Development of Uranium Reference Particles to Support Nuclear Safeguards

Controlled hydrolysis of certified uranium hexafluoride, carried out in a specifically constructed aerosol chamber, leads to the production of uranyl fluoride particulates which ¿deposited on a graphite planchet support ¿ may be used as a quality control reference material. The particle size and surface distribution depends on several parameters, such as relative humidity inside the aerosol chamber and the concentration of gaseous uranium hexafluoride, determined by the distillation conditions. The best quality samples were obtained a RH ranging from 55 to 70% and low gas amount. These improvements for a single deposition were used to prepare a reference sample with two different uranium enrichments. Preliminary SIMS measurements proved good results with respect to both type of particles. It was also revealed that ¿ in some case ¿ the micrometer size uranium particles are accompanied by a large medium background.JRC.DG.D.2-Reference material

Development of Uranium Reference Particles for Nuclear Safeguards and Non-proliferation Control

In the oversight of the nuclear Non-Proliferation Treaty and as part of the Additional Protocol of the International Atomic Energy Agency, environmental sampling has become an important tool for the detection of non-declared nuclear activities. One extensively developed technique in environmental sampling (ES) makes use of pieces of cotton cloth called swipes to wipe surfaces in and around a nuclear facility. The dust collected on these swipes typically contains micrometer-sized uranium particles with an isotopic composition characteristic for the processes at the inspected facility. Since its implementation in the 1990s, ES has proven to be a very effective tool in the detection of clandestine activities owing to a number of highly sensitive and selective techniques, including secondary ion mass spectrometry and thermal ionisation mass spectrometry. However, considering the potential consequences of the ana lyses, these measurements need to be subjected to a rigorous quality management system. In a continuous effort to improve the accuracy and detection efficiency of the uranium isotope ratio measurements, uranium particle reference materials are being developed by different research groups. It was concluded however, that the existing methods for the production of particulate reference materials generally do not reproduce the particles recovered from swipe samples. For this reason, we developed the aerosol deposition chamber at the Institute for Reference Materials and Measurements for the production of reference uranium particles that are representative of the particles collected at enrichment facilities. This method is based on the controlled hydrolysis of milligram amounts of uranium hexafluoride with a certified uranium isotopic composition. After optimization of the experimental set-up, the particles produced by the aerosol deposition chamber were characterized by scanning electron microscopy, transmission electron microscopy, mu-Raman spectroscopy and secondary ion mass spectrometry. The particle morphology and composition were found to be dependent on the relative humidity of the air, the exposure to ultraviolet light and the time elapsed after formation. Possible correlations between the relative amount of fluorine and the age of the particles were investigated. These results were the starting point for the first inter-laboratory measurement evaluation programme (NUSIMEP) on uranium particles.JRC.D.4-Isotope measurement

The Preparation of Uranium Oxyfluoride Particles for Inter-Laboratory Measurement Campaigns

Environmental sampling is based on the collection and analysis of micrometer-sized uranium particles from nuclear installations. In order to improve the accuracy and detection efficiency, reference uranium particles are needed that are similar to the particles found in swipes. The method developed at IRMM is based on the controlled hydrolysis of UF6 with a certified isotopic composition. The morphology and composition of the particles was dependent on the relative humidity of the air, the exposure to ultraviolet light and the time elapsed after formation. The particles will be used for nuclear signatures inter-laboratory measurement evaluation programs (NUSIMEP).JRC.D.4-Isotope measurement

Fluorine as a Safeguards Tool for Age Dating of Uranium Oxyfluoride Particles?

The connection between the fluorine in the particles and their age has been investigated. The influence of ultraviolet light exposure combined with the time elapsed after preparation has been investigated on uranium oxyfluoride particles prepared at the IRMM using an aerosol deposition chamber. The presence of fluorine in particles was confirmed by energy-dispersive X-ray analysis. Semi-quantititative information was obtained by evaluating peak height ratios obtained by secondary ion mass spectrometry. Possible correlations existing between the amount of fluorine and the particle age could be investigated, taking into account the environmental conditions to which the uranium oxyfluoride particles were exposed.JRC.D.4-Isotope measurement

Fluorine as a Safeguards Tool for Age-Dating of Uranium Oxyfluoride Particles?

When uranium hexafluoride (UF6) is released into the environment, it reacts with the atmospheric moisture forming uranium oxyfluoride particles and HF. Fluorine-containing compounds such as HF are recognized as signatures for enrichment activities, but in this study the connection between the fluorine in the particles and the age of the particles has been investigated. Establishing this link is not evident, as uranium oxyfluoride particles are highly hygroscopic and little is known about their reaction mechanisms and stability. There are various environmental factors such as time, humidity and light or heat exposure that could affect the amount of fluorine, and even after the particles are collected the fluorine could decrease further due to sample preparation or storage conditions. The influence of (ultraviolet) light exposure combined with the time elapsed after preparation has been investigated on uranium oxyfluoride particles prepared at IRMM using the aerosol deposition chamber. The presence of fluorine in single particles was confirmed by SEM-EDX. Semi-quantitative information was obtained by evaluating specific SIMS peak height ratios. With these measurements, possible correlations existing between the amount of fluorine and the age of a particle could be investigated, taking into account the environmental conditions to which the uranium oxyfluoride particles were exposed.JRC.D.4-Isotope measurement

Micro-Raman Spectroscopy of Uranium Oxyfluoride Particulate Material for Nuclear Safeguards

Environmental samples collected by safeguards organizations such as IAEA often contain particles of uranium oxyfluoride. UO2F2 is a compound formed from the hydrolysis of uranium hexafluoride. In order to investigate its changes in the environment, uranium oxyfluoride particulate material was measured by micro-Raman spectroscopy. The uranyl symmetric stretching frequency was detected in the majority of the spectra. Depending on the conditions under which the particles were prepared and stored, additional peaks and features appeared in the spectrum. Experiments were carried out using different incident laser wavelengths at the lowest possible power setting, as the particles appeared to be very heat-sensitive.JRC.DG.D.2-Reference material