3D microscopic X-ray fluorescence and diffraction analysis of a fuel-cladding interface of a highly irradiated boiling water reactor fuel rod

During irradiation of nuclear fuel in light water reactors (LWRs), the fuel cladding creeps onto the uranium oxide fuel pellets due to the high temperature and pressure in the reactor core. With increasing burnup, the uranium fission leads to a surplus of oxygen that creates an oxide layer at the inner surface of the cladding which causes a fuel-cladding bonding. This bonding layer, that incorporates fission products, can be important for the pellet-cladding mechanical interaction. The present study focuses on the fuel-cladding interface (FCI) region of a high burnup spent fuel irradiated in a Swiss Boiling Water Reactor (61 GWd/tU). A microstructural characterization was carried out using a combined microbeam X-ray fluorescence (µXRF) and X-ray diffraction (µXRD) experiment in tomographic mode. All the measurements were carried out at the microXAS beamline of the Swiss Light Source synchrotron, at the Paul Scherrer Institute, PSI. A small micrometer-scale sample of a size of approximately 34 µm x 34 µm x 18 µm of the region of interest at the FCI was prepared using the focused ion beam (FIB) technique and then analyzed. The µXRF maps, obtained with high spatial resolution, were used to correlate the chemical composition with the crystalline phases identified using XRD. The results show that tetragonal zirconia is the main phase present in the bond layer between fuel and cladding, although it is a metastable form under the temperature and pressure conditions of a LWR. Lattice parameters evolve through the zirconia layer, with smaller parameters at the Zr/ZrO₂ interface and larger ones at the ZrO₂/UO₂ interface. This may indicate phase stabilization by stresses and/or fission products. XRD analysis allowed evaluating the lattice parameters of the identified phases, among others for UO₂ near the cladding, which was 5.474 Å. In addition, XRF analyses revealed the presence of Kr bubbles within the zirconia layer.