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

by Christian Mark G Salvador, Jason M Richards, Shannon M Mahurin, Meng-dawn Cheng, Joshua Hubbard
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Reaction Chemistry & Engineering
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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.