SIMULATION OF THE THERMOMECHANICAL BEHAVIOR OF (U, Gd)O2 FUEL ELEMENTS IN THE RTOP-CA CODE

The paper presents a description of the RTOP-CA code models designed for simulating the behavior of (U, Gd)O2 fuel rods, presents the examples of program verification using experimental data obtained at the research reactor in Halden and in the post-irradiation examinations of WWER fuel. The experience of operating WWER fuel has shown that differences in the thermomechanical behavior of (U, Gd)O2 fuel elements and UO2 fuel rods during the first - second fuel cycles can be observed. In particular, an accelerated increase in the length of the (U, Gd)O2 fuel rod cladding as compared to UO2 fuel elements is possible. To simulate the behavior of (U, Gd)O2 fuel rods under irradiation in the RTOP-CA code, models that describe the features of uranium-gadolinium fuel were developed. To calculate the evolution of heat release in (U, Gd)O2 fuel rods in the RTOP-CA code, a model of burnup of gadolinium isotopes was implemented. The developed model makes it possible, without using of detailed neutron-physical calculations, to simulate the burnup of gadolinium and the evolution of heat generation along the radius of the pellets. The models take into account the effect of the gadolinium oxide content on the thermophysical and mechanical properties of the fuel. The calculations performed using the RTOP-CA code are in good agreement with the results obtained during the experiments in Halden on the dynamics of the central fuel temperature, gas pressure in the (U, Gd)O2 fuel rod, and the elongation of the fuel column and cladding. To simulate the accelerated elongation of (U, Gd)O2 fuel rods in comparison with standard WWER fuel elements at the initial stages of fuel operation at nuclear power plants in the RTOP-CA code, a model of mechanical interaction of pellets and cladding was developed taking into account the misaligned arrangement of pellets.