SIMULATION OF CSI DECOMPOSITION IN FUEL CLADDING GAP

There is a strong radiation field in operating fuel pins, which affect s the chemical state of fission products. The effect of the radiation field on the iodine partial pressure in operating fuel pins has been t heoretically studied. The decomposition rate of CsI in operating fuel pins has been calculated by a Monte Carlo code and the radiation effec t on iodine partial pressure in a fuel-cladding gap has been evaluated . It has been found that the iodine partial pressure calculated with c onsideration of the radiation effect is higher than that in the therma l equilibrium without radiation. The probability of CsI decomposition has been calculated as a function of distance from the fuel surface. T he calculation results show very large decomposition probability at th e near surface of the fuel, where low energy particles are dominantly emitted. When the gap is closed, the very high partial pressure of iod ine is expected in a remaining small space between fuel and cladding.