An out-of-reactor method was developed for the investigation of the migration of fission products in nuclear fuel. This permitted the effects of thermal diffusion, radiation damage and local segregation to be assessed independently. Desired concentration profiles of any desired species were first created, in the near-surface regions of polished samples, by using ion implantation. The effect of thermal annealing or simulated fission was then precisely determined by means of depth profiling, using secondary ion mass spectrometry. A comparison of I migration in UO2 wafers, which had been ion-implanted to fluences that spanned 5 orders of magnitude, revealed subtle radiation-damage effects and a pronounced concentration dependence of the thermal diffusion. At concentrations above 1016/cm3, much of the I became trapped; probably in microscopic bubbles. The true thermal diffusion coefficients for I in polycrystalline UO2 were derived by modelling low-fluence data.

Migration Behaviour of Iodine in Nuclear Fuel. W.H.Hocking, R.A.Verrall, I.J.Muir: Journal of Nuclear Materials, 2001, 294[1-2], 45-52