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Uranium silicides, in particular U 3 Si 2 , are being explored as an advanced nuclear fuel with increased accident tolerance as well as competitive economics compared to the baseline UO 2 fuel. Here we use density functional theory calculations and thermochemical analysis to assess the stability of U 3 Si 2 with respect to non-stoichiometry reactions in both the hypo- and hyper-stoichiometric regimes. We find that the degree of non-stoichiometry in U 3 Si 2 is much smaller than in UO 2 and at most reaches a few percent at high temperature. Non-stoichiometry impacts fuel performance by determining whether the loss of uranium due to fission leads to a non-stoichiometric U 3 Si 2±x phase or precipitation of a second U-Si phase. We also investigate the U 5 Si 4 phase as a candidate for the equilibrium phase diagram. [ABSTRACT FROM AUTHOR]

The elastic and magnetic properties, thermodynamical stability, deviation from stoichiometry and order/disorder transformations of phases that are relevant to Be alloys were investigated using density functional theory simulations coupled with phonon density of states calculations to capture temperature effects. A novel structure and composition were identified for the Be–Fe binary ε phase. In absence of Al, FeBe 5 is predicted to form at equilibrium above ∼1100 K, while the ε phase is stable only below ∼1500 K, and FeBe 2 is stable at all temperatures below melting. Small additions of Al are found to stabilise FeBe 5 over FeBe 2 and ε , while at high Al content, AlFeBe 4 is predicted to form. Deviations from stoichiometric compositions are also considered and found to be important in the case of FeBe 5 and ε . The propensity for disordered vs ordered structures is also important for AlFeBe 4 (which exhibits complete Al–Fe disordered at all temperatures) and FeBe 5 (which exhibits an order–disorder transition at ∼950 K). [ABSTRACT FROM AUTHOR]

The effect of cluster configuration on vacancy mediated uranium migration is investigated for stoichiometric UO 2 and hyper-stoichiometric UO 2+ x . The minimum enthalpy pathway involves reconfiguration of the most stable cluster to a metastable configuration, in stoichiometric and hyper-stoichiometric systems. Additionally, a much larger number of alternative metastable configurations were identified for UO 2+ x compared to UO 2 . This would lead to a larger contribution to the Arrhenius pre-exponential term with hyper-stoichiometry. Consequently, the often made assumption that re-orientation of the ground state cluster offers access to the migration activation energy cannot be assumed to be valid. [ABSTRACT FROM AUTHOR]

Abstract: Accommodation of excess oxygen in CeO2, ThO2 and UO2 has been investigated using ab-initio modelling. Calculations indicate that hyperstoichiometry is preferentially accommodated by the formation of peroxide species in CeO2 and ThO2 but not in UO2, where oxygen interstitial defects are dominant. Migration of the excess oxygen defects was also studied; the peroxide ion in CeO2 and ThO2 is transported via a different mechanism, due to the formation of peroxide molecules, to the oxygen interstitial in UO2. Frenkel pair defects were investigated to understand if the interstitial component could assume a peroxide like configuration in the vicinity of the vacancy. While it was already expected that this would not be the case for UO2 since peroxide was not stable, it was also not found to be the case for CeO2 and ThO2 with the peroxide disassociating into a lattice species and a separate interstitial ion. [Copyright &y& Elsevier]

Abstract: The accommodation of trivalent oxides (M2O3) in uranium dioxide has been investigated using atomic scale simulation. Calculations suggest that all trivalent oxides studied preferentially enter UO2 by associating the substitutional ion with an oxygen vacancy, larger cations forming the cluster . Solution into hyper-stoichiometric UO2+x was accompanied by the formation of the cluster and smaller solution energies than into stoichiometric UO2. Solubility is a particularly strong function of hyper-stoichiometry for smaller cations such as Cr3+, which has implications for the use of Cr2O3 as a grain enlarger but not so for larger cations such s Gd3+. [Copyright &y& Elsevier]

Abstract: CrUO4 and (Cr,Al)UO4 have been fabricated by a sol–gel method, studied using diffraction techniques and modelled using empirical pair potentials. Cr2O3 was predicted to preferentially form CrUO4 over entering solution into hyper-stoichiometric UO2+x by atomic scale simulation. Further, it was predicted that the formation of CrUO4 can proceed by removing excess oxygen from the UO2 lattice. Attempts to synthesise AlUO4 failed, instead forming U3O8 and Al2O3. X-ray diffraction confirmed the structure of CrUO4 and identifies the existence of a (Cr,Al)UO4 phase for the first time (with a maximum Al to Cr mole ratio of 1:3). Simulation was subsequently used to predict the partition energies for the removal of fission products or fuel additives from hyper-stoichiometric UO2+x and their incorporation into the secondary phase. The partition energies are consistent only with smaller cations (e.g. Zr4+, Mo4+ and Fe3+) residing in CrUO4, while all divalent cations are predicted to remain in UO2+x . Additions of Al had little effect on partition behaviour. The reduction of UO2+x due to the formation of CrUO4 has important implications for the solution limits of other fission products as many species are less soluble in UO2 than UO2+x . [Copyright &y& Elsevier]

Abstract: Nuclear nonproliferation, medical imaging and high energy physics activities require next generation scintillator materials with improved energy resolution, stopping power, linearity of response and speed. Such materials can be obtained either through the discovery of new compounds or through the optimization of existing materials. Atomistic simulation techniques can make valuable contributions to the optimization of scintillator materials by improving the fundamental understanding of defect structure. Once the defects that contribute to performance limitations have been identified, it becomes easier to remove them from the system; atomistic simulation can also be used to guide the process. In this paper, defects originating from intrinsic disorder, non-stoichiometry, activation, impurities and intentional co-doping in oxide scintillators are discussed. In particular, results are presented for the material systems: bixbyites, perovskites and garnets (where RE represents a 3+ cation ranging in size from to ). The propensity for these point defects to cluster is also discussed. [Copyright &y& Elsevier]