SHAPE CHANGE OF PROCESS PORES IN KERNELS OF HIGH-TEMPERATURE URANIUM DIOXIDE BASED FUEL ELEMENTS

A computational method and results for calculating the shape change of an initially spherical vacuum pore with migration in uranium dioxide fuel during operation of a high-temperature fuel element are presented. The conditions for a spherical fuel element to be elongated and flattened in the direction of the temperature gradient, i.e., its transformation into prolate and lenticular, are determined. It is shown that the sign of the second derivative of the saturated vapor pressure of uranium dioxide with respect to the radius of the fuel kernel is decisive. It is established that the region of formation of the prolate pores is located at the periphery of the cross section of a fuel kernel and that of lenticular pores at the center. As the thermal neutron fraction in the reactor spectrum increases, the formation region becomes narrower for prolate pores and wider for lenticular pores.