Measuring fluorine in uranium oxyfluoride particles using secondary ion mass spectrometry for nuclear forensics

Abstract

Abstract Gaseous uranium hexafluoride (UF6) is used for the enrichment of uranium and is known to be a very reactive and volatile gas. When a small amount of UF6 is released in the environment, it forms uranium oxyfluoride (UO2F2) particles and hydrogen fluoride from the reaction with atmospheric moisture. The UO2F2 particles settle on various surfaces within the enrichment facility where they are collected by safeguards organizations such as the International Atomic Energy Agency (IAEA) through a technique called environmental sampling. Despite their small size, the uranium isotopic composition of these particles is used to determine whether an enrichment facility is compliant with its declarations. Additional and complementary information on the particles' source, formation process and exposure history can be obtained from elemental, molecular and morphological analysis. As fluorine is not always detected in uranium particles originating from enrichment facilities, it is assumed that UO2F2 is unstable under certain environmental conditions. This study aims to determine how those environmental conditions affect the relative amount of fluorine in UO2F2 particles. Expressly for the purpose of this work, we prepared a set of UO2F2 particle samples from the hydrolysis of UF6 and stored these samples in environmental chambers at different temperature, humidity and lighting conditions. The NanoSIMS secondary ion mass spectrometer, with a nanometer-scale spatial resolution, was used to measure the F+/U+ secondary ion ratio of individual particles immediately after sample preparation, and at different time intervals. NanoSIMS analyses on single particles stored up to 7 months in dry air did not detect a significant decrease in the relative amount of fluorine. The exposure to high humidity however, resulted in a much lower F+/U+ ratio, up to 2 orders of magnitude below that of the particles stored in dry air. It should be noted that fluorine was still detected in these samples, even after 7 months of high humidity exposure.