3D reconstructions of irradiated U[sbnd]Mo fuel to understand breaching effects in ZrN diffusion barriers

Abstract

To understand interaction layer behavior in UMo fuel kernels coated with physical vapor deposition (PVD) ZrN barriers, two cubes of irradiated fuel were serial sectioned and imaged in 50 and 200 nm increments using a focused ion beam (FIB) instrument. Locally, the fuel underwent a burnup of 52% U235 or a fission density of 4.0 × 1021 fissions/cm3. 3D reconstructions were created from the serial sectioned images. 3D reconstructions revealed the morphology of multiple locations where UMo/Al interaction layer formed between the Al matrix and the UMo fuel. These locations are associated with breaches in the ZrN barrier and not from diffusion of Al through the ZrN barrier. When not compromised, the ZrN barrier successfully impedes UMo fuel interaction with Al cladding. The ZrN barrier has decreased from a nominal pre-irradiated thickness of 1.16 μm to a thickness of 0.69 μm. A ZrN-rich phase adjacent to the ZrN barrier was observed on the Al matrix side of the coating. This phase is likely the original ZrN barrier and Al matrix that decomposed with increasing burnup as a result of fission recoil induced mixing. Nominally, the thickness of the irradiated ZrN barrier and the ZrN-rich Al matrix is 1.36 μm.