Quantifying Local Atomic Distortions in UO2 Grain Boundaries: Correlation with Energetic and Electronic Properties.

UO2, as a key material in the nuclear industry, is composed of grains or crystallites in real applications. Their interfaces, known as grain boundaries (GBs), significantly impact thermal conductivity, corrosion resistance, and mechanical response. Here, utilizing Hubbard-corrected density functional theory, we systematically examine the local cluster structures, energetic stabilities, and electronic properties of five typical tilt UO2 GBs ranging from Σ 3 to Σ 11. We categorize all possible distorted U- and O-centered clusters at these GBs and identify their cluster morphologies and radial and angular distortions. Our results highlight the abundance of new U-O bonds stretching to "medium-range", a feature often overlooked in conventional coordination analysis. To quantitatively describe these distorted clusters, we use smooth overlap of atomic positions (SOAP) to represent the structural and chemical local environments, which takes into account both radial (2-body) and angular (3-body) distortions. We define a dissimilarity index by computing the inner product of SOAP descriptors between the distorted and the perfect motif in ideal UO2. Our findings show that the medium-range SOAP dissimilarity correlates well with the GB excess energy, outperforming metrics such as dangling bonds or bonding strain. Furthermore, it is found that the band gaps in sufficiently high-energy GBs are shortened, with excess states primarily contributed by the distorted U clusters. Our results present a comprehensive gallery of the local distorted clusters introduced by typical UO2 GBs and have implications on the structure-property relations of GBs and other interfaces.

First-Principles Study on the Adsorption Behavior of O2 on the Surface of Plutonium Gallium System.

To deeply understand the adsorption process of oxygen on the surface of a plutonium gallium system and to reveal the chemical reaction mechanism at the initial stage of oxidative corrosion on the surface of plutonium gallium alloy at a theoretical level, the adsorption behavior of oxygen molecules on the surface of a plutonium gallium system was investigated by a first-principles approach based on density flooding theory. The results show that the molecular bond length increases and finally breaks when the surface oxygen molecule is adsorbed on the surface of plutonium gallium system and dissociates into two atomic states. The most likely adsorption position of oxygen molecules on the surface of plutonium gallium system is hole-site vertical adsorption with the adsorption energy size of 10.7 eV. The bonding between oxygen atom and surface is mainly due to the overlapping hybridization of Pu-6s, Pu-7s, Pu-6d, Ga-3d and O-2p orbitals. Oxygen molecules mainly interact with the atoms of the first layer on the surface of the plutonium gallium system. The oxygen atoms after stable adsorption are able to diffuse to the subsurface of the plutonium gallium system after overcoming the energy barrier of 16.7 eV and form a stable structure. The research results reveal the initial reaction process and adsorption law of oxygen on the surface of plutonium gallium system from microscopic level, which is helpful to further explore the surface corrosion prevention technology of plutonium gallium system and improve the reliability and safety of plutonium gallium alloy components.