Change of WWER-440 fuel rod geometry under operation up to 65 MW-day/kgU fuel burnup

Full text: The paper describes results of analysis and summary of data on post-irradiation examination of 13 WWER-440 fuel assemblies irradiated up to the fuel burnup of 22-65·MW/kgU. The mechanism of changes of diameter, length, fuel-to-cladding gap in fuel rods, as well the processes causing such changes are in focus of the paper. Experimental data were analyzed using a multi-component radiation-thermal creep model. Significant changes in the mode of deformation at the fuel-to-cladding interaction are described. At this stage of operation the swelling fuel influences the cladding to increase its diameter up to ∼ 35 μm at the burnup of ∼65 MW·day/kgU. The discovered peculiarities of the fuel rod length changes (increase of the elongation rate at the burnup of 50 MW·day/kgU and shortening at the burnups higher than 50 MW·day/kgU) are due to the fuel-to-cladding interaction. The described effects were shown to occur at lower burnups than in case of rated power in the fuel rods that were operated at the reduced linear power. As far as the limit value criterion is concerned, the values of the observed strains are acceptable and do not reduce efficiency of the fuel rods. (author)