AbstractThe fission-product release of spherical monogranular UC2particles is studied by postirradiation annealing of neutron-activated samples and by exposing samples to a steady rate of photofission. The release of the several fission products studied appears to follow the kinetics and temperature-dependence of an activated diffusional process. The anomalous fast release of a portion of the Xe133in annealing experiments may be interpreted as being due to release of attached or trapped xenon atoms by a process not following Fick's law.Analysis of the data obtained gives the following diffusion coefficients for krypton, tellurium, iodine, xenon and barium in UC2(in cm2/sec):The diffusion coefficient for xenon in UC2shows agreement with the diffusion coefficient for xenon in UO2. A negative activation entropy (‒13 eu) for diffusion is indicated.