Nuclear fuel constitutes an essential component of the performance and safety of nuclear reactors. It is composed of complex materials with specific properties and is subjected under irradiation to a large number of diverse but interconnected phenomena. A better understanding of the properties of fuel materials and of the mechanisms underlying their changes under irradiation is key to the development of more accurate and predictive codes for the simulation of fuel elements. An efficient approach to improve the understanding of fuel behaviour is to complement the examination of neutron-irradiated materials by a basic research approach combining separate effect experiments on model, surrogate or ion-irradiated materials and multiscale modelling from the atomic to the mesoscopic scale. We will present the approach developed on (U,Pu)O2 fuels in the INSPYRE H2020 project and illustrate the results obtained on significant operational issues and on the development of constitutive laws for the behaviour of nuclear fuels. INSPYRE has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 754329. This project is part of the research activities portfolio of the Joint Programme on Nuclear Materials.