U(Mo) grain refinement induced by irradiation with high energy iodine

Grain refinement in U(Mo) has been achieved out-of-pile by irradiating an U(Mo) foil with 80 MeV high-flux 127I ions at 140°C up to a high dose of ~ 2.1 × 1018 ions/cm² or a fission density equivalent of ~ 3.7 × 1021 f/cm³. Several locations in the irradiated area were analysed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) in order to study the dose-dependent evolution of the irradiation induced restructuring. The refined U(Mo) grains formed in proximity of the sample surface, where a strong irradiation induced oxidation was also observed. The spatial concomitance between the irradiation induced grain refinement and sample oxidation points out a possible connection between the two phenomena. The EBSD maps suggest that polygonization was the underlying mechanism driving the grain refinement based on the observation of many low-angle grain boundaries surrounding the newly refined grains. In a later stage, the grain boundaries transformed from low-angle to high-angle under the effect of raising local lattice stresses induced by the ion implantation. The estimated ion irradiation dose triggering the polygonization is approximately one order of magnitude lower than what is observed in-pile. This experiment shows that ion irradiation is capable of simulating the high burnup structure forming in U(Mo) in-pile. Ion irradiation therefore once more proves to be a powerful tool to study the fundamental behaviour of neutron irradiated U(Mo) or other fuel materials.