Impact of iodide ions on the speciation of radiolytic transients in molten LiCl-KCl eutectic salt mixtures

The fate of fission-product iodine is critical for the deployment of next generation molten salt reactor technologies, owing to its volatility and biological impacts if it were to be released into the environment. To date, little is known on how ionizing radiation fields influence the redox chemistry, speciation, and transport of iodine in high temperature molten salts. Here we employ picosecond electron pulse irradiation techniques to elucidate for the first time the impact of iodide ions (I-) on the speciation and chemical kinetics of the primary radiation-induced transient radicals generated in molten chloride salt mixtures (e(S)(-) and Cl-2(-)) as a function of temperature (400-700 degrees C). In the presence of I- ions (>= 1 wt% KI in LiCl-KCl eutectic), we find that the transient spectrum following the electron pulse is composed of at least three overlapping species: the e(S)(-) and the Cl-2(-) and ICl- radical anions, for which a deconvoluted spectrum of the latter is reported here for the first time in molten salts. This new transient spectrum was consistent with gas phase density functional theory calculations. The lifetime of the e(S)(-) was unaffected by the addition of I- ions. The newly observed interhalogen radical anion, ICl-, exhibited a lifetime on the order of microseconds over the investigated temperature range. The associated chemical kinetics indicate that the predominate mechanism of ICl- decay is via reaction with the Cl-2(-) radical anion. The iodine containing product of this reaction is expected to be ICl2-, which will have implications for the transport of fission-product iodine in MSR technologies.

Automated summary

The impact of iodide ions on the speciation and chemical kinetics of radiation-induced transient radicals in high temperature molten salts was studied using picosecond electron pulse irradiation techniques. It was found that the presence of iodide ions resulted in the formation of at least three overlapping species: e(S)(-), Cl-2(-), and ICl-. The newly observed interhalogen radical anion, ICl-, exhibited a lifetime on the order of microseconds over the investigated temperature range, and its decay mechanism was determined to be reaction with the Cl-2(-) radical anion. These findings have implications for the transport of fission-product iodine in molten salt reactor technologies.