Uncertainty quantification study on gas phase iodine release from Fukushima Daiichi accident in unit 3

Abstract

A significant amount of radioactive iodine was release to the environment during the accident in the Fukushima Daiichi nuclear power plant. In this numerical study, we used the IMPAIR 2.2 code to simulate the iodine release during containment venting from the suppression chamber gas space and drywell leakage from unit 3 of the power plant. The IMPAIR code was coupled to the Dakota software and uncertainty quantification study and sensitivity analysis of the simulations was performed. The results indicated that both molecular iodine and organic methyl iodide were formed in the water volumes of the plant, where it was transferred to the gas phase of the modeled structures. The sensitivity analysis indicated that the water pH was the most sensitive parameter on the gaseous iodine formation and release during the venting or leakage periods. Additionally, the water pH was shown to affect the speciation of the gas phase iodine. Overall, the simulations performed in this work showed that iodine volatilized from the contaminated water phase in the suppression chamber and the drywell of Fukushima Daiichi unit 3 have an important contribution to the iodine source term to the environment.