RELEASE KINETICS OF VOLATILE FISSION-PRODUCTS UNDER SEVERE ACCIDENT CONDITIONS

An analytical model has been developed to describe the kinetic release behavior of the volatile fission product species (e. g., cesium) from uranium dioxide fuel. This treatment is based on the analysis of a series of out-of-pile annealing tests with bare fuel specimens, at temperatures of 1200 to 1800-degrees-C, performed under a wide range of atmospheric conditions that are characteristic of a severe reactor accident. The physically based model accounts for the changing fuel stoichiometry. A more general framework is therefore provided to detail the release kinetics in reducing and oxidizing environments. Solid-state diffusion in the fuel matrix is shown to be the rate-controlling mechanism of release in atmospheres of either hydrogen or argon. On the other hand, in addition to the slower diffusion component, it is demonstrated that a "burst-release" process also occurs in a steam environment, in accordance with first-order rate theory, where fission products are rapidly released at small values of the stoichiometry deviation.