Uranium dioxide mass transfer in the central channel of the fuel kernel of a fuel element of a single-element energy-generating channel

Abstract

The methods and computer program for calculating, as well as the computational results obtained, the remaining-life changes occurring in the profile of the central channel of the kernel of a fuel element in a single-element electricity generating channel because of uranium dioxide mass transfer, taking account of the redistribution of the fuel components and the effect of the spacers, are presented. It is shown that the measured and a priori computed profiles of the central channel are close after prolonged testing. It is shown that the superstoichiometric composition of the initial uranium dioxide with oxygen coefficient close to 2.003-2.005 shifts in time to the stoichiometric composition. This is due to the removal of oxygen outside the bounds of the fuel kernel. The calculations show that when uranium dioxide with substoichiometric composition is used or a perforated screen is introduced into the central channel the rate of the axial mass transfer of uranium dioxide is decreased by almost a factor of 2.The radial mass transfer of uranium dioxide in the gap between the kernel and cladding of a ventilated fuel element of a single-element electricity generating channel (EGC) results in elimination of this gap or makes the real contact close to an ideal thermal contact ( Fig. 1) [1]. Previous computational-experimental studies have shown that radial mass transfer in the entire range of the realizable working temperatures of the cladding and the thermal power of the fuel element occurs in a freemolecular regime and is completed in a short time (tens-hundreds of hours) as compared with the service life [2]. Thus, direct loading on the cladding by a swelling kernel occurs irrespective of the initial construction of the fuel element.Longitudinal mass transfer in the central channel of the kernel changes the configuration of the fuel arrangement with the possibility of uranium dioxide condensate blocking the channel. As a response, local constrictions which appear in the initial channel increase the deformation rate of the cladding; the formation of a closed vacuum-sealed cavity results in unacceptable loads on the cladding by gaseous fission products leaving the fuel. Because of small axial temperature gradients, longitudinal mass transfer is a slow process, comparable in duration to the service life, and therefore requires detailed studies taking account of the following concomitant factors: cyclic participation of oxygen in transfer with superstoichiometric composition of the uranium dioxide, change of the oxygen coefficient of uranium dioxide as a result of oxygen outflow from the system into the gas phase and inflow of additional oxygen from the dioxide with uranium burnup, local leaks of heat from the kernel through the system of spacers, as well as a change of the fuel temperature during radial mass transfer.Since strengthened emitter cladding is used in a single-element EGC, effectively redistributing the volume change, during swelling of the uranium dioxide, in the direction of the central channel of the kern...