The material of the study is the uranium dioxide (UO2) fuel used in pressurized water reactors (PWR). Under irradiation and steady state, this polycrystalline oxide exhibits intragranular and intergranular bubbles. Their sizes and their shapes vary according to the burn up and their location inside the fuel. They are saturated by ssion gases. This work presents the modeling of the damage of a porous material and its over- fragmentation under dynamic loading during RIA (Reactivity Initiated Accident, a fast thermal transient). This numerical study is performed on a fuel fragment generated under steady state of irradiation and submitted to RIA. A new cohesive zone model is devel- oped following the micromechanical approach of Nkoumbou Kaptchouang and al. [1] and using the viscoplastic model for uranium dioxide of Salvo and al. [2]. This new cohesive model is temperature, porosity and strain-rate dependent. The traction-separation law enables to simulate the damage of the fuel until the creation of new cracks within the fuel fragment and their propagations. [1] N.B. Nkoumbou Kaptchouang, Y. Monerie, F. Perales, and P.G. Vincent. Cohe- sive GTN model for ductile fracture simulation. Engineering Fracture Mechanics, 242:107437, 2021. [2] M. Salvo, J. Sercombe, T. Helfer, P. Sornay, and T. Desoyer. Experimental character- ization and modeling of UO2 grain boundary cracking at high temperatures and high strain rates. Journal of Nuclear Materials, 460:184{199, 2015.