Experimental characterisation and modelling of the fission products implantation at the fuel-cladding interface of high burnup PWR fuel rods

During irradiation in light water reactors, numerous fission products (FPs) are produced by the fission reactions. At the periphery of the fuel, these elements implant by recoil in the cladding inner oxide layer (zirconia) located at the fuel-cladding interface (FCI). In the present study, two irradiated UO 2 fuel rods with high burnups (61.4 and 62.7 GWd/tU) have been analysed using EPMA. The aim of this work was to understand the mechanisms behind zirconia growth, FPs implantation at the FCI and their impact on the interface. In this objective, the measured elemental concentration profiles of U and Zr at the FCI is first compared to Fission Enhanced Diffusion (FED) models derived from out-of-pile experiments, showing good agreement. It is then shown that the implantation of metallic fission products (Nd, Mo, Pd, Ru) in the zirconia layer at the FCI can be described by a ballistic mechanism. Volatile fissions products, such as Xe, are found to affect differently the interface as they gather in bubbles. This explains the zirconia growth in form of protuberances called “circumvolutions” that are typically found in high burnup fuel rods. An analytical model describing zirconia swelling due to gaseous FP is proposed and compared successfully to measurements of zirconia thickness and circumvolution height.