DETERMINATION OF A POROSITY CORRECTION FACTOR FOR THE THERMAL-CONDUCTIVITY OF IRRADIATED UO2 FUEL BY MEANS OF THE FINITE-ELEMENT METHOD

The influence of porosity on the thermal conductivity of irradiated UO 2 fuel has been determined with the Finite Element Method (FEM). The p ore shape and the pore distribution are entered in the FEM program fro m light-microscopy photographs. The two-dimensional (2D) thermal condu ctivity in the plane of the photograph is obtained from the FEM calcul ations. The 2D thermal conductivity, that has no physical meaning, is the lower limit of the three-dimensional (3D) thermal conductivity. Fo r three well defined pore shapes the relation is determined between th e 2D and the 3D thermal conductivity. From these computations a simple relation is obtained that transfers the 2D into the 3D thermal conduc tivity, independent of the pore shape. The influence of porosity on th e 3D thermal conductivity of irradiated UO2 and Nb2O5 doped UO2 fuels was computed with the FEM.