Experiments on the release of gaseous iodine from gamma-irradiated aqueous CsI solution and influence of oxygen and methyl isobutyl ketone (MIBK)

Abstract

Volatile iodine production due to radiation chemical effects is known to be an important uncertainty source in the evaluation of the source term for severe accidents of light water reactors (LWRs). The gaseous release fractions of molecular iodine and organic iodine from gamma-irradiated 10-4 mol/L cesium iodide aqueous solution were measured with the dose rate ̃ 7 kGy/h at room temperature. The solution was buffered with 0.1 mol/L boric acid and sodium hydroxide (pH ̃7) and contained up to 10-3 mol/L methyl isobutyl ketone (MIBK) as an organic additive. The concentrations of MIBK in the solution and oxygen in the sweep gas were changed as experimental parameters. The total iodine release fraction of the original aqueous inventory and the fraction released as organic iodine were 2-47 and 0.02-1.5%, respectively, at the end of 2 h of irradiation. They were dependent on both the aqueous MIBK concentration and oxygen concentration in the sweep gas. Under a constant cover gas condition, the total iodine release showed a decreasing trend and the organic iodine release showed an increasing trend when the MIBK concentration increased. This behavior can be explained by the branching of the reaction path of radiolytic degradation of ketones depending on the availability of dissolved oxygen, and competition between iodine and organic compounds on the consumption of radicals produced by water radiolysis. © Atomic Energy Society of Japan.