Some questions of uranium dioxide mass transfer influence on high-temperature fuel element behaviour

A characteristic feature of thermionic fuel elements is the necessity of venting the central void to prevent buildup of fission gas (FG) pressure. The simplest system providing FG removal is a cylindrical central hole along the fuel rod. However, when using uranium dioxide having relatively high vapor pressure, fuel evaporation in the hottest, central part of the rod and fuel condensation on the cold parts results in formation of a closed void. Another factor complicating mass transfer is the incongruent evaporation of nonstoichiometric uranium dioxide. Redistribution of oxygen associated with incongruent evaporation adds to oxygen redistribution due to temperature gradients within the rod. In addition fuel stoichiometry changes in the process of uranium fission and at the expense of oxygen escaping through the venting hole. Methods and results of computing mass transfer in the central hole are given. The results can be used to determine the temperature limit for using the central-hole gas venting system. The effect of uranium dioxide mass transfer on fuel element lifetime is also studied