Loss of ability to cool the fuel in a water-cooled nuclear reactor will lead eventually to overheating of the reactor fuel, fuel cladding rupture and release of fission products. These fission products can migrate through the reactor coolant system (RCS) into the reactor containment and perhaps leak into the environment where they pose a threat to the public health and safety. Iodine was one of the major fission product elements released from the damaged reactors during the accident at the Fukushima Daichi nuclear power plant in 2011. Iodine has a complex chemistry that determines its chemical form and, consequently, its volatility in the containment. Although iodine is expected to be released from the RCS mainly as iodide aerosols, radiolytic reactions occurring in containment water pools or on surfaces can lead to the release of volatile species into the containment atmosphere. These in turn will be subject to radiolytic reactions in the atmosphere, resulting in the conversion of the gaseous species into involatile iodine oxides, which may deposit on surfaces or re-dissolve in water pools. The concentration of airborne iodine in the containment will, therefore, be determined by the balance between the reactions contributing to the formation and destruction of volatile species, as well as by the physico-chemical properties of the iodine oxide aerosols which will influence their longevity in the atmosphere. This paper summarises work that has been reported elsewhere on iodine chemistry in international programmes including Phebus-FP, EC SARNET (Severe Accident Research Network) and the OECD BIP projects, from which a new paradigm has emerged for describing the behaviour of iodine in a post-accident reactor containment.
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