Impact of hyperstoichiometry on the solubility and diffusion rate of iodine in fuel

A novel out-reactor method has been developed over the past few years for investigating the migration behaviour of fission products in oxide nuclear fuels. This process allows the effects of thermal diffusion, radiation damage and local segregation to be independently assessed. Tailored fission-product concentration profiles are first created in the near-surface region of polished wafers by ion implantation. The impact of thermal annealing or simulated fission is then precisely determined by depth profiling with high-performance secondary ion mass spectrometry (SIMS). Thermal diffusion of iodine at a peak temperature of either 1200oC or 1400oC under slightly oxidizing conditions to achieve a nominal O/U ratio of either 2.01 or 2.02 was found to cause an increase in both the solubility and diffusion rate of iodine by two orders of magnitude compared to stoichiometric fuel. These effects are consistent with the increase in the number of uranium lattice vacancies predicted by a thermodynamic model for the defect structure of the uraninite lattice. (author)