Transport phenomena in nuclear fuels under severe temperature gradients

The fundamental concepts involved in atomic transport phenomena in the presence of temperature gradients are reviewed in the context of irreversible thermodynamics. The basic theory for the bodily migration of macroscopic particles and pores is then developed. These theoretical concepts are applied to various phenomena of importance in nuclear fuels, including lenticular void motion and central void formation, fission-gas swelling and gas release, solid fission product redistribution and the redistribution of fuel constituent species. In some cases there is good quantitative agreement, as in lenticular void motion. In other cases, as in fuel constituent and solid-fission-product redistribution, the basic processes seem qualitatively in hand but uncertainties in the values of key parameters preclude quantitative comparisons. In the area of fission-gas swelling and gas release, considerable controversy remains as to the importance of the motion of small bubbles at low and intermediate temperature. At the higher temperatures of the equiaxed grain-growth regime, and within the columnar grains, evidence for significant bubble migration along the thermal gradient seems incontrovertible. For the latter situation, semi-quantitative agreement between theory and bubble migration velocities has been obtained but some disagreement remains as to the relative importance of this phenomenon in determining the magnitudes of swelling and gas release. (Auth.)