In-pile migration of fission product inclusions in mixed-oxide fuels

Migration velocities of fission-gas bubbles and solid fission-product inclusions in uranium-plutonium mixed oxide under irradiation have been obtained. Trails behind migrating second-phase particles are observed upon etching a polished metallographic section from an irradiated fuel pin. It was determined from the microstructure and the reactor power history that these trails were produced during one reactor power cycle of known duration. Particle velocity was computed from the observed trail length and the duration of the power cycle. Velocities varied between 1 and 10 Å/sec for particles with diameters between 1 and 10 μ. The temperature was between 1585 and 1850°C, and the average temperature gradient was [inverted lazy s]5400∘C/cm. In all cases, the particles moved in the direction of increasing temperature. The measured velocities for both bubbles and solid inclusions are shown to be in agreement with the calculated velocities based on a surface-diffusion mechanism with Qs*, the surface heat of transport, equal to 100 kcal/mole.