Polycrystalline uranium dioxide (UO2), formed into pellets, is used as nuclear fuel in Pressurised Water Reactors (PWRs). At high-temperature, its viscoplastic behaviour is governed by the dislocation flow. Traditionally, the dislocations that generate viscoplastic deformations are referred to as Statistically Stored Dislocations (SSDs). On the other hand, experimental observations performed on deformed pellets show a development of sub-grains. The formed sub-grains are induced by lattice rotation, and are associated to the presence of Geometrically Necessary Dislocations (GNDs). Thus, to model a viscoplastic behaviour of UO2 polycrystals, we use the frame of the Field Dislocation Mechanics (FDM) theory, which allows us to characterise the plastic flow of both populations of dislocations: SSDs and GNDs. For this purpose, we have built a FDM model by homogenisation. In a first step, each dislocation is described at the microscopic scale by a Nye tensor, associated with a plastic deformation field. Fundamental equations concerning the dislocation motion and the related plasticity are presented. The microscopic equations are then homogenised on a larger scale. The homogenisation process reveals the densities of SSDs and GNDs as far as the classical laws of crystal plasticity (Orowan's law, etc.), adapted for both populations. It also provides balance equations describing how SSDs are transformed by the dislocations motion into GNDs, and vice versa. The built model has been applied to simulate the compression of polycrystalline aggregates of UO2. Mechanical equilibrium and the transport of the GNDs are solved using Fast Fourier Transform (FFT) method. Our results show the accumulation of GNDs on the grain boundaries, their migration within grains, and the formation of sub-grains, which qualitatively reproduces the experimental observations. The proposed model predicts good average behaviour of polycrystals, as well as takes into account influence of experimental conditions : strain rate and temperature.