Gas Phase Oxidation of Elemental Iodine in Containment Conditions Available to Purchase

The aim of the study was to verify the possible formation of iodine aerosol when gaseous iodine is exposed to radiation. The effect of oxygen, ozone and iodine concentration as well as that of radiation intensity on IO_X aerosol formation was determined. Experimental evidence on kinetics of particle formation, growth and transport was gained. Experiments were carried out using EXSI facility at VTT. Gaseous iodine was produced from I₂-KI-water solution and carried into the facility with N₂-O₂ mixture. Oxygen concentration was varied between 2–50 %-vol. Due to the production technique flow contained always some humidity. UVC lamps were used as source of ionizing radiation. Ozone concentration was also varied with an ozonator. The reactions took place in a flow furnace heated to 120°C. Reaction and aerosol kinetics were studied by varying the residence time between about 2–7 seconds. The formed particles were collected on filters, after which gaseous iodine was trapped in NaOH-water solution. The amount of iodine in filters and in trapping bottles was analysed with ICP-MS. Particle concentration and size distribution were measured online using aerosol instrumentation. Ozone concentration was measured online with FTIR. The morphology as well as the elemental composition of the particles were analysed with SEM-EDX. The conversion of gaseous iodine to aerosol particles was rapid and almost complete even with low ozone concentration. The fraction of gaseous iodine increased though with decreasing residence time. Diameter of the formed primary particles was approximately 10 nm. Particles grew by agglomeration and by surface reaction. Agglomerate size distribution was log-normal with NMD varying between 60–120 nm. Size of the agglomerates increased and number concentration decreased with increasing residence time. Agglomerate size also increased with iodine concentration. Presence of ozone promoted retention of iodine in the facility probably by surface reaction. Some iodine deposited on surfaces evaporated later and formed particles in the gas stream. According to SEM-EDX analysis, particles deposited on carbon foil contained iodine and oxygen. However, the exact speciation is not known. Particles deposited on copper grids had probably reacted to copper iodide.