Power ramped cladding stresses and strains in 3D simulations with burnup-dependent pellet–clad friction

This paper presents 2D(r, θ) plane strain and 3D simulations of PCI during base irradiation and ramp tests. Inverse analysis is used to estimate the evolution of friction at the pellet–clad interface with burnup. The number of radial cracks that form during power ramp tests in seventeen UO2-Zy4 rodlets with burnups in the range 20–60 GWd/tU is the main parameter on which inverse analysis is based. It is shown that the sole evolution of the friction coefficient with burnup is sufficient to capture the radial crack pattern of the rodlets after power ramping. A simple relation between the friction coefficient and the burnup variation after initial pellet–clad contact is thus proposed and used in 3D simulations of PCI. The delayed gap closing at mid-pellet level with respect to inter-pellet level leads to an axial variation of the friction coefficient, with maximum values near the pellet ends. The consequences in terms of PCI failure propensity are then discussed.