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,daDepartment 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
a b s t r a c t
a r t i c l e i n f o
Article history: Received 31 March 2016
Received in revised form 28 February 2017 Accepted 3 March 2017
Available online 6 March 2017
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 × 50 μm2) in the operator breathing zone.
Charac-teristic 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.
© 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/). Keywords: Uranium Aerosol Impactor Microscopy X-ray 1. Introduction
In fabrication of nuclear fuel, the presence of uranium aerosols can prove hazardous with respect to radiation and chemical toxicity follow-ing 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 urani-um 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 bio-logical excretion[1]. Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) is suitable for distinguishing uranium aerosols from other aerosols present in an in-dustrial environment.
Characterization studies of uranium particles have been reported in numerous articles and reports over the last 50 years. In thefield of nu-clear fuel fabrication, much research has focused on the production pa-rameters 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, natu-rally, between stages in the nuclear fuel cycle[2–7]. Hence, the proper-ties 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.
The UO2powder for production purposes can be characterized by the mass median 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 aero-dynamic particle diameter, dae, is described as
dae¼ de ρ= ρ½ ð 0χÞ1=2 ð1Þ
where deis the diameter of the spherical particle with the same volume as the particle considered,ρ (g/cm3) is the density of the irregular par-ticle,ρ0the reference density (1 g/cm3) andχ 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 the particle 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-ticle is assumed to be 50% higher than for a spherical parpar-ticle with the same volume[8,10].
⁎ Corresponding author at: Department for Radiation Protection and Environment, Westinghouse Electric Sweden AB, 72163 Västerås, Sweden.
E-mail address:hanssoea@westinghouse.com(E. Hansson).
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/).
Contents lists available atScienceDirect
Spectrochimica Acta Part B
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / s a bMass and activity distributions of aerosols often, but not always, fol-low log-normal distributions[8,12]. Such tendencies for UO2aerosols have previously been reported[13–15]. Several authors have reported AMADs from nuclear fuel workplaces, discussing particle size distribu-tions of the sampled material in various detail[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 spec-troscopy, the uranium aerosol source term was characterized with re-spect 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 im-portant in order to correctly carry out risk assessments with respect to inhalation exposure of workers and the public.
2. Materials and methods 2.1. Uranium source description
Several production methods are available for production of UO2 pel-lets for light-water nuclear reactors. The fabrication plant in the present study is run by Westinghouse Electric Sweden AB and processes hun-dreds of tons of uranium annually in the different workshops: conver-sion, powder preparation, pelletizing and burnable absorber (BA) pelletizing.
The conversion is carried out using a wet chemical process where UO2is formed from uranium hexafluoride (UF6) via ammonium uranyl carbonate (AUC). UF6is added to a vessel, where AUC is formed and pre-cipitated after an exothermic reaction with ammonium carbonate. After drying, the AUC powder is fed into afluidizing bed furnace, where UO2is 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: uranylfluoride (UO2F2), ammonium diuranate (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 UO2powder with a larger average particle size than from the alternative processes (dry route conversion of UF6to UO2or wet chemical ADU conversion)[4,5]. The site in the present study produces UO2powder with an MMD of typically 20μm, as measured by laser diffraction[23]. It has been shown that UO2 aero-sols from the AUC route of conversion are larger than aeroaero-sols from the ADU route of conversion[24]. Interestingly, we have not found any re-ports on the characterization of airborne AUC.
The powder preparation workshop prepares the UO2powder 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 work-shop is milled. Waste materials such as grinding waste and defect pel-lets from the pelletizing workshop are oxidized to U3O8to be used for powder blending. Milled UO2powder and oxidized waste have MMDs of 3–4 μm and 5–7 μm, respectively[23,25].
The main pelletizing workshop produces the majority of the fuel pel-lets by pressing UO2into 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 andfinally 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 lubri-cant is added. BA pellet waste is oxidized and recycled at the workshop. The waste has an 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. The235U enrichment of the uranium handled at the site varies between depleted uranium (b0.71%) up to 4.95% (mass percentages). The aver-age235U enrichment for the year of 2014 was approximately 3.7%, as measured by inductively coupled plasma mass spectrometry[26]. Pres-ence of236U and232U is negligible (b0.1% of alpha activity).
2.2. Aerosol sampling
Cascade impactors accomplish a separation of particles based on aerodynamic diameter by pumping air through the impactor, which is divided into several stages. Airflow velocity is increased at each stage, which is prepared with an impaction substrate. Each impactor stage has a specific cut-point, defined as the aerodynamic diameter of parti-cles with 50% probability of impaction[8]. The inertia of large particles will cause them to impact onto substrates at the early stages of the im-pactor, whereas small particles require higher velocities for impaction to occur. All air isfiltered through a final collection filter (typically glassfiber material) before leaving the impactor, collecting the remain-ing particles that were not deposited by impaction.
Marple 298 impactors (Thermo Scientific, Prod. No. SE298) which operate at 2.0 L/min, were used for sampling in the present study (ex-cept Sampling 2 which was carried out for SEM imaging only). The choice of impactor was based on the following merits: 1) its portability enabled sampling in the operator breathing zone, 2) eight impaction stages (A–H) with relevant cut-points (21.3, 14.8, 9.8, 6.0, 3.5, 1.6, 0.9 and 0.5μm) and 3) the impactor has been verified in the literature to have sharp cut-points[14,27].
A Gilian 5000 pump was used and calibrated with an Alicat MB-50 SLPM-D orifice flow controller in accordance with the instruction man-ual[28]. Theflow rate was checked before and after each sampling cam-paign using the same orifice flow controller and a rotameter. Flow rates showed negligible variation (typicallyb1%).
Eight sampling campaigns were conducted to collect aerosols to be analyzed with SEM/EDX with the following objectives:
1. Investigate aerosol morphologies at all four workshops (breathing zone and complementary sampling at sites of particular interest). 2. Determine the size distribution, elemental composition and dynamic
shape factor,χ, of uranium aerosols in the operator breathing zone at the pelletizing workshop. This workshop was prioritized from a ra-diological risk assessment perspective.
3. Evaluate SEM/EDX as a method for determining the elemental com-position of aerosols at the conversion workshop with respect to fluo-rine and nitrogen, giving an indication of material chemical form.
The following impaction substrates were used: sticky carbon tape (Ted Pella, Inc., Prod. No. 16085-1) (Sampling 1, 3, 4 and 8), mixed cellu-lose ester membrane (MCE) (Thermo Scientific, SEC-290-MCE) (Sam-pling 2 and 5) and glass fiber (Thermo Scientific, SEC-290-MCE) (Sampling 6 and 7). Carbon tape is, due to its conductivity, ideal for SEM/EDX analysis, and was chosen for all breathing zone sampling. MCE and glassfiber were used for complementary sampling. Final collec-tionfilter were glass fiber filters (Whatman GF/A, 1.6 μm pore size). Each sampling campaign was designed to sample enough particles for a repre-sentative SEM/EDX analysis, but avoiding particle overlap on the impac-tion substrates and was thus based on knowledge of airborne uranium levels at the site. The sampling campaigns are summarized inTable 1, and a full description is given in Table S1 (Appendix). Impaction patterns were generally homogeneous, as illustrated in Fig. S1 (Appendix).
2.3. SEM/EDX analysis
SEM/EDX analyses of sampled aerosols were carried out using a Carl Zeiss EVO LS15 Scanning Electron Microscope at the International Atomic Energy Agency (IAEA) Environment Laboratories. The X-ray de-tector used for the EDX analyses was an Oxford Instruments X-Max 50 detector (electrically cooled silicon drift detector with an active surface of 50 mm2and equipped with a polymer window) with a resolution of 125 eV on the manganese Kαline. The software used for SEM imaging was SmartSEM (version 5.06) and INCAFeature (version 5.03) for the EDX analyses. The INCAFeature software used a cobalt standard for cal-ibration of the EDX system and quantification was carried out by peak deconvolution, digitalfiltering and a matrix correction using the XPP method (exponential model of Pouchou and Pichoir Matrix Correction, correcting for effects of atomic number, absorption andfluorescence on X-ray emission from a sample)[29]. The XPP method is designed to improve quantification of low Z elements in high Z matrices. A 20 kV accelerating voltage was used to enable excitation of both urani-um L and M electrons.
Additional SEM/EDX measurements of reference particles (Section 2.3.5) were carried out at the Carl Zeiss France S.A.S Laboratories using the same microscope model, combined with an Oxford Instruments X-Max 80 detector (80 mm2active surface). The same software versions of SmartSEM and INCAFeature were used.
For sets using carbon tape, images were acquired by the backscattered electron detector (BSD) in high vacuum mode. For sets using MCE and glassfiber, images were acquired in variable pressure mode in order to minimize charging effects on the sample.
2.3.1. All workshops– aerosol morphology
Uranium aerosol morphology was evaluated by scanning impaction surfaces andfinal collection filters from all sampling campaigns. Parti-cles of unknown or deviating morphologies were analyzed using INCAFeature to verify uranium contents. Uranium particles were im-aged and non-uranium particles ignored. Point EDX measurements were carried out to determine the elemental composition. This informa-tion was used to determine the origin of each particle studied.
2.3.2. Pelletizing workshop - aerosol size distribution and elemental composition
The INCAFeature software was used to determine particle size distri-bution and elemental composition of aerosols at the pelletizing work-shop (Sampling 4) by automatically scanning five impaction substrates (Stages C–G). Stages A, B and H could not be automatically scanned due to setup difficulties (Stages A and B contained too few par-ticles over too large areas, and Stage H had too much particle overlap). Thefinal collection filter was not scanned due to the uneven glass fiber surface.
In order to detect uranium particles and discriminate non-uranium particles, the SEM was operated with low brightness and high contrast settings which were optimized for each sample. The optimization was carried out by ensuring that the number of uranium particles detected
by the software agreed with manual counting with point EDX measure-ments during scan set-up.
The acquisition time for EDX measurements was set to 6 s per parti-cle. The particle discrimination level was set by adjusting the magni fica-tion so that objects larger than 0.1 μm could be detected. Visual inspections showed that the number of particles smaller than 0.1μm was very small. Each impaction substrate held thousands of particles, so in order to perform scans within a reasonable time frame, a represen-tative section of each impaction substrate was selected for scanning. Each section was defined in INCAFeature as an area, consisting of a num-ber offields of fixed size. Each scan was run until all non-discriminated particles in allfields had been analyzed, in total 0.6–1.2 h per impactor substrate. A total of 377, 239, 167, 478 and 392 (impaction Stages C–G, respectively) uranium particles were automatically scanned and ana-lyzed using INCAFeature.
The equivalent circle diameter (ECD) was used as an indicator of particle size. Particle thickness was not evaluated. Elements considered in the elemental composition analysis from the automatic counting were: oxygen,fluorine, sodium, aluminum, silicon, calcium, chromium, manganese, iron, nickel, copper, zink and uranium. Carbon was exclud-ed due to its abundance in the impaction substrate. Results were expressed as normalized weight percentages, as per INCAFeature de-fault. The cobalt standard was used to regularly measure beam intensity and check detector calibration.
2.3.3. Conversion workshop– aerosol elemental composition
Particles from impaction Stages B, E and F from Sampling 1 were ran-domly chosen for evaluation of elemental composition, especially nitro-gen andfluorine. These impaction stages correspond to inhalable (Stage B) and respirable (Stages E–F) size fractions. Automatic quantification of these elements could not be carried out using INCAFeature due to low nitrogen andfluorine concentrations in combination with scan settings. Instead a manual inspection of EDX spectra was carried out. EDX spectra frequently had to be re-acquired due to inhomogeneous distributions of elements within each particle and alteration of particle structure due to interaction with the electron beam. The latter phenomenon occurred by burning of low-Z components of the particle. In those cases, peaks were only visible in the beginning of the spectrum acquisition, before they faded into the continuum of the spectrum. Electron beam interaction with AUC has previously been observed to cause reduction to UO3 and/or U3O8[21]. Spectra were regularly acquired by focusing the elec-tron beam on the impaction substrate, with no particles nearby, in order to rule out distortions from the sample support. Nitrogen andfluorine was never observed in the background support spectra.
2.3.4. Pelletizing workshop– aerosol dynamic shape factor
The dynamic shape factor was calculated for the pelletizing work-shop (Sampling 4), impaction Stages C–G. Particle volume was assumed to equal the median projected particle area (generated by the INCAFeature software) multiplied by particle height which was as-sumed to equal half median particle width (also generated by INCAFeature). Equivalent spherical diameters, de, were derived and shape factor estimates were carried out for different densities using Eq.(1). Aerosol densities of 1.6 g/cm3, 2.4 g/cm3, 5.7 g/cm3 and 10.5 g/cm3were assumed as they correspond to milled UO
2powder fill density, regular UO2powderfill density, un-sintered pellet density and sintered pellet density, respectively[23,25].
2.3.5. Reference particles
The potential bias (further discussed inSection 3.2) from electron beam interaction with the support material was investigated using cer-tified high purity U3O8reference particles (Standard Reference Material U-010, National Bureau of Standards). Particle sizes ranged between ≅0.5–10 μm and were attached to sticky carbon tape (Ted Pella, Inc., Prod. No. 16085-1). Agglomerates of particles were present and particle shapes were somewhat irregular.
Table 1
Brief description of the conducted sampling campaigns. Sampling no. Description
1 Conversion workshop, operator breathing zone 2 Conversion workshop, ventilation air
3 Powder preparation workshop, operator breathing zone 4 Pelletizing workshop, operator breathing zone 5 Pelletizing workshop, pellet inspection work station 6 Pelletizing workshop, pellet grinding
7 Pelletizing workshop, ventilation air
Scans were carried out using the same support material and a 20 kV accelerating voltage. The INCAFeature protocol for automated scans described inSection 2.3.2was used with a 2 s acquisition time due to the larger detector area. Particles below 2μm ECD were difficult to ana-lyze using the automated protocol due to contrast settings and particle overlap. Small particles were thus analyzed in manual mode. A total of 76 and 27 particles were analyzed in automatic and manual mode, respectively. The uranium weight percentages estimated by INCAFeature were compared to the expected theoretical level of 85% for pure U3O8.
3. Results and discussion
3.1. All workshops– aerosol morphology
Particle morphology was observed to vary with impaction stage cut-point and sampling location. The pre-pelletizing workshops, i.e. conver-sion and powder preparation, showed a looser structure, more agglom-eration and a more irregular morphology compared to the pelletizing and BA pelletizing workshops. Late impaction stages (E–H) showed less agglomeration than the early stages. On the individual aerosol level, the different workshops showed remarkable variation, especially at impactor Stages A–B. Structures of the uranium aerosols included, apart from discrete particles, many different geometrical structures, ag-glomerates of particles and particles attached to lower-Z materials.
Some structures, especially from the conversion workshop, were dif-ficult to interpret.Fig. 1a, from Sampling 2 shows a uranium aerosol were the EDX analysis indicated a presence of iron, traces of nitrogen
but nofluorine. The amorphous structure suggests an early process step, perhaps related to precipitation of particles. We are not aware of similar uranium aerosols described in the literature.
Rod-like uranium aerosols were observed at several impaction stages (including thefinal collection filter) at the conversion workshop (Sampling 1–2) either as agglomerates (Fig. 1b) or individual particles (Fig. 1c). EDX analyses showed a presence offluorine and nitrogen. The origin of rod-like aerosols is unknown and we are not aware of sim-ilar uranium aerosols described in the literature. Possibly it originates from the handling of uranyl peroxide from the hydrogen peroxide pro-cess system, where uranium is extracted for recycling from propro-cess so-lutions containingfluorine and nitrogen. The formation of particles is a highly complex process, depending on numerous parameters. A forma-tion of somewhat similar cylindrical particles has been demonstrated for UO4particles from a hydrogen peroxide solution, as well as UO3 and U3O8under specific conditions[30–31]. The EDX analyses of the re-maining particles inFig. 1c, including the square-shaped particle, showed nitrogen traces, but nofluorine, indicating AUC. AUC crystals have been shown to occur in shapes similar to the particle in this image[2,32].
Anotherfinding at the conversion workshop, Sampling 1–2, was the presence of spherical uranium particles (Fig. 1d). EDX analyses showed, in addition to uranium, traces offluorine and nitrogen. The spherical shape and size strongly indicates UO2F2. Several studies have shown that formation of UO2F2from UF6in a humid environment results in spherical shapes of about≅0.2–2.0 μm diameter[33–35]. The presence of nitrogen is slightly surprising, but could be explained by attachment of nitrogen-rich materials present in the atmosphere. The low-contrast
Fig. 1. Aerosol morphology at different sampling locations. a–e: conversion workshop, Sampling 1–2; f–h: powder preparation workshop, Sampling 3; i–k: pelletizing workshop, Sampling 4–7; and l: BA pelletizing workshop, Sampling 8.
particles in the lower part of the image contained iron but no uranium. Remaining particles contained uranium and nitrogen, indicating AUC.
Fig. 1e shows an example of a particle consisting of uranium particles attached to a low-Z particle. This particular particle contained nitrogen, iron, chromium, aluminum andfluorine. The particle origin is unknown, but the presence offluorine and nitrogen indicate early conversion steps or uranyl peroxide.
Particles sampled at the powder preparation workshop were more straightforward to identify as pellet shards and waste from pellet grind-ing from the pelletizgrind-ing workshop, and un-milled UO2powder from the conversion workshop.Fig. 1f shows typical pellet grains and pores from sintering in the pelletizing workshop. Some particles showed a more rounded shape, as illustrated byFig. 1g, which is typical for UO2powder from the conversion workshop. The small particles inFig. 1g are likely to originate from waste material from pellet grinding, and have possibly undergone oxidation, suggesting U3O8. Some particles showed more obvious indications of oxidation, such asFig. 1h, where the rugged sur-face strongly suggests partial oxidation of a pellet shard, with obvious pores from the sintering process showing.
Most particles at the pelletizing and BA pelletizing workshop could be correlated to sintered material (e.g.Fig. 1i). The aerosol morphology varied less compared to the other workshops but showed a very large size range. A large cluster of aerosols of varying sizes (Fig. 1j) sampled near one of the pellet grinders contained some very small (≅0.1 μm) particles. The largest sampled aerosol was a pellet shard (Fig. 1k) collected in the operator breathing zone at the pelletizing workshop, measuring almost 100 × 50μm2(thickness unknown). Both pelletizing workshops showed agglomeration of particles, as illustrated inFig. 1l. Agglomeration tended to be more frequent at the BA pelletizing work-shop, which could be explained by the addition of lubricant. The rate of agglomeration was not further investigated.
3.2. Pelletizing workshop – aerosol size distribution and elemental composition
Fig. 2shows aerosol ECD distributions and uranium weight percent-ages as counted by INCAFeature for Sampling 4. Overlap between the impaction stages is evident and also expected by cascade impactor
Fig. 2. Distribution of uranium aerosol equivalent diameter (de) and uranium weight percentage for Sampling 4. Stages C–G correspond to 377, 237, 167, 478 and 392 particles,
respectively. Note that de(ECD) does not equal dae(aerodynamic diameter) (Eq.(1)). A bin width of 0.25μm was used in each histogram. For clarity, y-axes in Panels a) and i) were
adjusted. The values of these bins were 0.32, 0.55 and 0.26, respectively. Three particles of 10.2, 11.2 and 13.8μm ECD (Stage C) are outside the scale of the x-axes. Data have not been corrected for the oxygen bias discussed in the text.
cut-point definition. Variations in particle shape and density will affect the aerodynamic properties of particles, and may further add to the ob-served overlap (Eq.(1)). Similar patterns were observed in a pre-study at the same plant[36]. The ECD distributions must be interpreted with care, since particle counting is dependent on several parameters, e.g. brightness/contrast settings that must be optimized for each sample. Small differences in brightness/contrast can affect detection limits, however the electron beam was stable in the presented measurements and we believe the effects of such a bias is small in the present work. It was noted that some large particles near the scannedfield edges were not counted by INCAFeature, possibly biasing particle size distributions. Agglomerates of uranium particles and small uranium particles at-tached to large non-uranium particles were sometimes counted as mul-tiple individual uranium particles. This is the explanation to the many particlesb1 μm inFig. 2a–b.
Observed uranium weight percentages were lower than expected (theoretically 88% for UO2and 85% for U3O8). The uranium weight per-centages inFig. 2appeared to be lower for small particles, and were ob-served to drop with each impaction stage. This is probably due to a bias from oxygen in the impaction substrate (carbon was not included in the quantification) and a presence of other elements. For micron-sized par-ticles a majority of the electron beam interaction may come from inter-actions in the impaction substrate, which will affect measured X-ray intensities[37]. An observation similar toFig. 2has been reported else-where for uranium particles when the same accelerating voltage was used[38].
As illustrated inFig. 3, automated measurements of U3O8 refer-ence particles (ECDN 2 μm) showed uranium weight percentages of 70.3 ± 2.8% (1 standard deviation), i.e.≅ 15 percentage points lower than the theoretical level (85%). Manual scans of particles smaller than 2μm indicated lower uranium weight percentages, i.e. a more pronounced bias from oxygen in the substrate.
Uranium weight percentages were further investigated by studying particles of the same ECD (2–4 μm, a size range present at all impaction stages) present on different impaction stages (Fig. 4). Particles on the later impactor stages (lower cut-points) unexpectedly appeared to have lower uranium weight percentages. We cannot explain this with the aforementioned oxygen bias since reference particle measurement indicated an approximately uniform bias of 15 percentage points in the 2–4 μm ECD size interval. We hypothesize that 2–4 μm ECD particles collected at late impaction stages have a looser structure (and thereby a lower effective density and aerodynamic diameter of particles collected at late impaction stages (Eq.(1)) than equally sized particles at early im-paction stages. This would be in agreement with cascade impactor
operating principles and could indicate a presence of multiple uranium compounds. Quantification of elemental composition of uranium parti-cles using SEM/EDX is well-known to be challenging, as emphasized by e.g. Ciurapinsky et al.[39]. Our measurements are semi-quantitative and thus the results must be interpreted with care.
Uranium aerosols at the pelletizing workshop frequently contained iron and, to some extent, chromium and nickel, as shown inTable 2. These elements probably attach to the uranium pellets as they slide along transportation bands. Nickel is also found in the grinding equip-ment. In certain batches, chromium is added to the powder to improve sintering properties. Weight percentages of these elements in uranium aerosols were in many cases substantial (N50%) (Table S2 (Appendix)).
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. Elemen-tal composition alone is insufficient to distinguish between the many potentially present uranium compounds, but a presence of nitrogen and/orfluorine does, however, give an indication of chemical form.
Fig. 5a shows the EDX spectrum of a particle with an obvious presence of nitrogen andfluorine, indicating UO2F2, AUC and/or uranium perox-ide.Fig. 5b shows the EDX spectrum of a uranium particle where nitro-gen andfluorine peaks are absent, indicating uranium oxide.
It was found that for Sampling 1, impaction Stages E and F, about 50% of the uranium aerosols contained nitrogen and/orfluorine. The re-maining 50% indicated uranium compounds such as UO2and/or U3O8. At impaction Stage B,N90% of the uranium particles contained nitrogen and/orfluorine. For Sampling 2, about 70% of uranium aerosols indicat-ed nitrogen and/orfluorine, but were not correlated to impactor cut-point.
Fig. 3. Uranium weight percentages of U3O8reference particles.“+” and “o” markings
represent automated and manual scans, respectively. The dashed line represents the theoretical uranium weight percentage for U3O8. Two particles with ECD 37.2μm and
61.3μm are outside the scale of the x-axis. Their corresponding uranium weight percentages are 74.7% and 75.6%, respectively.
Fig. 4. Uranium weight percentage of uranium aerosols with 2–4 μm ECD collected at impaction Stages C–G. Error bars correspond to ±1 standard deviation. The solid line represents the expected, uncorrected, measured uranium weight percentage for U3O8
aerosols, based on our reference particle measurements. The dotted and dashed lines represent the theoretical uranium weight percentage for UO2and U3O8, respectively.
Data have not been corrected for the oxygen bias discussed in the text.
Table 2
Percentage of uranium aerosols (Sampling 4) that also contained either iron, chromium or nickel at impactor Stages C–G.
Impactor stage Uranium aerosols containing either Fe, Cr or Ni
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%
3.4. Pelletizing workshop - aerosol dynamic shape factor
Calculating the aerosol dynamic shape factor using Eq.(1)for the pelletizing workshop proved to be difficult; in particular the estimates of deand particle density assumptions.
Fig. 6illustrates shape factor estimates for densities corresponding to milled UO2powderfill density, regular UO2powderfill density, un-sintered pellet density and un-sintered pellet density as described in
Section 2.1. A dynamic shape factor below 1 is not possible by definition, which suggests faulty deestimates 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 sup-ported by the high chromium contents at Stage C (Table S2).
A density of 10.5 g/cm3corresponds 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 aero-sols of 0.5–1.5 μm aerodynamic diameters[40]. However, these were long chains of particles and the particles in the present work are expect-ed to be associatexpect-ed with a lower shape factor.
Fig. 6suggests that uranium aerosols in the pelletizing workshop have a density lower than 10.5 g/cm3and a shape factorN1.5. This is supported by the electron microscope images frequently showing high-ly irregular particles. The correlation between dae, de, density and shape factor appears to be very complex.
4. Conclusions and perspectives
The present work constitutes, to the best of our knowledge, thefirst 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 late im-paction stages. This is partially explained by a bias from electron beam interactions with oxygen in the impaction substrate. Measurements of U3O8reference 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 2μm ECD. The bias is greater for smaller particles. However, aerosols in the same size interval (2–4 μm ECD, where the bias was approximate-ly uniform), but collected at different impaction stages, unexpectedapproximate-ly showed different uranium weight percentages. This could indicate a variable density of uranium aerosols, but thefinding requires further in-vestigation. The aerosol dynamic shape factor proved difficult to evalu-ate, but there are indications that the generally assumed value of 1.5 is an underestimation. Size distribution and shape factor need further in-vestigation for other workshops.
Many particles at the conversion workshop showed traces of nitrogen andfluorine. 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 perspec-tives include studies of chemical form and crystalline states using micro X-rayfluorescence (µXRF) and micro X-ray diffraction (µXRD) analysis. Acknowledgements
The authors are grateful to the International Atomic Energy Agency for allowing use of their SEM/EDX equipment at the Environment Labo-ratories 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 (grant number SSM2014-4843). Special thanks go to Isabelle Levy and Francois Oberhansli at the IAEA for assistance with the SEM, Magnus Hedberg
Fig. 5. SEM/EDX spectra of a: a uranium aerosol containing nitrogen andfluorine; 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.
Fig. 6. Shape factor estimates for the pelletizing workshop (Sampling 4) for impaction Stages C–G. Error bars correspond to estimated uncertainty (±15%) in de.
and Noëlle Albert at the European Commission, DG Joint Research Cen-tre, Directorate G - Nuclear Safety and Security for their commenting on the manuscript, and to Jörgen Gustafsson and numerous colleagues at Westinghouse Electric Sweden AB for invaluable discussions and comments.
Appendix A. Supplementary data
Supplementary data to this article can be found online athttp://dx. doi.org/10.1016/j.sab.2017.03.002.
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