A model for lattice and grain boundary diffusion in polycrystalline materials was used to assess the potential for the use of fission gas release experiments to measure the lattice and grain boundary diffusion coefficients in metallic nuclear fuel materials. The assessment made was that, assuming that grain-boundary diffusion in metallic fuels was similar to that in other metals, it was reasonable to expect that lattice diffusion coefficients could be determined from short-term release experiments and that the product of the grain boundary diffusion coefficient, the segregation factor, and the boundary width could be extracted from gas release experiments at longer times. Under the same assumption, activation energies could be deduced from the temperature dependence of the measured diffusivities. The proposed experiment was a so-called separable-effects one which was used to underpin simulations and models. In this case, the aim was to provide input to fission-gas source term models. Such models seek to capture, to the extent possible, all of the relevant physics of fission-gas release. Because of the integral nature of in-pile experiments, some parameters were obtained through fitting to integral experiments. To achieve predictive capability, a stronger physical basis had to be brought to bear upon the parameters; including diffusivity. As with all separate-effects experiments, the diffusivities that were derived had to be scaled to the reactor environment by simulation and modelling or by other separate supporting effects such as radiation-enhanced diffusion and re-solution. The model provided information on the optimization of grain boundary versus bulk diffusion data, using temperature, diffusion time, and grain size. The assessment was that, assuming grain boundary diffusion in metallic fuels was similar to that in other metals, experiments could be designed in which lattice diffusion coefficients could be determined from short-term gas-release experiments, and the triple product could be extracted from gas release at longer times. In an experiment, counting statistics could require trade-offs between tracer concentration and measurement precision. Under the same assumption, activation energies could be deduced from the temperature dependence of the measured diffusivities. In contrast to the case of infinitesimal tracer experiments where segregation coefficients could be measured, to account for effects of segregation of Xe to the grain boundaries in a fission gas release experiment, some supporting first-principles calculations may be needed.

The Potential to Use Fission Gas Release Experiments to Measure Lattice and Grain Boundary Diffusion in Metallic Fuels. W.E.King, M.Robel, G.H.Gilmer: Journal of Nuclear Materials, 2011, 411[1-3], 97-111