since the new particle formation was not observed without irradiation or when the atmosphere was pure nitrogen. The formed particles were highly water soluble and volatile. These findings could also partially explain the constant concentration of iodine, which was observed in the gas phase of containment at the end of every Phébus FP test.