Microstructure of U₃Si₂ fuel plates submitted to a high heat flux

In order to gain insight on the performance limits of U₃Si ₂ fuel with Al cladding, fuel plates with a fissile material density of 5.1 and 6.1 gU/cm³ were irradiated in the BR2 reactor of SCK•CEN in Mol. The plates were intended to be subjected to severe conditions leading to a cladding surface temperature of 180-200 °C and fuel temperatures of 220-240 °C. The irradiation program was stopped after the second cycle based on the visual inspection and wet sipping tests of the elements, correspondingly showing degradations on the outer Al surfaces of the U₃Si₂ plates and the release of fission products. The maximum fuel burn-up was 29% and 25% ²³⁵U, respectively. In a PIE program the microstructural causes for this degradation are analysed. It is found that the failure of the plates is entirely related to the corrosion of the Al cladding, which has caused temperatures to rise well beyond the calculated values. In all stages, the fuel grains have retained their integrity and, apart from the formation of an interaction phase with the Al matrix, they do not demonstrate deleterious changes in their physical properties.