Exploring the iodine adsorption and deposition behavior: An out-of-pile experiment on accident conditions of lead-bismuth fast reactor

Iodine is the primary fission product of the lead-cooled fast reactor (LFR) and is often used as an early environmental indicator for severe accidents. Understanding radioactive iodine behavior is crucial for effective radiation hazard control under accident conditions. In this study, the adsorption and deposition mechanisms of iodine (evaporated from lead-bismuth eutectic) onto fused silica and 316 L stainless steel (SS316L) under a dry argon atmosphere were investigated using a high-temperature adsorption testing platform (HATP), which was specifically designed and implemented based on thermochromatography and thermosublimatography. The adsorption enthalpies were determined by Monte Carlo simulations, yielding values of −145 ± 10 kJ/mol and −148 ± 10 kJ/mol (as a peak deposition temperature of 448 ± 11°C and 440 ± 12°C) for fused silica and SS316L, respectively. Further thermodynamic calculations indicated that PbI 2 (g) was the predominant iodine species under experimental conditions. These results provide insights into the chemical speciation and migration of radioactive iodine released from LFR, which is crucial for improving safety assessments and nuclear emergency response strategies.