Ab initio molecular dynamics study of the reaction of U+ and U2+ with H2O in the gas phase: direct classical trajectory calculations

Experimental Investigation of Uranium Volatility during Vapor Condensation

The predictive models that describe the fate and transport of radioactive materials in the atmosphere following a nuclear incident (explosion or reactor accident) assume that uranium-bearing particulates would attain chemical equilibrium during vapor condensation. In this study, we show that kinetically driven processes in a system of rapidly decreasing temperature can result in substantial deviations from chemical equilibrium. This can cause uranium to condense out in oxidation states (e.g., UO₃ vs UO₂) that have different vapor pressures, significantly affecting uranium transport. To demonstrate this, we synthesized uranium oxide nanoparticles using a flow reactor under controlled conditions of temperature, pressure, and oxygen concentration. The atomized chemical reactants passing through an inductively coupled plasma cool from ∼5000 to 1000 K within milliseconds and form nanoparticles inside a flow reactor. The ex situ analysis of particulates by transmission electron microscopy revealed 2-10 nm crystallites of fcc-UO₂ or α-UO₃ depending on the amount of oxygen in the system. α-UO₃ is the least thermodynamically preferred polymorph of UO₃. The absence of stable uranium oxides with intermediate stoichiometries (e.g., U₃O₈) and sensitivity of the uranium oxidation states to local redox conditions highlight the importance of in situ measurements at high temperatures. Therefore, we developed a laser-based diagnostic to detect uranium oxide particles as they are formed inside the flow reactor. Our in situ measurements allowed us to quantify the changes in the number densities of the uranium oxide nanoparticles (e.g., UO₃) as a function of oxygen gas concentration. Our results indicate that uranium can prefer to be in metastable crystal forms (i.e., α-UO₃) that have higher vapor pressures than the refractory form (i.e., UO₂) depending on the oxygen abundance in the surrounding environment. This demonstrates that the equilibrium processes may not dominate during rapid condensation processes, and thus kinetic models are required to fully describe uranium transport subsequent to nuclear incidents.

Can water continuously oxidize the PuO molecule? Mechanisms, topological analysis and rate constant calculations

A detailed description of the PuO continuously oxidized by water in the gas-phase.

Theoretical study of Ni+ assisted C-C and C-H bond activations of propionaldehyde in the gas phase

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The DFT investigation of Ni⁺ assisted decomposition of propionaldehyde has been applied.

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Interestingly, two competitive reaction paths have been found.

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The C—C activation path is kinetically more favorable than the C—H activation path.

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The bonding analysis indicated that the initial complex, Ni⁺(C₃H₆O), is formed by electrostatic interaction.

The reactions of Ni⁺ with propionaldehyde in the gas phase have been systematically investigated using density functional theory at the B3LYP/def2-TZVP level. The decomposition reaction mechanism has been identified. Our calculations indicated that Ni⁺ can assist decomposition of propionaldehyde to form Ni⁺CO and C₂H₆ through two types of reaction channel: C—C bond activation and C—H bond activation. In addition, charge decomposition analysis (CDA) was carried out to obtain a deeper understanding for orbital interaction of the initial complex. The bonding properties of the species involved were discussed by means of diverse analysis methods including electron localization function (ELF) and atoms in molecules (AIM).

Gas-phase COS activation by U+: Reaction mechanisms and bonding analysis

Density functional theory (DFT) calculations were used to investigate the gas phase reaction of U[Formula: see text] with COS to produce US[Formula: see text]CO and UO[Formula: see text]CS. It is shown that the two reactions are exothermic and the formation of UO[Formula: see text]CS has the lower energy barrier which agrees with the experimental result that UO[Formula: see text] is the main product. The reaction mechanisms and the potential energy profiles (CPEPs) considering different spin states were presented in detail. Diverse analyses including atoms in molecules, natural bond orbital were used to study the bonding properties of all the involved species.