Large Grain Size UO2 Sintered Pellets Obtaining used for Burn up Extension

The actual tendency all over the world is to manufacture fuel bundles capable to resist high burn-up. The main factors affecting the burn-up increase are: the pellet-cladding mechanical interaction (PCMI), the oxidation and hydriding of the Zircaloy-4 sheath, the increase of internal pressure, stress corrosion cracking, Zircaloy-4 irradiation growth, fuel swelling. A way to increase fuel burn-up is to diminish the elements internal pressure by an adequate large grain UO2 fuel pellet structure. In the large grain size UO2 pellets, fission gas release rate decreases and the elements internal pressure increase slowly. One of the ways to increase pellets grain size without increasing sintering temperature and time is the addition of small quantities (< 1 % wt M/U) of sintering additives. By the addition of certain dopants in the UO2 powder (TiO2, Nb2O5, Cr2O3, CaO, V2O5,) the grain size, the porosity and the mean free diffusion path are increased, whereas the grain boundary area is reduced. At the Institute for Nuclear Research (ICN)- Pitesti a project for obtaining large grains size UO2 pellets using dopants was developed. Two UO2 non-free flowing powders, manufactured by ADU and IDR routes, were mixed with dopants (TiO2, Nb2O5, Cr2O3), content of metals being 0.05-1 % wt M/U. The green pellets obtained by pressing UO2 powders were directly sintered (4 hours at 1700°C) in a sintering furnace