The thermal conductivity of UO2 is an important parameter in the design of nuclear fuel assemblies. The thermal conductivity can be reduced by radiation-induced porosity, leading to increased safety risks. In the literature, an analytical equation has been suggested to describe the influence of randomly ordered ellipsoidal porosity on thermal conductivity. However, in the case where the shape and the distribution of the pores is very complex, as in irradiated nuclear fuel, this equation is less well suited. The finite element method is introduced as a computational technique to take into account the influence of complex porosity structures on the thermal conductivity. Using the combination of image analysis and the finite element method, an equation has been obtained that describes the relation between the average elongated form of the pores and the overall thermal conductivity. Both the finite element method and image analysis are tools to estimate the thermal conductivity of high-burnup nuclear fuel.