Pore migration under high temperature and stress gradients

An analytical model is proposed for the evolution of ellipsoidal pores by surface diffusion under the influence of large temperature gradients and the associated thermoelastic stress field. It is found that both of these influences affect the migration velocity of a pore as well as its shape. The shape of a pore is determined by competition between the thermoelastic stress field, the interfacial energies of the pore and grain boundaries, and the kinetics of the mass transport process. The dramatic case of void migration in a uranium dioxide nuclear fuel rod is considered, in which very large temperature gradients of the order of 4 × 105 K/m are predicted. It is found that crack-like pores are expected to form on the radial grain boundaries prevalent in the fuel rod microstructure, and that these crack-like pores lead to the eventual structural failure of the component. This agrees with experimental observations. The temperature gradient is the dominant driving force for the migration of near-spheroidal and prolate pores, whereas the stress field is the dominant driving force for oblate crack-like pores.