  This paper deals with the comparison between thermal and Fission Enhanced
Diffusion (FED) of uranium into zirconia, representative of the inner face of
cladding tubes. The experiments under irradiation are performed at the Institut
Laue Langevin (ILL) in Grenoble using the Lohengrin spectrometer. A thin
$^{235}UO\_2$ layer in direct contact with an oxidized zirconium foil is
irradiated in the ILL high flux reactor. The fission product flux is about
10$^{11}$ ions cm$^{-2}$ s$^{-1}$ and the target temperature is measured by an
IR pyrometer. A model is proposed to deduce an apparent uranium diffusion
coefficient in zirconia from the energy distribution broadening of two selected
fission products. It is found to be equal to 10$^{-15}$ cm$^2$ s$^{-1}$ at
480$\circ$C and compared to uranium thermal diffusion data in ZrO$\_2$ in the
same pressure and temperature conditions. The FED results are analysed in
comparison with literature data.
