In irradiated polycrystalline uranium dioxide (UO2), the pressure generated at high temperature by the noble gases in the intergranular bubbles as well as the thermomechanical stresses due to temperature gradients cause the fracture of the grain boundaries. In this study, one analyzes through atomistic calculations of static and molecular dynamics type the properties of three UO2 grain boundaries and their behavior under uniaxial tensile loading up to fracture. In these atomistic simulations, the interactions between atoms are described using a many-body variable charge potential. Structural analysis, performed on the three grain boundary structures under uniaxial tensile stresses and over a wide temperature range, reveals no source of plasticity. This suggests a brittle behavior at fracture, in agreement with recent experimental studies carried out on micrometer size specimens containing grain boundaries. A method based on thermodynamic considerations is used to build normal cohesive zone laws for the UO2 grain boundaries using molecular dynamics simulation data. The cohesive zone laws will be employed in a future study in failure simulations at higher scales.