Aregbe, Y.

Kips, R.

Kraiem, M.

Richter, S.

Stefaniak, E.

Truyens, J.

Wellum, R.

UNT Digital Library

 Development of uranium reference particles to support nuclear safeguards    E.A. Stefaniak1, R. Kips2, J. Truyens1, Y. Aregbe1,  M. Kraiem1, S. Richter1, R. Wellum1   1 European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Geel, Belgium  2 Lawrence Livermore National Laboratory, Livermore, California, USA   IAEA Symposium on International Safeguards,  Vienna, Austria 5-10 November 2010   September 17, 2010 LLNL- PROC-455932 Disclaimer and auspices This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, by Pacific Northwest National Laboratory under contract DE-AC05-76RL01830, and under the auspices of the European Commission, DG Joint Research Centre under Action Sheet 36. The analyzed samples were generated by the IRMM expressly for the purpose of this experiment and we have no evidence that fugitive emissions from a uranium enrichment facility would, or would not, appear similar to the analyzed samples.   Development of uranium reference particles to support nuclear safeguards E.A. Stefaniak1, R. Kips2, J. Truyens1, Y. Aregbe1, M. Kraiem1, S. Richter1, R. Wellum1   1 European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Geel, Belgium  2 Lawrence Livermore National Laboratory, Livermore, California, USA   Abstract 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 the relative humidity inside the aerosol chamber and the concentration of gaseous uranium hexafluoride, determined by the distillation conditions. The best quality samples were obtained at 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 types of particles. It was also revealed that – in some cases – the micrometer size uranium particles are accompanied by a large uranium background  Introduction  So-called “environmental sampling” is a powerful tool for the implementation of the Additional Protocol for the application of nuclear safeguards [1] to verify completeness and correctness of a state’s nuclear activities. The swipe samples collected by safeguards inspectors contain U-bearing particles, which are analyzed by the International Atomic Energy Agency (IAEA) network of analytical laboratories (NWAL). In order to strengthen this technique and help them in their analytical performance, there is a need for the development of uranium reference particles that resemble real-life particles and can be used as quality control samples. Production of reference uranium particles has been carried out so far either by the controlled hydrolysis of uranium hexafluoride (UF6) [2, 3, 4] or via nebulization of uranium nitrate solution [5, 6]. In the latter case, the particles are formed from dried droplets of a nebulized aqueous solution.  They are monodisperse and their size depends on the aerosol quality. In case of UF6 hydrolysis, the process is performed in an aerosol chamber [2, 4], where some of the gaseous UF6 reacts with air moisture, forming UO2F2. A similar process also occurs in uranium enrichment plants when small amounts of UF6 are released inadvertently into the atmosphere. UF6 +2H2O → UO2F2 +4HF  The solid phase formed in this reaction is hydrated uranyl fluoride which settles down as sub-micron size particles and HF remaining in the gas phase [7]. Te diameter of uranyl fluoride particles varies with values ranging from 0.5 to 2.2 m and with a majority of particles having a diameter of around 0.7 m [3]. A major factor influencing the particle size is the relative humidity in the aerosol chamber - particles formed at a relative humidity of less than 15% are found to be shaped as branched chains of tiny spheres, while those at 70% RH appear as individual, isolated, regular spheres. A relative humidity of more than 70% leads to progressive dissolution of their surface layers.  One of the ongoing research projects at EC JRC IRMM to support nuclear safeguards in the field of environmental sampling is the production of new certified test samples for NUSIMEP (Nuclear Signatures Interlaboratory Measurement Evaluation Programme) inter-laboratory comparisons, such as uranium reference particles. The particles are prepared from a well-characterized IRMM uranium hexafluoride reference material, certified for major and minor uranium isotope ratios. The first IRMM inter-laboratory comparison devoted to particle analysis, NUSIMEP-6 [8] was very successful with a positive feedback from the participants. This paper focuses on the development of the uranium reference particle production using the preliminary results of the recently optimized procedure for single and double deposition.  Materials and methods  Production of uranium reference particles via UF6 hydrolysis has already been described elsewhere [2,3,4]. Briefly, a small amount of gaseous UF6 trapped in a glass bulb is released, by breaking the bulb,  into a specially-made aerosol chamber, where the RH is controlled by a humidity standard LiCl solution (Rotronic, Switzerland). The amount of UF6 is dependent on the distillation time and temperature; the ampoule with UF6  is kept at -3oC and the time of distillation varies from 20 to 40 s. Once the glass vial is broken inside the chamber, the particle supports, normally polished carbon disks,  are inserted into the chamber to collect the freshly formed particles. The time elapsed between the gas release and particle collection, as well as the time of particle deposition varies from 20 to 40 min and from 3 to 4 hours, respectively. Imaging of the single and double deposition samples was carried out using a tungsten-filament FEI Quanta 200 3D scanning electron microscope (SEM). Secondary ion mass spectrometry (SIMS) measurements were performed applying a NanoSIMS CAMECA SAS, with which  we obtained 10 µm x 10 µm ion images of particles deposited on the planchet with a spatial resolution of 300-400 nm using an O- primary ion beam of about 50 pA.  Results and discussion  NUSIMEP-6 participants’ feedback spurred the efforts made at IRMM to modify and improve the quality of the certified test samples used for inter-laboratory comparisons on uranium reference particles. Major concerns were the surface density of particles, the particle size and the uranium background detected on the disk surface around the particles. The reaction of UF6 hydrolysis appeared to be influenced by a number of factors, of which relative humidity, rate of formation and amount of uranyl hexafluoride are considered the most important.  Single deposition In order to decrease surface loading, we have changed the conditions of UF6 distillation to reduce the amount of hydrolyzed gas. As the particle size depends also on the relative humidity, it was necessary to keep the RH inside the aerosol chamber between 55% and 70%. Keeping the RH within an even smaller range is difficult to achieve since the hydrolysis is carried out in ambient air with a given level of humidity. During the experiments, the RH could be kept at about 60% for most of the time, which resulted in good quality samples.  Another factor influencing the U-reference sample quality is the rate of UO2F2 formation, related to how the UF6 is released into the air in the aerosol chamber. Fig 1 presents two SEM pictures of uranium oxyfluoride particles produced in two depositions. During the first one (Fig.1, left) UF6 slowly diffused out of the vial, while during the second one (Fig.1, right) it was released immediately. Although the amount of UF6 was doubled in the first case, the particle density was smaller than for the second deposition.         Fig.1. Two single depositions of uranyl fluoride particles The presence of uranium background is also related to the rate of UF6 release and hydration. According to the feedback of participants in NUSIMEP-6 the surface of some of the graphite disks was coated not only by particles but a uranium smear or film was also observed, creating a strong background signal (Fig.2).          Fig.2. SIMS image of an individual uranium particle, with a large background of uranium.  This “U-smear” was considered as the most limiting factor for proper particle analysis by the NUSIMEP-6 participants. Therefore major emphasis was placed in reducing this U-film. From the preliminary results of further experiments we tend to believe that this uranium smear is composed of very small nm-sized particles that settle down on the surface together with the larger ones. This occurs particularly when uranium hexafluoride is slowly released into the air, instead of rapidly. However, this assumption needs to be investigated further.  Double deposition  We applied the procedure described above, optimized for a single deposition, to prepare a sample with particles of two different enrichments. The sample was composed of low-enriched uranium with a n(235U)/n(238U)ratio of ca 2% (LEU) and high enriched uranium with a n(235U)/n(238U) ratio of 20% (HEU). The preliminary results, obtained from NanoSIMS measurements, showed that it is possible to distinguish particles with different uranium isotopic ratios on the same planchet (Fig.3.)  Fig.3 SIMS results of all the particles of n(235U)/n(238U)ratio of ca 2% (LEU) and n(235U)/n(238U) ratio of 20% (HEU). Blue lines indicate n(235U)/n(238U) ratio of the reference materials used for particle production. Conclusions By controlling the particle size and distribution of uranium reference particles on graphite supports, obtained by hydrolysis of UF6, the quality of test samples has been improved. Tighter control of relative humidity during the reaction and reduction of the UF6 concentration was an essential but not exclusive factor. The rate of gas release into the deposition chamber appears to be the determining factor for uranium background formation. However, this needs to be further investigated. Preliminary results on test samples with two uranium enrichments, i.e. 2% and 20% of U-235 proved that it is feasible to distinguish and analyse the particles with different isotopic compositions on the same graphite disk. This optimised procedure will be applied for the preparation of the certified test samples in the next NUSIMEP inter-laboratory comparison organised for the IAEA Network of Analytical Laboratories Acknowledgement This work was performed under a cooperative task (action sheet 36) between IRMM and the Lawrence Livermore National Laboratory (LLNL) under the Agreement between the European Atomic Energy Community (EURATOM), represented by the Commission of European Communities, and the United States Department of Energy (DOE) in the field of nuclear materials safeguards research and development. Prepared by LLNL under Contract DE-AC52-07NA27344. References 1. D.L. Donohue, “Strengthening IAEA safeguards through environmental sampling and analysis”, J. Alloys Compd., 271–273 (1998) 11–18. 2. R. Kips, A. Leenaers, G. Tamborini, M. Betti, S. Van den Berghe, R. Wellum, P. Taylor, “Characterization of Uranium Particles Produced by Hydrolysis of UF6 Using SEM and SIMS” Microsc. Microanal. 13 (2007) 156–164. 3. R. Kips, R. Wellum, “Fluorine as a Safeguards Tool for Age Dating of Uranium Oxyfluoride Particles?” ESARDA BULLETIN, No. 38, June 2008.  4. R. Kips, A.J. Pidduck, M.R. Houlton, A. Leenaers, J.D. Mace, O. Marie, F. Pointurier, E.A. Stefaniak, P.D.P. Taylor, S. Van den Berghe, P. Van Espen, R. Van Grieken, R. Wellum, “Determination of fluorine in uranium oxyfluoride particles as an indicator of particle age”, Spectrochimica Acta B 64 (2009) 199–207.  5. N. Erdmann, M. Betti, O. Stetzer, G. Tamborini, J.V. Kratz, N. Trautmann, J. van Geel, ”Production of monodisperse uranium oxide particles and their characterization by scanning electron microscopy and secondary ion mass spectrometry”, Spectrochimica Acta Part B 55 (2000) 1565-1575.  6. Y. Ranebo, N. Niagolova, N. Erdmann, M. Eriksson, G. Tamborini, M. Betti, “Production and Characterization of Monodisperse Plutonium, Uranium, and Mixed Uranium-Plutonium Particles for Nuclear Safeguard Applications”, Anal. Chem. 82 (2010) 4055–4062. 7. D.P. Armstrong, W.D. Bostick, W.H. Fletcher, “An FTIR Study of the Atmospheric Hydrolysis of Uranium Hexafluoride”, Applied Spectr. 45, 6 (1991) 1008-1016. 8. Y. Aregbe, J. Truyens, R. Kips, S. Richter, E. Stefaniak, H. Kühn, M. Kraiem, NUSIMEP-6 Interlaboratory Comparison Report, EUR 23702 EN (2008), http://irmm.jrc.ec.europa.eu/html/interlaboratory_comparisons/nusimep/index.htm   

Development of uranium reference particles to support nuclear safeguards

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Development of uranium reference particles to support nuclear safeguards

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