Atomic scale simulation of irradiated nuclear fuel

Abstract

Atomic scale simulations have been performed investigating various phenomena governing nuclear fuel performance during reactor operation and during post irradiation storage or disposal. Following a review of some of the key features of irradiated nuclear fuel, such as fission product distribution, two key factors were identified as the focus for this investigation: i) the role of uranium dioxide non-stoichiometry and ii) the effect of temperature. The former has been carried out using a previous pair potential model, whilst a new many-body potential was developed to enable temperature effects to be studied over the full range of temperatures of interest. Secondary oxide precipitates are known to exist in irradiated nuclear fuel with Ba, Sr and Zr precipitating to form the perovskite (Ba,Sr)ZrO3 grey phase. The binary BaO, SrO and ZrO2 may also be formed. The precipitation enthalpies of these oxides were predicted as a function of hyper-stoichiometry. Additionally, CrUO4 can also precipitate from Cr-doped UO2+x. The possibility of fission product segregation to the phases from UO2 or UO2+x was also investigated with a broad range of species preferring segregation from stoichiometric UO2. The role of defect cluster configuration on vacancy mediated uranium migration was investigated for UO2 and UO2+x. In both cases the lowest enthalpy migration pathway involved reconfiguration of the cluster to a metastable configuration. Furthermore, there were a very large number of alternative pathways that had similar migration enthalpies, especially for UO2+x. A new potential model was developed that uses a novel approach to include many-body interactions in the description of the actinide oxide series. This represents a significant improvement on previous models in the ability to describe the thermal expansion, specific heat capacity and elastic properties of CeO2, ThO2, UO2, NpO2, PuO2, AmO2 and CmO2 from 300 to 3000 K. Using the new model the thermal expansion, specific heat capacity, oxygen diffusivity and thermal conductivity of the mixed oxides (UxTh1-x)O2 and (UxPu1-x)O2 were predicted. Enhanced oxygen diffusion and a degradation in thermal conductivity were predicted in terms of the non-uniform cation sublattice.