Microstructural evolution investigation of irradiated oxide fuels by means of electron backscattered diffraction

In this paper, both irradiated UO ₂, together with a fresh UO _2.001 sample from the same batch as a reference, and irradiated FR-MOX U _0.55Pu _0.45O _1.99 (TRABANT 2/3) fuel were studied using Electron Back Scattered Diffraction (EBSD). Irradiation of both oxide fuels was carried out at the High Flux Reactor (HFR) Petten, the UO ₂ at a moderate maximum Linear Heat Rate (LHR) of 200 W/cm to a burn-up of 5.4 % FIMA (fission per initial metal atom) or 50 MWd/kgUO ₂, and the MOX at a high maximum Linear Heat Rate (LHR) of 374 W/cm to a burn-up of 1.7% FIMA. In the irradiated UO ₂, the grain growth was observed in the fuel centre. The irradiated MOX (U, Pu)O ₂ showed the typical FBR fuel microstructure zones, i.e. as-fabricated (at the fuel edge), equi-axed and columnar grains as observed from the fuel-clad region towards fuel central hole. The LAGB (Low Angle Grain Boundary), 3° < θ < 15°, as an initial state for the HBS structure was observed on both UO ₂ and MOX. This restructuring mechanism for both oxide fuels is proposed to follow the well-known polygonization mechanism in which an arrangement of dislocations at the grain boundaries occurs.