Enhanced Oxygen Isotope Determination in Uranium Oxides Using BrF5 Fluorination

Abstract

A new method for accurate determination of oxygen isotopes in uranium oxides encountered in the nuclear fuel cycle was developed using the conventional BrF5 fluorination technique. Laser‐assisted fluorination was tested for comparison. We focused on fine powders of triuranium octoxide (U3O8), uranium dioxide (UO2±x with 0 ≤ x ≤ 0.25), uranium trioxide (UO3.nH2O, with 0.8 ≤ n ≤ 2) and diuranates (M2U2O7.nH2O, with M = NH4, Na or Mg0.5 and 0 ≤ n ≤ 6). Fluorination at room temperature and heating under vacuum at 150 °C are shown to eliminate both adsorbed and structural water from the powder samples. Precision fit for purpose of δ18O values (± 0.3‰, 1s) and oxygen yields (close to 100%) were obtained for U3O8 and UO2 where oxygen is only bound to uranium. A lower precision was observed for UO3.nH2O and M2U2O7.nH2O where oxygen is both present in the structural H2O and bonded to uranium and where the extracted O2(g) can be contaminated by NF3 and NOx compounds. Laser‐assisted fluorination gave shifted δ18O values between +0.8 and +1.4‰ for U3O8, around −0.8‰ for UO3.nH2O and between −3.9 and −4.5‰ for M2U2O7.nH2O (± 0.3‰, 1s) compared with the conventional method.