Oxygen distribution in the rod cladding tubes during a Loss of Coolant Accident (LOCA) situation is known to play a key role regarding the post-quench mechanical properties of fuel rods. Analytical solutions of the diffusion equations in the complex multilayer and multiphase systems describing the cladding material tube during high temperature oxidation are limited to stationary isothermal conditions. To address the problem of non-stationary transient LOCA situations, a numerical tool (named DIFFOX) has been developed. In the first part, the main assumptions and main features of the code are described. In parallel to the code development, an experimental program has been launched to provide a validation database as well as to input data for improvement of the code models. Zircaloy-4 cladding samples have been pre-oxidized at low temperature then vacuum annealed or steam oxidized at high temperature. Oxygen distribution profiles are investigated by Electron Probe Microanalysis. The ability of the code to describe the chemical reduction of the pre-oxide layer and oxygen diffusion into the metal during high temperature annealing is demonstrated. Regarding high temperature steam oxidation, the pre-oxide layers are observed to be either protective or not on a same given sample. While a good code to experimental data agreement is obtained in the first case, the growing of a high temperature oxide underneath the pre-oxide is not well reproduced by the code. The effect of hydrogen has also been investigated by performing small scale oxidation tests on 600 wppm hydrogen charged Zy-4 cladding. Samples have been long-term annealed for homogenization after oxidation. The main purpose of these experiments is to provide quantitative data on the modification of the α/α+β and α+β/β transus of the Zy4-O pseudo binary phase diagram induced by the presence of hydrogen. A variation of the transus locations deduced from these experiments is proposed.