Consequences of 131I Transmutation in Gas Phase Radioiodine Molecules and Adsorbed on Graphite Surface

The release of the radioisotope I-131 represents a major health concern because of its absorption by the thyroid gland. Capture materials, such as activated carbon, provide a means to efficiently mitigate I-131 emissions during nuclear facility operations. In this study we use ab initio molecular dynamic calculations to investigate the effects of I-131 radioactive decay on the molecular stability of I-2, acid iodine (HI, HIO, HIO2, and HIO3), and methyl iodide species in both the gas phase and adsorbed on graphite. Gas phase calculations show that I-2 is the least resistant to radiolytic decomposition, followed by the HIOx family of molecules, then HI and CH3I which are the most resistant. However, the graphite surface is found to greatly enhance the fragmentation of adsorbed molecular species. While the identification of gas phase fragments can offer new pathways for chemical kinetics models, the fragmentation products formed at the graphite surface can react with surface carbon atoms. Over time, this could reduce the number of adsorption sites available for iodine capture and degrade the abatement capability.