The morphology of interlinked porosity in nuclear fuels

Abstract

Micro-examination of irradiated uranium dioxide has shown that pores formed along triple-point grain edges play an important part in the swelling of the fuel. When the volume change is greater than 7 % this porosity has the form of long tunnels linking adjacent grain corners and forming an interconnected network throughout the body of the fuel. The formation of these tunnels is important in two respects. Firstly, despite the high density of individual gas bubbles that are also present in the fuel, by far the major contribution to the observed fuel swelling is from the grain edge tunnels. Secondly the interlinked porosity provides a direct route for the release of fission gas from interior regions of fuel. In nuclear reactor fuel elements both of these features can be responsible for damaging the fuel containment. The analogy between the observed morphology of the grain edge porosity and that of cylindrical soap films is used in developing models describing the formation and interlinkage of the tunnels. In particular a method of calculating the equilibrium pore volume in materials of arbitrary grain boundary and free-surface energies, and subjected to a hydrostatic external loading, is presented. The analysis is also used to determine the potential for sintering once the gas has escaped. A significant result of the theory is that in any material there exists a minimum level of swelling, independent of the applied pressure on the fuel, below which interconnected tunnels are unstable. For uranium dioxide a minimum swelling of 5 % is predicted which is in excellent agreement with experimental observations.