Interactions of iodine and methyl iodide with reactive metals
In the event of a severe nuclear reactor accident, iodine will be released from the fuel into the containment building. If released to the environment, iodine represents a biological hazard due to the concentration in the thyroid gland. Being a reactive element, iodine will undergo reactions with water, oxygen, organic material, surfaces etc. present in the containment. Large amounts of reactive metals like copper, zinc and aluminium present in a boiling water reactor are likely to react with iodine species in an accident situation. Experiments within this work have studied reactions of iodine and methyl iodide with these three metals under accident conditions.
Copper, zinc and aluminium reacted similarly with gaseous iodine under humid conditions, with reaction rates of the same magnitude. The adsorption of iodine on theses surfaces could be divided into two steps; fast initial adsorption during less than one hour followed by slower adsorption during the rest of the experiment. Only the mass transfer of gaseous iodine to the metal surface limited the initial adsorption while the subsequent adsorption was limited by the diffusion of iodine through the metal iodide layer built up on the metal surface. Adsorption rates increased with temperature.
Experiments with iodine and metals in water showed that copper only continuously adsorbed iodine on the surface. The solubility of zinc iodide and aluminium iodide is too high to allow an increasing metal iodide concentration on the surface.
Zinc and aluminium only showed a minor uptake of methyl iodide on the surface for temperatures up to 80C. The uptake of methyl iodide on copper was low at 25 and 50C but continuously increased at 80C.