Radiation effect on partial pressure of fission product iodine

Abstract

The knowledge of the partial pressure of fission product iodine is important for evaluating the FCCI (fuel cladding chemical interaction) in FBR. Thermodynamical analysis shows that iodine is believed to react with cesium so strongly that the partial pressure of iodine is kept too low to cause FCCI. A molecule of cesium iodide is, however, excited by radiation in the reactor and dissociates into radical and ion. In this work, the radiation effect on the partial pressure of iodine is theoretically studied for the gaseous cesium-iodine system. Fission fragment flux is considered as the radiation and the iodine partial pressure is evaluated based on the theory of gaseous reaction. The results of the calculation show that the partial pressure of iodine can be increased enough to cause FCCI in the radiation field. In the case of the FBR fuel pin, the partial pressure of iodine under radiation conditions can be calculated to be ~ 10 −2 Pa (~10 −7 atm), which is higher than that of the non-radiation condition ~10 −9 Pa (~10 −14 atm). These calculations were carried out under the assumption that the cladding inner surface temperature was 873 K (600°C) and the oxygen potential was −418 kJ/mol (−100 kcal/mol).