The finite element method (FEM) is used to compute the conductivity of a matrix that contains a dispersed phase, which can be porosity or inclusions. The FEM accounts for the influence of shape, orientation and distribution of the dispersed phase on the conductivity. From these computations the two dimensional conductivity is obtained, that represents a useful lower limit of the real conductivity which is three dimensional. A relation has been derived that transfers the two dimensional conductivity into the real conductivity. As an example the influence of the microstructure of irradiated UO2 on the conductivity is calculated.