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

Investigating chemical and molecular changes in uranium oxyfluoride particles using NanoSIMS and Micro-Raman Spectroscopy

Processes involving the treatment of nuclear material inherently release small amounts of this material to the environment. Environmental sampling is therefore an important tool in the detection of undeclared nuclear activities. Environmental samples taken at enrichment facilities typically contain particles of uranium oxyfluoride (UO2F2) formed from the hydrolysis of UF6. Somewhat surprisingly, these samples were also found to contain uranium-bearing particles without a measurable amount of fluorine, suggesting UO2F2 is unstable with respect to the loss of fluorine. As environmental sampling depends upon laboratory analysis of nuclear material that has often been exposed to the environment after it was produced, it is important to understand how those environmental conditions might have changed the material over time. Previous studies have shown that exposure to high temperature, high relative humidity and ultraviolet light accelerates the loss of fluorine in UO2F2 particles, yet the conditions under which this occurs and the chemical and molecular changes that result from this decomposition are not well understood. This work aims to provide a better understanding of the chemical and molecular changes in UO2F2 particles from the exposure to specific environmental conditions, and in particular from the exposure to air of different relative humidity. To this end, UO2F2 particulate material was prepared from the controlled hydrolysis of UF6 expressly for the purpose of these experiments. Particles were measured by ultrahigh spatial resolution secondary ion mass spectrometry (NanoSIMS) and micro-Raman spectroscopy before and after exposure to air at different relative humidity. These measurements demonstrated that even though the decomposition of UO2F2 is very slow, NanoSIMS and Raman spectroscopy can be applied to distinguish subtle differences depending on the environment to which the samples were exposed. The experiments described in this paper also demonstrated that a combination of analytical techniques is the best approach to characterize UO2F2 particles produced from UF6 hydrolysis.JRC.D.2-Standards for Innovation and sustainable Developmen

"Multi-Dynamic" Isotope Ratio Measurements for Uranium and Plutonium using the "Multiple Ion Counting" Detection System of the TRITON TIMS at IRMM

A "Multiple Ion Counting" (MIC) detection system installed into a thermal ionization mass spectrometer (TIMS) allows the simultaneous detection of up to seven small ion beams with currents of 10-19 - 10-14 Ampere in ion counting mode, corresponding to count rates of 1-60.000 cps (counts per second). In order to circumvent complicated inter-calibration routines for the given set of 7 ion counters the principle of multi-dynamic measurements was implemented in combination with the "Multiple Ion Counting" (MIC) system. The multi-dynamic measurement procedure was applied to diluted samples of isotope reference materials for uranium and plutonium with concentrations at the ppb and the ppt level, respectively. The results for uranium measurements on samples IRMM-186 and NBL-U500 clearly show an improved precision by a factor of ca. 3 for the multi-dynamic measurement procedure compared to measurements carried out using one single ion counter in peak-jumping mode. When using the "Multiple Ion Counting" (MIC) system, the multi-dynamic procedure is also superior compared to the static total evaporation technique, as demonstrated by plutonium measurements on samples of NBL-CRM137. As a conclusion, the multi-dynamic procedures provide improved precision and accuracy compared to previous TIMS measurement techniques. The multi-dynamic measurement technique in combination with the "Multiple Ion Counting" (MIC) system of the TRITON TIMS is suitable for the isotopic analysis on low-level uranium and plutonium samples as well as single uranium or plutonium oxide particles for nuclear safeguards purposes, and will be applied also for the verification analysis of reference particles made of UF6 as produced by IRMM.JRC.D.4-Isotope measurement

New insight into UO2F2 particulate structure by micro-Raman spectroscopy

Uranyl fluoride particles produced via controlled hydrolysis of uranium hexafluoride have been deposited on different substrates: polished graphite disks, silver foil, stainless steel and gold-coated silicon wafer, and measured with micro-Raman spectroscopy (MRS). All three metallic substrates enhanced the Raman signal delivered by UO2F2 in comparison to the graphite one. The fundamental stretching of the U-O band appeared at 867cm-1 in case of the graphite substrate, while in case of the others it was shifted to lower frequencies (847-839 cm-1). All applied metallic substrates showed the expected effect of Raman signal enhancement with respect to Uranyl stretching vibration; however the gold layer appeared most effective. Application of this improved method for MRS analysis of uranyl fluoride particles could be found in the field of nuclear safeguards and environmental analysis.JRC.D.2-Standards for Innovation and sustainable Developmen

NUSIMEP-6: Uranium Isotope Amount Ratios in Uranium Particles

The IRMM Nuclear Signatures Interlaboratory Measurement Evaluation Programme (NUSIMEP) is an external quality control program organised by IRMM with the objective of providing materials for measurements of trace amounts of nuclear materials in environmental matrices. Measurements of the isotopic ratios of the elements uranium and plutonium in small amounts, such as typically found in environmental samples, are required for nuclear safeguards, for the control of environmental contamination and for the detection of nuclear proliferation. Participants in NUSIMEP compare their reported measurement results with independent external certified reference values with demonstrated traceability and uncertainty, as evaluated according to international guidelines. NUSIMEP-6 focused on measurements of Uranium Isotope amount rations in uranium particles aiming to support laboratories involved in uranium particles analysis. It was the first NUSIMEP on particle analysis and particularly also organised for the IAEA network of analytical laboratories of environmental sampling (NWAL) as part of the IRMM activities in the frame of the EC support programme to IAEA. The NUSIMEP test samples were prepared by controlled hydrolysis of well certified uranium hexafluoride in an aerosol deposition chamber at IRMM. Participating laboratories in NUSIMEP-6 received a test sample of uranium particles on a graphite planchet with undisclosed isotope amount ratios values n(234U)/n(238U), n(235U)/n(238U) and n(236U)/n(238U). The uranium isotope amount ratios were to be measured using their routine analytical procedures. Measurement of the major ratio n(235U)/n(238U) was obligatory; measurements of the minor ration n(234U)/n(238U) and n(236U)/n(238U) were optional. 15 Institutes reported measurement results, among those 7 NWAL laboratories. The analytical methods applied were SIMS, (FT)-TIMS, LA-ICP-MS and alpha spectrometry. The participants' measurement results were evaluated against the certified reference values. The results of NUSIMEP-6 confirm the capability of laboratories in measuring the major ratio n(235U)/n(238U) in uranium particles. More difficulties were observed for the minor isotope ratios n(234U)/n(238U) and n(236U)/n(238U). In addition feedback from the participants was collected in view of improvements and optimisation of future NUSIMEP interlaboratory comparisons for uranium isotope amount ratios.JRC.D.2-Reference material