Migration behaviour of iodine in nuclear fuel

A novel out-reactor method has been further developed for investigating the migration behaviour of fission products in UO2 nuclear fuel, which allows the effects of thermal diffusion, radiation damage and local segregation to be independently assessed. Tailored concentration profiles of any desired species are first created in the near-surface region of polished samples by ion implantation. The impact of either thermal annealing or simulated fission is then precisely determined by depth profiling with high-performance secondary ion mass spectrometry (SIMS). Comparison of iodine migration in UO2 wafers that had been ion-implanted to fluences spanning five orders of magnitude has revealed subtle radiation-damage effects and a pronounced concentration dependence for thermal diffusion. At concentrations above ∼1016 atoms/cm3 much of the iodine became trapped, likely in microscopic bubbles. True thermal diffusion coefficients for iodine in polycrystalline UO2 have been derived by modelling the low-fluence data.