Under Loss Of Coolant Accident (LOCA) conditions, the temperature evolution within the fuel pellets combined with a reduction of the cladding confinement can lead to fuel fragmentation. This phenomenon provides additional fission gas release, inducing a higher rod internal pressure and possibly an additional driving force to disperse the smallest fuel fragments out of the cladding when the cladding balloons and bursts. Experiments show that the pellets are fractured in many fragments, with size ranges varying from a few millimetres to a few microns. Usually the hypothesis used to explain fuel pellet fragmentation during transient, is grain cleavage induced by over pressurized fission gas bubbles, located at the grain boundary. This work focuses on the pellet rim, where bubbles density increases owing to a higher irradiation level. This area, called “High Burn-up Structure” (HBS), has a specific behaviour due to a microstructure reorganization composed of small grains about 100 nm compared to 10 μm for initial UO2 fuel. The aim of this study is to define a macroscopic fragmentation model based on a micro mechanical approach to have a better understanding of the fuel mechanical behaviour at lower scale: size and volume fraction of fragments. This paper introduces a stepwise micromechanical method: firstly, we detail how to model the HBS microstructure including pressurized porosities, based on experimental or numerical data and define a Representative Volume Element (RVE). Then we use 3D full field computations in order to determine crack snapshot. Elastic computations are performed to find the bubbles pressure level which is required to reach the cracks initiation threshold. Then nonlinear computations, using a failure local behavior law, are conducted to identify the failure snpashot. The latters will be used as an input data of the homogenization (“macroscopic”) model. This model is exposed in the last section.