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The thermal and magnetic properties of uranium dioxide, a prime nuclear fuel and thoroughly studied actinide material, remain a long standing puzzle, a result of strong coupling between magnetism and lattice vibrations. The magnetic state of this cubic material is characterized by a 3-k non-collinear antiferromagnetic structure and multidomain Jahn-Teller distortions, likely related to its anisotropic thermal properties. Here we show that single crystals of uranium dioxide subjected to strong magnetic fields along threefold axes in the magnetic state exhibit the abrupt appearance of positive linear magnetostriction, leading to a trigonal distortion. Upon reversal of the field the linear term also reverses sign, a hallmark of piezomagnetism. A switching phenomenon occurs at ±18 T, which persists during subsequent field reversals, demonstrating a robust magneto-elastic memory that makes uranium dioxide the hardest piezomagnet known. A model including a strong magnetic anisotropy, elastic, Zeeman, Heisenberg exchange, and magnetoelastic contributions to the total energy is proposed.

Accurately predicting changes in the thermal conductivity of light water reactor UO 2 fuel throughout its lifetime in reactor is an essential part of fuel performance modeling. However, typical thermal conductivity models from the literature are empirical. In this work, we begin to develop a mechanistic thermal conductivity model by focusing on the impact of gaseous fission products, which is coupled to swelling and fission gas release. The impact of additional defects and fission products will be added in future work. The model is developed using a combination of atomistic and mesoscale simulation, as well as analytical models. The impact of dispersed fission gas atoms is quantified using molecular dynamics simulations corrected to account for phonon-spin scattering. The impact of intragranular bubbles is accounted for using an analytical model that considers phonon scattering. The impact of grain boundary bubbles is determined using a simple model with five thermal resistors that are parameterized by comparing to 3D mesoscale heat conduction results. However, when used in the BISON fuel performance code to model four reactor experiments, it produces reasonable predictions without having been fit to fuel thermocouple data.

The stabilities of selected fission products-Xe, Cs, and Sr-are investigated as a function of non-stoichiometry x in UO(2 ± x). In particular, density functional theory (DFT) is used to calculate the incorporation and solution energies of these fission products at the anion and cation vacancy sites, at the divacancy, and at the bound Schottky defect. In order to reproduce the correct insulating state of UO(2), the DFT calculations are performed using spin polarization and with the Hubbard U term. In general, higher charge defects are more soluble in the fuel matrix and the solubility of fission products increases as the hyperstoichiometry increases. The solubility of fission product oxides is also explored. Cs(2)O is observed as a second stable phase and SrO is found to be soluble in the UO(2) matrix for all stoichiometries. These observations mirror experimentally observed phenomena.

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The aluminate perovskites YAlO3 (YAP) and LuAlO3 (LuAP) have been identified as potential scintillator materials due to their high light output and short decay time. However, the performance of these materials is significantly reduced by point defects. In this paper, atomistic simulations provide insight into the types of point defects that are expected under various conditions in YAP and LuAP, as well as other REAlO3 compounds (where RE denotes a rare earth ion ranging from Lu to La or Y). For example, we predict that cation antisites are the dominate intrinsic defect for smaller REAlO3 compounds, with the concentration of Schottky-type defects increasing for compounds with larger RE ions. We also predict that cation vacancies will be present in association with the oxidation of the Ce activator. From these results, we show how defects affect different aspects of the scintillation process. Our aim is to provide information that can be used to aid in the intelligent optimization of this family of scintillator compounds.

The ionic compound cesium chloride adopts a cubic crystal structure bearing the same name. However, ab initio electronic structure calculations based on density functional theory methods using generalized gradient approximation functionals do not predict that cesium chloride adopts this phase. In this paper we apply semiempirical methods (density functional theory plus a pairwise dispersion correction) to account for missing van der Waals interactions within cesium chloride. The C 6 and R 0 dispersion parameters for cesium are established within Grimme's DFT + D2 formalism. Inclusion of the dispersion corrections is found not only to improve the quality of structures in comparison to experiment for all cesium halides, but also leads to the correct prediction of the ground-state phase under ambient conditions.

It is well known that Xe, being insoluble in UO 2 , segregates to dislocations and grain boundaries, where bubbles may form resulting in fuel swelling. Less well known is how sensitive this segregation is to the structure of the dislocation or grain boundary. In this work, we employ pair potential calculations to examine Xe segregation to dislocations (edge and screw) and several representative grain boundaries (Σ5 tilt, Σ5 twist and random). Our calculations predict that the segregation trend depends significantly on the type of dislocation or grain boundary. In particular, we find that Xe prefers to segregate strongly to the random boundary as compared to the other two boundaries and to the screw dislocation rather than the edge. Furthermore, we observe that neither the volumetric strain nor the electrostatic potential of a site can be used to predict its segregation characteristics. These differences in segregation characteristics are expected to have important consequences for the retention and release of Xe in nuclear fuels. Finally, our results offer general insights into how atomic structure of extended defects influence species segregation.I. INTRODUCTION Segregation can be understood as the interaction between an isolated zero dimensional defect (in this case, an impurity) and multidimensional defects such as dislocations (1D), grain boundaries (GB) and free surfaces (3D). 11 The driving force for this process is the energy difference (segregation energy) between the isolated impurity in the bulk and that associated with an extended structural defect. If the energy at the structural defect site is lower than in the bulk, the local impurity concentration is higher at the structural defect than in bulk and conversely, if the energy at the defect site is higher than in the bulk, the impurity will remain in the bulk. Segregation phenomena influence many material properties such as ion transport (which has a strong effect, for example on sintering rates), electrical and chemical reactivity and grain growth.2 At higher concentrations, segregating impurities are known to form glassy or ordered phases and can cause structural transformations of the GB. 2,3 In the case of UO 2 nuclear fuel, fission gases such as xenon (Xe) are insoluble in the fuel matrix. 45 Therefore, Xe tends to segregate to dislocations and GBs forming fission gas bubbles. Xe may diffuse along the short circuit paths provided by these two defects and be released into the plenum region between the fuel rod and the cladding. 5 If the gases are released from the fuel, they contribute to the gaseous atmosphere within the fuel pin and the fuel pin internal pressure correspondingly increases; this can contribute to failure of the fuel pin. If these gases are retained inside the fuel, they form bubbles, which lead to swelling of the fuel matrix. Swelling is detrimental to fuel performance as it contributes to fuel-cladding mechanical interaction (FCMI); the resulting stresses can shorten the lifetime of the pin. 7 Swelling and release are compleme...