Determination of intermediates and products of the uranyl aerosol formation in UF6 hydrolysis in the gas phase†

The reaction pathway of hydrolysis of UF₆ to form UO₂F₂ particles is an essential insight in nuclear fuel processing; however, it is still limited to theoretical calculations. Herein, we present the identification of the intermediates and products using various gas precursor concentrations and molecular beam mass spectrometer (MBMS). Compounds containing different uranium atom counts were identified by exposing 300 and 2323 ppm water to 200 ppm UF₆. Non-uranium compounds (e.g., (HF)₃(H₂O)H, (HF)₄H, and (H₂O)₂(HF)₃) dominate the mass spectra in terms of absolute signal intensity. These compounds were dependent on the initial concentration of UF₆ based on the linear relationship observed between products and gas reactant. Uranium compounds were characterized by UF₆, UO₃, and UO₂F₂ core molecules, with each species existing predominantly in a certain water concentration. Monomeric compounds (e.g., UF₆(HF)₂(H₂O)₇, UO₂F₂(HF)₇H, and UO₂F₂(HF)₅(H₂O)₃) or species with one uranium atom had high fluorine to uranium ratio (F/U) due to several HF units bonded with the uranium core. Dimeric (e.g. (UO₂F₂)₂(H₂O) and (UF₆)₂(H₂O)4(HF)₃H) and trimeric (e.g., (UO₃)(UO₂F₂)₂(HF)(H₂O)₃ and (UO₂F₂)₂UF₆H₂F) compounds persisted in high masses with low F/U and H/U ratios. Moreover, ramping of UF₆ concentration (50–231 ppm) at fixed water content (1.3% Rh or 300 ppm) showed different trends among 949 ions, with some following consistently with molecular identification (e.g., (UO₃)₃(HF)₂(H₂O)H). Overall, this study provided important information regarding the formation pathway of UO₂F₂, which will be essential in chemical modelling studies. The vast information generated from mass spectrometric runs merits cluster evaluation and factorization to yield more information on the U–O–F system.