Formation Behavior of Gaseous Iodine from Sodium Iodide under SFR Severe Accidental Condition

Abstract

Many experimental and analytical studies on the source term such as fission products released from nuclear fuels to the external environment have been carried out since the severe accident in Fukushima Daiichi Nuclear Power plant. Currently, source term research is being also focused in the field of advanced nuclear power plant such as sodium-cooled fast reactor (SFR). In SFR, sodium (Na) chemistry should be considered for source term evaluation because Na is used as coolant in terms of excellent heat transport capability. Iodine as one of important fission products is reactive with Na. Therefore, sodium iodide (NaI) as reaction product is formed in the reactor vessel under SFR severe accidental condition. If considering the transport of NaI from the reactor vessel to the containment vessel with air atmosphere, almost NaI is likely to behave as aerosol. However, it was reported that gaseous iodine as non-aerosol was formed from some NaI depending on the atmospheric conditions.

In this study, thermal behavior of NaI in different atmospheric conditions was investigated using thermal analysis technique to understand the formation behavior of gaseous iodine. Based on the results, it was revealed that the thermal behavior of NaI in inert atmosphere is phase change and approximately 10 mass% of I₂ could be formed from NaI in 20 vol% oxygen atmosphere. The chemical equilibrium simulation was also performed to understand the mechanism of I₂ formation. The estimated value of I₂ formation in 20 vol% oxygen atmosphere was comparable to the result of thermal analysis.