Simulation of neutron-physical processes in the surface layer of a fuel kernel

A large change in the structure, density, and chemical and phase composition occurs in nuclear fuel with deep burnup, and an edge zone is formed. Simulation of the formation of an edge zone in a fuel kernel of thermal reactors will make it possible to suggest ways to decrease its influence on the characteristics of a fuel element. In the present work, the neutron-physical processes occurring in the peripheral layer of a fuel kernel are simulated. The distribution of nuclear reaction rates along the radius of a fuel pellet is calculated using the SCALE-4.3, MCNP-4B, and UNK computer programs. The radial dependence of the local breeding ratio is calculated. It is shown that for fresh fuel BR > 1 for fissile nuclei in a 100 µm thick layer, while the initial BR averaged over a pellet is no more than 0.5. The volume energy distribution in a 100 µm thick peripheral layer is 30% higher than the average value over a pellet. A combined pellet, where the central part possesses the standard enrichment (4–5% 235U) and the peripheral layer contains less than 0.7% 235U, is proposed to decrease the influence of the edge zone on the properties of fuel.