Shape change of process pores in kernels of high-temperature uranium dioxide based fuel elements

Abstract

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.The initial process porosity of uranium dioxide strongly affects the evolution of the structure, strength properties, and radiation characteristics. For this reason, the shape change of the pores as they migrate in a temperature gradient field is of great interest [1][2][3][4]. Large pores which initially are approximately the same size as a grain become prolate as they migrate in the direction of a temperature gradient right up to the formation of cylindrical channels. The observed migration of lenticular pores forms a columnar structure of uranium dioxide grains. The sources of such pores are rounded microcracks in uranium dioxide, which arise in the starting-stopping regimes of a reactor [3]. Lenticular pores can also form during migration of grain boundaries oriented perpendicular to the direction of a temperature gradient with pores arranged on the boundaries and acquiring the shape of a lens as the curvature of the grain boundary increases as it migrates together with the pore [5].As a supplement to the indicated mechanism, the present work examines the effect of the character of the radial temperature distribution and temperature gradient in a high-temperature fuel element on the shape change of the technological pores distributed in the volume of a fuel pellet. A ventilated fuel element of a thermionic converter reactor with single-element electricity-generating channels based on uranium dioxide with mostly open porosity with ten-micron pores stabilized to thermal sintering is examined [6].Computational estimates show that vacuum pores with the indicated sizes at the operating temperature migrate as a result of the transfer of uranium dioxide vapors in the pore volume in the free-molecular regime. In this case, the migration rate and pore shape are most sensitive to the local values of the temperature and the temperature gradient in the fuel-element kernel which vary in the migration process. The effect of these factors on the porosity of a uranium dioxide kernel was studied in the present work.Mass transfer in a vacuum spherical pore with an arbitrary temperature distribution on its surface was investigated in [7]. The intensi...