Trimolecular Reactions of Uranium Hexafluoride with Water

Abstract

The hydrolysis reaction of uranium hexafluoride (UF 6 ) is a key step in the synthesis of uranium dioxide (UO 2 ) powder for nuclear fuels. Mechanisms for the hydrolysis reactions are studied here with density f unctional theory a nd t he Stuttgart s mall-core scalar relativistic pseudopotential and associated basis set for uranium. The reaction of a single UF 6 molecule with a water molecule in the gas phase has been previously predicted to proceed over a relatively sizeable barrier of 78.2 kJ·mol -1 , indicating this reaction is only feasible at elevated temperatures.Given the observed formation of a second morphology for the UO 2 product coupled with the observations of rapid, spontaneous hydrolysis at ambient conditions, an alternate reaction pathway must exist. In the present work, two trimolecular hydrolysis mechanisms are studied with density functional theory: (1) the reaction between two UF 6 molecules and one water molecule, and (2) the reaction of two water molecules with a single UF 6 molecule. The predicted reaction of two UF 6 molecules with one water molecule displays an interesting "fluorine-shuttle" mechanism, a significant energy barrier of 69.0 kJ·mol -1 to the formation of UF 5 OH, and an enthalpy of reaction (ΔH 298 ) of +17.9 kJ·mol -1 . The reaction of a single UF 6 molecule with two water molecules displays a "proton-shuttle" mechanism, and is more favorable, having a slightly lower computed energy barrier of 58.9 kJ·mol -1 and an exothermic enthalpy of reaction (ΔH 298 ) of -13.9 kJ·mol -1 . The exothermic nature of the overall UF 6 + 2·H 2 O trimolecular reaction and the lowering of the barrier height with respect to the bimolecular reaction are encouraging; however, the sizable energy barrier indicates further study of the UF 6 hydrolysis reaction mechanism is warranted to resolve the remaining discrepancies between the predicted mechanisms and experimental observations. Lind et al.,

Kinetic investigation of the hydrolysis of uranium hexafluoride gas

“…Previous efforts to characterize this reaction have focused on spectroscopic measurements [5][6][7] and computational studies. [8][9][10][11][12][13] The experimental studies have focused on the spectroscopic detection of uranium species but lacked the temporal resolution to observe the reaction or identify any transient species. Unfortunately, there are major discrepancies between the experimentally observed reaction and computationally predicted mechanisms.…”

“…Unfortunately, there are major discrepancies between the experimentally observed reaction and computationally predicted mechanisms. For example, the computational studies suggest large thermodynamic energy barriers that would likely prevent the reaction of UF 6 and water vapor, [9][10][11][12][13] yet the hydrolysis reaction is known to occur quickly and completely under ambient conditions. [5][6][7] Additionally, one of the long-suspected intermediates of this reaction, UOF 4 , has not been successfully identied under extreme conditions designed to optimize its production such as reacting water with a 5000-fold excess of UF 6 .…”

Formation of Aerosol Nanoparticles by Gas-Phase Hydrolysis Reaction of Uranium Hexafluoride

“…The uranyl ion forms solid salt species (e.g., UO2F2) (called "particles" or "particulate matter" hereafter) that are generally not a commodity to the nuclear industry but play a key intermediate role in UF6 applications, such as in the production of UO2 powder for nuclear fuel production (Hou et al 2007, Lind et al 2010 or in the verification of weapons declarations as a unique tracer for nuclear sites (Kemp 2008(Kemp , 2016. Therefore, it is of great importance to improve our understanding of the formation mechanism and the evolution of properties of this particulate species in nuclear manufacturing processes and its fate as they are produced in relevant environmental conditions.…”

“…When ω ≥ 3, the first step of hydrolysis becomes exothermic, and the HF tends to associate with UF5OH and H2O tightly. Lind et al (2010) showed that hydrolysis of one UF6 and two H2O molecules is the favorable pathway as a tri-molecular reaction. One water molecule provides the hydrogen to the nascent HF in the transition state that accepts one hydrogen from the other water molecule that is reacting with UF6 to form UF5OH.…”

Morphologic and chemical characterization of products from hydrolysis of UF6

“…These studies suggested a complex reaction pathway from UF 6 to UOF 4 through a UF 5 OH intermediate, and reported binding energies of formation and vibrational frequencies of molecular fragments [7][8][9]. A subsequent study using DFT with a small-core, scalar relativistic pseudopotential proposed that trimolecular mechanisms are likely involved in formation of the UF 5 OH intermediate [10]. While the structure and vibrational spectroscopy of UOF 4 has been reported, the UOF 4 intermediate has not been measured directly during hydrolysis of UF 6 in the presence of either excess or substoichiometric water concentrations [5,6 and refs therein].…”