Zirconium surface modification under fission product irradiation. Application to nuclear fuel cladding tubes

Nuclear fuel devices of pressurised water reactors are composed of uranium oxide pellets which are enclosed in zircaloy-cylinders. During reactor operation in the contact with the fuel the zircaloy oxidises non-uniformly in depths of the order of some micrometers. Furthermore, energy deposition of fission recoils leads to sputtering of uranium onto the inner surface of the cladding material. Thus, sputtered uranium ions start to migrate outwards. Experiments were performed at Institut Laue Langevin (ILL) on the Lohengrin mass spectrometer in order to simulate the alteration of cladding tubes in contact with the nuclear fuel during irradiation. A thin UO2 deposition in direct contact with a zirconium foil was put into the ILL high flux reactor (5×1014 neutrons cm−2 s−1). The variation of energy loss of selected fission products in the foil put in evidence a zirconium oxidation whose kinetics constant is deduced from these experiments. Following oxidation the fission product mean kinetic energy is stabilised, but still a broadening of the energy distribution is observed, which is characteristic of uranium diffusion inside the zirconia target. Using Fickʹs model, a uranium diffusion coefficient in zirconia under irradiation is deduced.