The effect of the chemically inert fission products on the chemistry of irradiated UO2

It is widely assumed that the fission gases and the volatile fission products play little or no role in influencing the chemical state of irradiated UO2sub(2+x). We show this to be incorrect when even a small quantity of these fission products is retained within the fuel grains. Prior to intragranular release most of the gases and volatiles occupy neutral trivacancy sites in the host lattice. However, these complexes can bind oxygen to be converted into fission product atoms residing in cation vacancies or (cation + one anion) vacancy complexes, and this results in reduction of the oxide. Here we analyse this behavior and its effect on the oxygen potential and cation volume self-diffusion coefficient. (author)