sem edx characterization of uranium aerosols at a nuclear fuel fabrication plant

Accepted Manuscript SEM/EDX characterization of uranium aerosols at a nuclear fuel fabrication plant E Hansson, H.B.L Pettersson, C Fortin, M Eriksson PII: DOI: Reference: S0584-8547(17)30136-2 doi: 10.1016/j.sab.2017.03.002 SAB 5215 To appear in: Spectrochimica Acta Part B: Atomic Spectroscopy Received date: Revised date: Accepted date: 31 March 2016 28 February 2017 March 2017 Please cite this article as: E Hansson, H.B.L Pettersson, C Fortin, M Eriksson , SEM/ EDX characterization of uranium aerosols at a nuclear fuel fabrication plant The address for the corresponding author was captured as affiliation for all authors Please check if appropriate Sab(2017), doi: 10.1016/j.sab.2017.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT SEM/EDX characterization of uranium aerosols at a nuclear fuel fabrication plant E Hanssona,b,* H.B.L Petterssona C Fortinc M Erikssona,d a RI PT Department of Medical and Health Sciences, Linköping University, 58185 Linköping, Sweden b Westinghouse Electric Sweden AB, Bränslegatan 1, 72163 Västerås, Sweden c Carl Zeiss SAS, 100 route de Versailles, 78160 Marly-le-Roi, France d Swedish Radiation Safety Authority, 17116 Stockholm, Sweden * NU SC Corresponding author at: Department for Radiation Protection and Environment, Westinghouse Electric Sweden AB, SE 72163 Västerås, Sweden Tel.: +46 732 367445 E-mail address: hanssoea@westinghouse.com (E Hansson) Abstract AC CE PT E D MA Detailed aerosol knowledge is essential in numerous applications, including risk assessment in nuclear industry Cascade impactor sampling of uranium aerosols in the breathing zone of nuclear operators was carried out at a nuclear fuel fabrication plant Collected aerosols were evaluated using scanning electron microscopy and energy dispersive X-ray spectroscopy Imaging revealed remarkable variations in aerosol morphology at the different workshops, and a presence of very large particles (up to ~100 x 50 µm2) in the operator breathing zone Characteristic X-ray analysis showed varying uranium weight percentages of aerosols and, frequently, traces of nitrogen, fluorine and iron The analysis method, in combination with cascade impactor sampling, can be a powerful tool for characterization of aerosols The uranium aerosol source term for risk assessment in nuclear fuel fabrication appears to be highly complex Keywords Uranium; Aerosol; Impactor; Microscopy; X-ray ACCEPTED MANUSCRIPT Highlights Combined cascade impactor sampling with SEM/EDX analyses of uranium aerosols  First characterization of uranium aerosols in AUC route nuclear fuel fabrication  Uranium complexes/aerosols with novel morphology descriptions  Variable uranium concentrations in aerosols AC CE PT E D MA NU SC RI PT  ACCEPTED MANUSCRIPT INTRODUCTION PT In fabrication of nuclear fuel, the presence of uranium aerosols can prove hazardous with respect to radiation and chemical toxicity following inhalation exposure The main risk scenarios are chronic exposure of workers, acute exposure of workers, exposure of the public from normal operations and exposure of the public due to accidental release of uranium compounds The aerosol source term description is fundamental in such risk assessments as it predicts the behavior of aerosols with respect to dispersion as well as deposition in the airways and subsequent biological excretion [1] Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) is essential to distinguish uranium aerosols from other aerosols present in an industrial environment NU SC RI Characterization studies of uranium particles have been reported in numerous articles and reports over the last 50 or so years In the field of nuclear fuel fabrication, much research has focused on the production parameters of the produced uranium dioxide (UO2) powder, e.g flowability, density and sinterability Such studies have shown that particle size distributions vary with production parameters and, naturally, between stages in the nuclear fuel cycle [2-7] Hence, the properties of uranium aerosols will differ between sites using different production methods, but detailed descriptions of uranium aerosols in nuclear fuel fabrication are scarce in the literature MA The UO2 powder for production purposes can be characterized by the Mass Median Equivalent Diameter (MMD), and airborne radioactive matter by the Activity Median Aerodynamic Diameter (AMAD) The latter can be evaluated by sampling aerosols with cascade impactors [8] The aerodynamic particle diameter, dae, is described as (1) CE PT E D where de is the diameter of the spherical particle with the same volume as the particle considered, ρ (g/cm3) is the density of the irregular particle, ρ0 the reference density (1 g/cm3) and χ the dynamic shape factor (dimensionless) [8-11] The dynamic shape factor depends on particle morphology, and is defined as the ratio of the drag force on particle of interest to the drag force of a spherical particle with the same volume 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 particle is assumed to be 50 % higher than for a spherical particle with the same volume [8,10] AC Mass and activity distributions of aerosols often, but not always, follow log-normal distributions [8,12] Such tendencies for UO2 aerosols have previously been reported [13-15] Several authors have reported AMADs from nuclear fuel workplaces without elaborating much on the particle size distribution of the sampled material [16-20] The elemental composition of aerosols might affect aerodynamic properties, and also serve as an indicator of chemical compound, which is important in many applications, including risk assessment [9] The present work is a case study of uranium aerosols sampled in the operator breathing zone at a nuclear fuel fabrication plant using cascade impactors Using electron microscopy and energy dispersive X-ray spectrometry, the uranium aerosol source term was characterized with respect to morphology, size distribution, elemental composition and dynamic shape factor To the best of our knowledge, no such studies have been carried out on uranium aerosols sampled in the operator breathing zone at a nuclear fuel fabrication plant The information is important ACCEPTED MANUSCRIPT in order to correctly carry out risk assessments with respect to inhalation exposure of workers and the public MATERIALS AND METHODS 2.1 URANIUM SOURCE DESCRIPTION PT Several production methods are available for production of UO2 pellets for light-water nuclear reactors The fabrication plant in the present study is run by Westinghouse Electric Sweden AB and processes hundreds of tons of uranium annually in the different workshops: conversion, powder preparation, pelletizing and burnable absorber (BA) pelletizing MA NU SC RI The conversion is carried out using a wet chemical process where UO2 is formed from UF6 via ammonium uranyl carbonate (AUC) UF6 is added to a vessel, where AUC is formed and precipitated after an exothermic reaction with ammonium carbonate After drying, the AUC powder is fed into a fluidizing bed furnace, where UO2 is formed by reduction The conversion workshop is complex, with several side processes which enable reuse of waste uranium and chemicals As a result, several additional uranium complexes can be present in the workshop: UO2F2, ADU (very small amounts from purification of waste uranium), uranium trioxide (UO3), uranium octoxide (U3O8), uranyl nitrate hexahydrate (UNH) and uranyl peroxide (UO4·2NH3·2HF·2H2O) [21,22] The wet chemical AUC route of conversion generates a UO2 powder with a larger average particle size than from the alternative processes (dry route conversion of UF6 to UO2 or wet chemical ADU conversion) [4,5] The site in the present study produces UO2 powder with an MMD of typically 20 µm, as measured by laser diffraction [23] It has been shown that UO2 aerosols from the AUC route of conversion are larger than aerosols from the ADU route of conversion [24] Interestingly, we have not found any reports on the characterization of airborne AUC CE PT E D The powder preparation workshop prepares the UO2 powder for pelletizing This is done by verification of low levels of humidity in the powder (for criticality safety reasons), blending to obtain the desired enrichment and blending with appropriate amounts of U3O8 (for sintering properties) In addition, powder for the BA pelletizing workshop is milled Waste materials such as grinding waste and defect pellets from the pelletizing workshop are oxidized to U3O8 to be used for powder blending Milled UO2 powder and oxidized waste have MMDs of 3-4 µm and 5-7 µm, respectively [23,25] AC The main pelletizing workshop produces the majority of the fuel pellets by pressing UO2 into pellets that undergo sintering at ~1700 °C in a hydrogen atmosphere to obtain the required density The ceramic pellets are then ground to the proper dimensions and finally undergo a visual inspection before encapsulation into fuel rods The BA pelletizing workshop produces pellets in a similar way to that of the main pelletizing workshop The already milled powder is blended with gadolinium oxide (Gd2O3) and U3O8 Before pressing, the powder goes through roller compacting and granulation, and lubricant is added BA pellet waste is oxidized and recycled at the workshop The waste has a MMD of 8-10 µm [25] At the conversion and powder preparation workshops most of the uranium is sealed, but various compounds might be exposed to the work environment due to small leakages, maintenance and sampling Open handling occurs in the pelletizing and BA pelletizing workshops The 235U enrichment of the uranium handled at the site varies between depleted ACCEPTED MANUSCRIPT uranium ( 50 %) (Table S2 (Appendix)) PT E Table Percentage of uranium aerosols (Sampling 4) that also contained either iron, chromium or nickel at impactor stages C-G AC CE Impactor Uranium aerosols containing either Fe, Cr or Ni stage Fe Cr Ni C 86 % 31 % 29 % D 79 % 2.5 % 2.1 % E 34 % 1.2 % 0.6 % F 49 % 0.6 % 0% G 18 % 0% 0% 3.3 CONVERSION WORKSHOP – AEROSOL ELEMENTAL COMPOSITION EDX analysis showed a much more complex elemental composition at the conversion workshop compared to the other workshops Elemental composition alone is insufficient to distinguish between the many potentially present uranium compounds, but a presence of nitrogen and/or fluorine does, however, give an indication of chemical form Fig 5a shows the EDX spectrum of a particle with an obvious presence of nitrogen and fluorine, indicating UO2F2, AUC and/or uranium peroxide Fig 5b shows the EDX spectrum of a uranium particle where nitrogen and fluorine peaks are missing, indicating uranium oxide 14 ACCEPTED MANUSCRIPT CE NU PT E D MA a) SC RI PT It was found that for Sampling 1, impaction stages E and F, about 50 % of the uranium aerosols contained nitrogen and/or fluorine The remaining 50 % indicated uranium compounds such as UO2 and/or U3O8 At impaction stage B, >90 % of the uranium particles contained nitrogen and/or fluorine For Sampling 2, about 70 % of uranium aerosols indicated nitrogen and/or fluorine, but were not correlated to impactor cut-point b) 3.4 AC Fig SEM/EDX spectra of a: a uranium aerosol containing nitrogen and fluorine; and b: a uranium oxide particle The uranium M-lines are seen in both particles, however, the intensity from the carbon support makes the uranium intensity appear lower in b) compared to a) This is auto scaled by INCAFeature PELLETIZING WORKSHOP - AEROSOL DYNAMIC SHAPE FACTOR Calculating the aerosol dynamic shape factor using Equation for the pelletizing workshop proved to be difficult; in particular the estimates of de and particle density assumptions Fig illustrates shape factor estimates for densities corresponding to milled UO2 powder fill density, regular UO2 powder fill density, un-sintered pellet density and sintered pellet density as described in section 2.1 A dynamic shape factor below is not possible by definition, which suggests faulty de estimates for Stage C This can be explained by that a few large chromium particles carrying numerous small uranium particles were misinterpreted by INCAFeature as multiple small uranium particles with high chromium contents The hypothesis is supported by the high chromium contents at stage C (Table S2) 15 SC RI PT ACCEPTED MANUSCRIPT NU Fig Shape factor estimates for the pelletizing workshop (Sampling 4) for impaction stages C-G Error bars correspond to estimated uncertainty (+/- 15 %) in de PT E D MA A density of 10.5 g/cm3 corresponds to sintered pellet material, and was believed to give the best shape factor estimate However, the shape factor estimates turned out to be much higher than the expected value of ~1.5, suggesting a lower density There have been reports of dynamic shape factors in the range of 2.2-3.6 for plutonium/uranium oxide aerosols of 0.5-1.5 µm aerodynamic diameters [40] However, these were long chains of particles and the particles in the present work are expected to be associated with a lower shape factor CE Fig suggests that uranium aerosols in the pelletizing workshop have a density lower than 10.5 g/cm3and a shape factor greater than 1.5 This is supported by the electron microscope images frequently showing highly irregular particles The correlation between dae, de, density and shape factor appears to be very complex CONCLUSIONS AND PERSPECTIVES AC The present work constitutes, to the best of our knowledge, the first SEM/EDX characterization of uranium aerosols sampled with cascade impactors in the operator breathing zone at a nuclear fuel fabrication plant These characterization studies will add to the understanding of uranium aerosols in nuclear fuel fabrication, affecting applications such as risk assessments Remarkable variations of uranium aerosol morphologies were found at the different workshops, and a presence of very large particles (up to ~100 µm) in the breathing zone of operators could be verified The exact formation process of all aerosols is not known, and needs further investigation At the pelletizing workshop, a clear size fractionation of aerosols was visible, as was an overlap over particle sizes It was shown that 18-86 % of the collected uranium aerosols also contained iron, chromium and/or nickel Uranium weight percentages appeared to be lower for 16 ACCEPTED MANUSCRIPT PT late impaction stages This is partially explained by a bias from electron beam interactions with oxygen in the impaction substrate Measurements of U3O8 reference particles indicated that the bias causes the software to generate uranium weight percentages 15 percentage points lower than the theoretical level of 85 % This is valid for particles larger than µm ECD The bias is greater for smaller particles However, aerosols in the same size interval (2-4 µm ECD, where the bias was approximately uniform), but collected at different impaction stages, unexpectedly showed different uranium weight percentages This could indicate a variable density of uranium aerosols, but the finding requires further investigation The aerosol dynamic shape factor proved difficult to evaluate, but there are indications that the generally assumed value of 1.5 is an underestimation Size distribution and shape factor need further investigation for other workshops SC RI Many particles at the conversion workshop showed traces of nitrogen and fluorine Such particles occurred much more frequently (90 %) at the early impaction stages compared to the later impaction stages (50 %) SEM/EDX is a powerful tool for evaluation of aerosol parameters such as size distribution, elemental composition and shape factor Future perspectives include studies of chemical form and crystalline states using µ-XRF and µ-XRD ACKNOWLEDGEMENTS PT E D MA NU The authors are grateful to the International Atomic Energy Agency for allowing use of their SEM/EDX equipment at the environmental laboratories in Monaco Westinghouse Electric Sweden AB is acknowledged for participation in the study, providing samples and funding Olav Axelsson Memorial Foundation is acknowledged for funding (grant number LIO-520711) The Swedish Radiation Safety Authority is thanked for funding of the pre-study to the present work Special thanks go to Isabelle Levy and Francois Oberhansli at the IAEA for assistance with the SEM, Magnus Hedberg and Noëlle Albert at the European Commission Joint Research Center Institute for 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plant Collected aerosols were evaluated using scanning... of uranium aerosols sampled with cascade impactors in the operator breathing zone at a nuclear fuel fabrication plant These characterization studies will add to the understanding of uranium aerosols