Iodine behaviour in the gas phase

The key reactant, the hydroxyl radical, OH, is formed by the radiolysis of water vapour. Concentrations of this key reactant are much lower in the gas phase than in die aqueous phase because of the higher density and higher radiation adsorption of water. Consequently, gas phase formation of organic iodides from molecular iodine is not usually an important process relative to the formation of organic iodides in the aqueous phase and partitioning of these iodides into the atmosphere. 

Both molecular iodine and organic iodides are susceptible to thermal and radiolytic decomposition in the containment atmosphere. Of particular interest is the reaction of iodine species with ozone formed by radiolytic processes in the atmosphere  

The adsorption of gaseous iodine species on surfaces has received much attention in the reactor safety literature. Varieties of metal, concrete and painted surfaces are in a reactor containment building. The gaseous forms of iodine will interact with these surfaces. The interactions are usually thought to involve a rapid physical adsorption followed by a slower chemical reaction. Dry (or moist, but not wet) surfaces of both stainless steel and organic (vinyl-, epoxy-, polyurethane-) paint have large capacities for absorbing molecular iodine. The adsorption rate of molecular iodine on the dry surfaces is close to the limit dictated by gas-phase mass transport. Intermediate-scale studies performed in the Radioiodine Test Facility have shown that about 75 to 95% of airborne molecular 

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