This paper is devoted to the study of the defect influence on uranium diffusion in zirconia in the context of nuclear waste disposal. The experiments in reactor conditions are performed at the Institut Laue Langevin in Grenoble using the Lohengrin spectrometer. A thin UO$_2$ layer in direct contact with a zirconium foil is irradiated in the ILL high-flux reactor. The fission product rate is around 3$\times10^{11}$ ions s^{−1}$ and the neutron flux is equal to 5$\times10^{14}$ n cm$^{−2}$ s$^{−1}$. The target temperature is controlled by an IR pyrometer and ranges from 470 to 490 \degresC. In these conditions, a zirconium oxidation is first observed before uranium diffusion. A model is proposed to deduce an apparent uranium diffusion coefficient in zirconia (ZrO$_2$) from the energy distribution broadening of a selected fission product (A=90). It is found to be equal to 10$^{−15} cm^2 s^{−1}$. The study of thermal diffusion is performed by using ion beam techniques (ion implantation and Rutherford Backscattering). ZrO$_2$ samples are implanted with 800 keV uranium ions at a dose of 10$^{16}$ ions cm$^{−2}$ and annealed at a pressure of 7.5$\times10^{−1}$ Pa. No uranium diffusion could be observed up to 800 \degresC. The influence of irradiation defects mainly due to fission products, both on zirconium oxidation and on uranium diffusion, is clearly demonstrated.