Formation of Nanostructures in UO2Fuel at High Burn-ups

In the present paper it is assumed that above a limiting value of fission fluency (burn-up) a more intensive process of irradiation induced chemical interaction occurs. A significant part of fission gas product is thus expected to be chemically bounded in the matrix of UO 2 fuel. The fission gas atoms substituting, for example, uranium atoms in the crystallographic lattice can form weak facets. At a certain saturation condition, division of the grains can occur at the weak facets and the increase in fission-gas-products release may be expected. The fact that the process of grain division for high burn-ups (70-80 MWd/kgU) forms an extremely fine structure up to a temperature as high as 1100°C and simultaneously the observed decrease in fission gas concentration in the fuel supports this concept. The analysis of fission gas concentration change due to the formation of nanostructures in UO 2 fuel at high burn-ups in terms of total surface area change in a function of burn-up and knock-out process is presented.