The chemistry of organic iodides under severe nuclear accident conditions in LWRs

Sammanfattning: For many countries nuclear fission is an important part of their energy portfolio. However, a great concern is the occurrence of nuclear reactor accidents, which can result in the emission of radioactivity into the environment. It has been proven in the past that the release of radioactivity can have environmental and health effects both locally, as well as in more distant locations, including those in other countries. As a result, severe nuclear accidents are of interest even to those countries that do not use nuclear fission as an energy source. One of the most important volatile fission products is iodine. In the event of a severe nuclear reactor accident iodine can be released from nuclear fuel and enter the containment. Iodine can be in different inorganic and organic chemical forms. The volatility of some iodine species is of importance as this enables the iodine to escape from a nuclear power plant and result in exposure of the general public. Radioiodine is re-concentrated in the small but vital thyroid gland and can induce cancer. While inorganic forms of iodine can be retained and their release into the environment to a large extent be prevented, organic iodides cannot be retained with the same efficiency by the filter systems used today. In this work the processes by which organic iodides can be formed inside the containment were studied. To this end methods were developed to produce I-131 labeled organic iodides that were used in the experiments. The formation of alkyl iodides other than methyl iodide, such as ethyl-, isopropyl-, butyl- and benzyl iodide from organic materials present in reactor containments was observed. It was found that organic materials can act as sources of organic iodides, but also consume them. The ability of a paint to retain and to generate organic iodides was found to decrease with thermal ageing of the paint. The iodine chemistry in paint films was investigated and organic iodides were found to bind to hydroxyl groups to form alkyl ethers with the iodine leaving group not being chemically bonded. A modified epoxy paint formulation was developed that can capture and retain more iodine than the bisphenol-A based epoxy paint Teknopox Aqua VATM that is currently used in Sweden. The removal of organic iodides from gas streams using solid absorbers was investigated but found to be not sufficient. It was found that trialkyl phosphines are particularly able to capture gaseous organic bonded iodine in experiments simulating the conditions in a Venturi scrubber.