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Uranium Aerosols at a Nuclear Fuel Fabrication Plant: Characterization Using Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy

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Uranium Aerosols at a Nuclear Fuel Fabrication Plant: Characterization Using Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy Spectrochimica Acta Part B 131 (2017) 130–137 Contents lists available at ScienceDirect Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab Uranium aerosols at a nuclear fuel fabrication plant: Characterization using scanning electron microscopy and energy dispersive X-ray spectroscopy E. Hansson a,b,⁎, H.B.L. Pettersson a, C. Fortin c, M. Eriksson a,d a Department of Medical and Health Sciences, Linköping University, 58185 Linköping, Sweden b Westinghouse Electric Sweden AB, Bränslegatan 1, 72136 Västerås, Sweden c Carl Zeiss SAS, 100 route de Versailles, 78160 Marly-le-Roi, France d Swedish Radiation Safety Authority, 17116 Stockholm, Sweden article info abstract Article history: Detailed aerosol knowledge is essential in numerous applications, including risk assessment in nuclear industry. Received 31 March 2016 Cascade impactor sampling of uranium aerosols in the breathing zone of nuclear operators was carried out at a Received in revised form 28 February 2017 nuclear fuel fabrication plant. Collected aerosols were evaluated using scanning electron microscopy and energy Accepted 3 March 2017 dispersive X-ray spectroscopy. Imaging revealed remarkable variations in aerosol morphology at the different Available online 6 March 2017 workshops, and a presence of very large particles (up to≅100 × 50 μm2) in the operator breathing zone. Charac- teristic X-ray analysis showed varying uranium weight percentages of aerosols and, frequently, traces of nitrogen, Keywords: fl Uranium uorine and iron. The analysis method, in combination with cascade impactor sampling, can be a powerful tool Aerosol for characterization of aerosols. The uranium aerosol source term for risk assessment in nuclear fuel fabrication Impactor appears to be highly complex. Microscopy © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license X-ray (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction particle size distributions vary with production parameters and, natu- rally, between stages in the nuclear fuel cycle [2–7].Hence,theproper- In fabrication of nuclear fuel, the presence of uranium aerosols can ties of uranium aerosols will differ between sites using different prove hazardous with respect to radiation and chemical toxicity follow- production methods, but detailed descriptions of uranium aerosols in ing inhalation exposure. The main risk scenarios are chronic exposure of nuclear fuel fabrication are scarce in the literature. workers, acute exposure of workers, exposure of the public from normal The UO2 powder for production purposes can be characterized by operations and exposure of the public due to accidental release of urani- the mass median diameter (MMD), and airborne radioactive matter um compounds. The aerosol source term description is fundamental in by the activity median aerodynamic diameter (AMAD). The latter can such risk assessments as it predicts the behavior of aerosols with respect be evaluated by sampling aerosols with cascade impactors [8]. The aero- to dispersion as well as deposition in the airways and subsequent bio- dynamic particle diameter, dae, is described as logical excretion [1]. Scanning electron microscopy (SEM) combined ¼ Á ½ρ=ðÞρ χ 1=2 ð Þ with energy dispersive X-ray spectroscopy (EDX) is suitable for dae de 0 1 distinguishing uranium aerosols from other aerosols present in an in- dustrial environment. where de is the diameter of the spherical particle with the same volume Characterization studies of uranium particles have been reported in as the particle considered, ρ (g/cm3) is the density of the irregular par- 3 numerous articles and reports over the last 50 years. In the field of nu- ticle, ρ0 the reference density (1 g/cm )andχ the dynamic shape factor clear fuel fabrication, much research has focused on the production pa- (dimensionless) [8–11]. The dynamic shape factor depends on particle rameters of the produced uranium dioxide (UO2) powder, e.g. morphology, and is defined as the ratio of the drag force on the particle flowability, density and sinterability. Such studies have shown that of interest to the drag force on a spherical particle with the same vol- ume. In an industrial environment, few particles are spherical, and a value of 1.5 is typically assumed, i.e. the drag force on the average par- ⁎ Corresponding author at: Department for Radiation Protection and Environment, Westinghouse Electric Sweden AB, 72163 Västerås, Sweden. ticle is assumed to be 50% higher than for a spherical particle with the E-mail address: [email protected] (E. Hansson). same volume [8,10]. http://dx.doi.org/10.1016/j.sab.2017.03.002 0584-8547/© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). E. Hansson et al. / Spectrochimica Acta Part B 131 (2017) 130–137 131 Mass and activity distributions of aerosols often, but not always, fol- the powder goes through roller compacting and granulation, and lubri- low log-normal distributions [8,12]. Such tendencies for UO2 aerosols cant is added. BA pellet waste is oxidized and recycled at the workshop. have previously been reported [13–15]. Several authors have reported The waste has an MMD of 8–10 μm [25]. AMADs from nuclear fuel workplaces, discussing particle size distribu- At the conversion and powder preparation workshops most of the tions of the sampled material in various detail [16–20]. The elemental uranium is sealed, but various compounds might be exposed to the composition of aerosols might affect aerodynamic properties, and also work environment due to small leakages, maintenance and sampling. serve as an indicator of chemical compound, which is important in Open handling occurs in the pelletizing and BA pelletizing workshops. many applications, including risk assessment [9]. The 235U enrichment of the uranium handled at the site varies between The present work is a case study of uranium aerosols sampled in the depleted uranium (b0.71%) up to 4.95% (mass percentages). The aver- operator breathing zone at a nuclear fuel fabrication plant using cascade age 235U enrichment for the year of 2014 was approximately 3.7%, as impactors. Using electron microscopy and energy dispersive X-ray spec- measured by inductively coupled plasma mass spectrometry [26].Pres- troscopy, the uranium aerosol source term was characterized with re- ence of 236Uand232U is negligible (b0.1% of alpha activity). spect to morphology, size distribution, elemental composition and dynamic shape factor. To the best of our knowledge, no such studies 2.2. Aerosol sampling have been carried out on uranium aerosols sampled in the operator breathing zone at a nuclear fuel fabrication plant. The information is im- Cascade impactors accomplish a separation of particles based on portant in order to correctly carry out risk assessments with respect to aerodynamic diameter by pumping air through the impactor, which is inhalation exposure of workers and the public. divided into several stages. Air flow velocity is increased at each stage, which is prepared with an impaction substrate. Each impactor stage 2. Materials and methods has a specific cut-point, defined as the aerodynamic diameter of parti- cles with 50% probability of impaction [8]. The inertia of large particles 2.1. Uranium source description will cause them to impact onto substrates at the early stages of the im- pactor, whereas small particles require higher velocities for impaction Several production methods are available for production of UO pel- 2 to occur. All air is filtered through a final collection filter (typically lets for light-water nuclear reactors. The fabrication plant in the present glass fiber material) before leaving the impactor, collecting the remain- study is run by Westinghouse Electric Sweden AB and processes hun- ing particles that were not deposited by impaction. dreds of tons of uranium annually in the different workshops: conver- Marple 298 impactors (Thermo Scientific, Prod. No. SE298) which sion, powder preparation, pelletizing and burnable absorber (BA) operate at 2.0 L/min, were used for sampling in the present study (ex- pelletizing. cept Sampling 2 which was carried out for SEM imaging only). The The conversion is carried out using a wet chemical process where choice of impactor was based on the following merits: 1) its portability UO is formed from uranium hexafluoride (UF ) via ammonium uranyl 2 6 enabled sampling in the operator breathing zone, 2) eight impaction carbonate (AUC). UF is added to a vessel, where AUC is formed and pre- 6 stages (A–H) with relevant cut-points (21.3, 14.8, 9.8, 6.0, 3.5, 1.6, 0.9 cipitated after an exothermic reaction with ammonium carbonate. After and 0.5 μm) and 3) the impactor has been verified in the literature to drying, the AUC powder is fed into a fluidizing bed furnace, where UO is 2 have sharp cut-points [14,27]. formed by reduction. The conversion workshop is complex, with several A Gilian 5000 pump was used and calibrated with an Alicat MB-50 side processes which enable reuse of waste uranium and chemicals. As a SLPM-D orifice flow controller in accordance with the instruction man- result, several additional uranium complexes can be present in the ual [28].Theflow rate was checked before and after each sampling cam- workshop: uranyl fluoride (UO F , ammonium diuranate (ADU) (very 2 2) paign using the same orifice flow controller and a rotameter. Flow rates small amounts from purification of waste uranium), uranium trioxide showed negligible variation (typically b1%). (UO ), uranium octoxide (U O ), uranyl nitrate hexahydrate (UNH) 3 3 8 Eight sampling campaigns were conducted to collect aerosols to be and uranyl peroxide (UO ·2NH ·2HF·2H O) [21,22]. The wet chemical 4 3 2 analyzed with SEM/EDX with the following objectives: AUC route of conversion generates a UO2 powder with a larger average particle size than from the alternative processes (dry route conversion 1.
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