Mechanical properties of nuclear fuel are a complex problem, involving many coupled mechanisms occurring at different length scales. Most of our knowledge regarding this mechanical behaviour is obtained by experiments on unirradiated fuel, or post-mortem analysis on spent fuel. Atomistic models can alternatively provide insight on the behaviour of the fuel in conditions inaccessible to current experiments, which are used as input parameter for models at larger length-scales. The present investigation considers recent studies on the evolution of the mechanical properties of UO$_2$ after irradiation damage by means of atomistic simulations with molecular dynamics method using empirical potentials.Firstly, we will describe the mechanical behaviour under tensile and compressive load in pristine UO$_2$ single-crystal.Secondly, mechanical properties and crack propagation mode will be investigated in large UO$_2$ single-crystal including point defects distribution arising from irradiation events. The influence of the crystallographic orientations, temperature, and dose will be discussed.Finally, we will present the influence of gas fission nanobubble distribution in the UO$_2$ matrix on the mechanical properties.