The swelling of the refractory nuclear fuels, UO2, UN, and UC, at temperatures of the order of 1700°C, is related to the behavior of the fission gases as these gases make their way out of the fuels. In all three fuels, the fission gases first precipitate to form a two-phase system consisting of solid fuel and gaseous precipitates. These precipitates or bubbles grow in-reactor mainly by the accretion of new fission gas atoms. New gas atoms diffuse to the older bubbles (which are formed in the first few minutes of irradiation) causing these bubbles to grow and to swell the fuel. In UC, this process continues with very little change to produce rather large amounts of swelling. However, at about 10 vol% swelling of UC, bubbles begin to overlap to form channels and gas release begins to accelerate. In UN and UO2 at about 1700°C, UN and UO2 vapors begin to migrate across the growing bubbles as a result of the temperature gradients usually found in fuels in-reactor. This process, which is a form of zone refining, results in rapid and efficient gas collection and release. This gas release may reduce the stresses applied by UN and UO2 to their claddings; but the zone refining process does not completely eliminate swelling stresses because swelling precedes gas release. Also, the zone refining or “restructuring” process tends to eliminate built-in porosity; so that while built-in porosity is very effective in limiting the swelling of UC, such porosity is relatively ineffective in reducing the swelling of UN and UO2 at temperatures where restructuring is rapid. These processes are described in terms of classical chemical and physical metallurgical models; and the validity of the models is illustrated, although not necessarily proved, by the results of high-temperature irradiation experiments.