[en] Highlights: • Fuel-cladding bonding in PWR fuel rods. • Zirconia layer formed at the fuel.|cladding interface. • Chemical bonding occurs between ZrO₂ and UO₂ at low burnups. • Mechanical bonding progressively occurs between ZrO₂ and UO₂ when increasing burnup. - Abstract: This study proposes a new and detailed description of the fuel-cladding bonding phenomenon occurring in PWR fuel rods. Very early and late bonding states were characterized on specimens of 35.3 GWd.t_U⁻¹ moderate burnup and of 64.5 GWd.t_U⁻¹ high burnup respectively. Results were then compared with those achieved on a re-created bonded situation obtained on a Zircaloy-4/hyper-stoichiometric UO_2+x model materials diffusion couple. These results tend to indicate that a chemical adhesion is probably at the origin of the PWR fuel-cladding bonding. In addition, the progressive formation of ZrO₂/UO₂ interfacial circumvolutions observed with increasing burnup, which lead to the physical anchorage of ZrO₂ and UO₂, is likely to lead to their mechanical adhesion. Thus, the in-reactor ZrO₂/UO₂ bonding could be considered, since the occurrence of the first bonded situations, as an adhesion phenomenon owning two components: an initial chemical progressively strengthened by a second mechanical.