Formation, partitioning and interactions of organic iodides under severe nuclear accident conditions

Licentiate thesis, 2012

During severe nuclear reactor accidents such as Chernobyl or Fukushima large amounts of radioactive iodine can be released in form of inorganic and organic iodine species into the environment. Volatile radioactive iodine species are harmful to human health when inhaled. The radioactive iodine species are concentrated in the thyroid gland where their beta particles may disturb the function to regulate the metabolism and may even induce cancer. Significant amounts of volatile organic iodides can be formed from released iodine and organic molecules which are released from paint coatings (epoxy paint top coatings, zinc primer) applied on containment surfaces, as well as from cable insulation materials. These reactions are caused by the pyrolysis and radiolysis during a severe accident scenario in light water reactors (LWR). In this work the formation and behaviour of organic iodides was investigated together with the formation of volatile and semivolatile organic substances from paints. Methyl iodide, the most feasible volatile alkyl iodide in the containment, is distributed between the water pools and the containment gas phase in such a way that significant amounts remain in the gaseous phase for potential release into the environment. Some countries like Sweden use wet-scrubber filter to reduce potential iodine release. The currently used wet-scrubber solutions are very efficient for removing elemental iodine but assumed to be less reactive towards organic iodides, such as methyl iodide. Thus these highly volatile short-chained organic iodides are likely to be released. Alternatively the release of iodine and organic iodides from the containment in case of a containment failure could be limited by sorption of those species on paint films. To be able to improve current used filter systems and paint types the reactivity of the relevant iodine species with the paint ingredients needs to be understood. Paint ingredients, such as solvents, of different paint types, with main focus on Teknopox Aqua VA paint used at Ringhals 2 nuclear power plant in Sweden, were identified and their reactions with iodine were studied. It was shown that the composition of different paint types varies significantly both qualitative and quantitative. Paints have a complex, heterogeneous chemical matrix that changes its properties with time, temperature, humidity and under the influence of irradiation. The impact on the volatile source term of iodine of the same type of paint was shown to be site and paint film age specific. Thus, freshly painted paint coatings will have a different contribution on the volatile, organic iodides chemistry during a severe nuclear accident than a paint which was for longer time exposed to reactor conditions or which is of different type.