Characterisation of aerosols produced in a simulated severe nuclear accident using electron microscopy

Characterisation of aerosols produced in a simulated severe nuclear accident using electron microscopy Contents lists available at ScienceDirect Journal of Aerosol Science journal homepage www elsevie[.]

Journal of Aerosol Science 106 (2017) 68–82 Contents lists available at ScienceDirect Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci Characterisation of aerosols produced in a simulated severe nuclear accident using electron microscopy K Knebela,b, ⁎,1 MARK , P.D.W Bottomleya, V.V Rondinellaa, A Lähdeb, J Jokiniemib a European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O Box 2340, D-76125 Karlsruhe, Germany University of Eastern Finland, Department of Environmental Science, Fine Particle and Aerosol Technology Laboratory, P.O Box 1627, FIN-70211 Kuopio, Finland b AB S T R A CT The chemical composition of aerosols formed by revaporisation and subsequent condensation of fission products and simulant material was investigated using electron microscopy A stage dedicated only to the collection of aerosol particles on carbon coated copper grids was designed and tested As raw material for these experiments core material from the TMI-2 accident, fission product deposits from the Phébus FPT3 tests and Cs2MoO4 powder were used The examination of aerosols was made by transmission and scanning electron microscopy and energy-dispersive Xray spectroscopy The results indicate the range of species encountered at different temperatures showed the necessity for a time dependent sampling method The investigated aerosols were in the nanoparticle range A clear pattern was found for the use of fission product deposits from the Phébus FPT3 experiments In oxidising atmosphere in the temperature range of 450 K to 910 K a species of nm sized spherical caesium-rhenium particles were observed that coagulated to spherical about 20 nm sized particles Other elements such as Mo and Te occurred at various temperature ranges and atmospheres Also the vaporisation of caesium molybdate, as predicted in thermodynamic equilibrium calculations, was examined Introduction In the case of a severe nuclear accident the main goal is to prevent the release of radioactive fission products into the environment After the Great Eastern Tsunami on March 11, 2011 in Japan, fission products (FP´s), especially the volatile species, leaked into the environment It is estimated that 0.6 −1.5×1019 Bq of 133Xe, 0.7 −5.0×1017 Bq of 131I and 1.0−5.0×1016 Bq 137Cs have been released into the atmosphere (Koo, Song & Yang, 2014) Traces of the released isotopes were measured in locations in Asia, for example China (Tuo et al., 2013) and Vietnam (Long et al., 2012) and also in Europe, for example Finland (Kettunen, Kontro, Leppanen & Mattila, 2013) and France (Evrard et al., 2012) This shows that such an accident is not a regional but a global threat To assess these threats, nuclear safety codes, such as ASTEC (Chatelard et al., 2016) are used to simulate possible scenarios and their impact Although the state of the art nuclear safety codes use thermodynamic equilibrium calculations to model the chemical reactions between the different elements interacting during an accident, they rely on experimental data for the chemical species formed (Allelein et al., 2009) The objective of this study is to enhance the knowledge of Cs behaviour in severe accident conditions Due to release mechanism of fission products vaporising from the degraded fuel and subsequent condensation an aerosol formation when released into the atmosphere, a special attention is given to vaporisation and condensation in this study In 1988 the Phébus FP ⁎ Corresponding author at: European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O Box 2340, D-76125 Karlsruhe, Germany E-mail address: KevinKnebel@gmx.de (K Knebel) present address: Hauptstraße 270, 50169 Kerpen, Germany http://dx.doi.org/10.1016/j.jaerosci.2017.01.008 Received May 2016; Received in revised form 27 January 2017; Accepted 31 January 2017 Available online 01 February 2017 0021-8502/ © 2017 Elsevier Ltd All rights reserved Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al programme was launched by the French ‘‘Institut de Radioprotection et de Sûreté Nucléaire’’ (IRSN) in cooperation with the European Commission (EC) and other international partners to investigate light water reactor accidents and the subsequent release of fission products (March & Simondi-Teisseire, 2013) As a follow-up to these experiments, parts of the experimental line with fission product deposits, were used for separate effect tests to examine the behaviour of 137Cs more closely 137Cs is of special interest as it is produced in high amounts during the fission process; it has a high volatility and with 30.2 years’ half-life that makes it a long term problem The initial facility at JRC-Karlsruhe was used to study the revaporisation of 137Cs of relatively active samples by monitoring the gamma-radiation emitted from fission products remaining on the sample inside the furnace (Auvinen, Bottomley, Jokiniemi, Knebel & Rondinella, 2014a) and the total deposits collected on filters to give the total revaporisation of all other (gamma –emitting) FP´s Thus a decision was made to upgrade the device with a means to collect the aerosols and gather information about the chemical composition and size of the aerosols that form during the rapid cooling of the revaporised fission products To get the particles in an untainted state the collection was made directly at their place of origin by impaction on to mm in diameter copper grids with a carbon substrate layer used for Transmission Electron Microscopy (TEM) In this paper two sorts of samples were examined: TMI-2 molten core samples and Phébus PF FPT3 vertical line deposits dominated by 137Cs In addition, two simplified experiments with Cs2MoO4 were also performed The TMI-2 and Phébus samples were chosen due to the fact that they represent the very rare case of deposits of irradiated material that was produced under accident conditions Cs2MoO4 was chosen because it represents the chemical form, which was predicted from thermodynamic calculations, but could not be observed in our previous experiments (Auvinen, Bottomley, Jokiniemi, Knebel & Rondinella, 2014b) The grids were exchanged during the sample heating, particularly at high revaporisation rates, and were examined by either TEM or SEM microscopy to determine size and composition where possible Cs as the major gamma emitting isotope was observed along with Re originating from thermocouples in the original Phébus tests Small amounts of Mo and Te also observed in the Phébus FPT3 samples Materials and methods 2.1 Experimental set-up 2.1.1 Fundamental set-up The basic design of the revaporisation set-up is shown in Fig and consists of a furnace, a diluter and a filter stage that are connected to a process gas and a diluter gas flow A stainless steel (1.4301) tube with 375 mm length and 21.5 mm diameter runs through the furnace The tube can be opened to insert the sample into the furnace and has the process gas connected at the furnace inlet The furnace can reach up to 1273 K while being continuously provided with a steady gas flow through the process gas inlet Hydrogen, air, nitrogen and argon are available as process gases with 0.1 to l/min flow rates Additionally, steam is available at to 120 g/h mass flow rates (~0.1 to 0.5 l/min nitrogen is needed as carrier gas for the steam) Any vapour produced during the experiment is transported down the steel tube in the furnace and passes to the diluter The diluter consists of metallic cylinder housing a porous sinter metal tube The outside of the porous tube is provided with a flow of 150 l/min nitrogen gas at room temperature which flows through the porous tube and mixes with the hot process gas flow resulting in a rapid cooling of the process gas along its length and any vapour transported by it As the temperature of the process gas is lowered from up to 1173 K (the process gas never reaches the maximum furnace temperature of 1273 K as there is always an offset between furnace and sample temperature) down below 300 K most condensable species solidify and form aerosols while remaining suspended in the turbulent gas flow These aerosols are collected inside the aerosol sample stage, which is connected to the diluter outlet The gas flow finally enters the filter stage where it flows along a high flow rate quartz tube filter to collect any aerosols on its inner cylindrical surface before the gas flow Fig Schematics of the revaporisation set-up used to examine radioactive sample with dose rates up to 25 mSv/h The surrounding mm thick lead shielding is not shown 69 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al Fig The upper photo shows the initial sampling method, in the middle a CAD drawing of the improved section is shown and at the bottom is a photo of the resulting combined filter and sampling section leaves the glove box drawn by a vacuum pump into the glove box exhaust system The vacuum pump also ensures a steady flow in the system as well as preventing any fission product from contaminating the glove box It also prevents any hydrogen from leaking during the experiments into the glove box and thus prohibits the potential build-up of an explosive atmosphere Additionally, the revaporisation set-up is equipped with a scintillation gamma-ray detector above the glove box used to measure the revaporisation rate of detectable fission products A more detailed description of the set-up can be found in (Knebel et al., 2014a) 2.1.2 Aerosol sampling The first step in examining the aerosols produced in the diluter was to take small pieces of the inner surface of the cylindrical fibrous quartz filter where they were collected and examine them with scanning electron microscopy (SEM) This proved to be unsuccessful, as single particles could not be identified due to their small size and the uneven structure of the quartz fibres Nevertheless, the energy-dispersive X-ray spectroscopy (EDS) showed the presence of traces of uranium and caesium Due to the fast cooling process in the diluter and the turbulent flow conditions, it was anticipated that the condensed aerosols that form are in the submicron region and therefore TEM combined with EDS would be a more successful analysis method This also has the benefit that the samples produced are very small and thus also emit lower radiation than accumulated samples produced with a cascade impactor At first a simple collection device using TEM-grids was used (Fig 2) and the grids did have deposits of fission product aerosols; however, a shorter stage was designed that was able to exchange the samples during the experiment As the furnace is heated slowly over several hours with K/min this enables the aerosols collected to be assigned to a corresponding temperature This filter stage is shown in the middle of Fig The TEM-grid is placed on a mm wide stainless steel sample holder and is fixed with a small hinge and screw The aerosol sampling stage itself consists of a short stainless steel pipe with flanges on each side, a hole on its upper side to insert the TEM-grid and a simple locking mechanism to fix the holder of the TEM-grid while sealing the system Finally polyoxymethylene was used instead of steel for the sampling stage This has the benefit that one flange connection is required The assembled aerosol sampling stage / filter cartridge combination is shown at the bottom of Fig Each combined sampling stage is now used only for one experiment and discarded afterwards The experiments showed that even within the limited space of the glove box, the exchange of a TEM-grid is possible in about The combination of this short amount of time and the under pressure produced by the vacuum pump excluded any significant contamination of the glove box 2.2 Fission product samples The initial material for the aerosols examined in this study originated from two different sources The first were degraded fuel samples from the TMI-2 severe nuclear accident that were examined during an OECD/NEA project (Akers, 1992) The TMI-2 samples were taken from the completely molten central mass of the degraded core and were in the form of a fine dark powder The initial examination (Bottomley & Coquerelle, 1992) showed three different oxide phases; these were a uranium-rich [(U,Zr)O2], a zirconium-rich [(Zr,U)O2] and a ferrous phase containing structural material from the reactor core, (iron, chromium, nickel and aluminium) The examination showed also the overall atomic composition of the metallic phases of this sample to be 18% U, 23% Zr and 59% (Fe, Cr, Ni, Mn and Al) The core samples were in powder form since they are remnants from the cutting performed in the initial examination These samples were used for the experiments as they still contained a significant amount of radioactive 137Cs, which is the main target of the revaporisation studies 70 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al The second type of sample were fission product deposits formed on the upper vertical line of the Phébus FPT3 experimental circuit (Haste, Payot, & Bottomley, 2013; Haste et al., 2013; Hache, Schwarz, von der Hardt, 1999), which was kept at a temperature of 973 K during the Phébus experiments The samples from the Phébus FPT3 experiment were of a different nature as they contained fuel only in trace amounts They consist of layers of fission products that first vaporised from the test bundle during the FPT3 experiment and then condensed on the inside of the stainless steel/Inconel pipes in the upper vertical line just above the degrading bundle Ring-shaped pieces were cut out of the piping and used as the source material for these experiments The rings were cut as needed to smaller pieces and then fixed with alumina glue on a stainless steel sample holder with the fission product deposit facing upward The fission product deposit is very brittle and substantial amounts are lost during the cutting process Nevertheless, the activity emitted by the samples is still quite high at about 1–20 mSv/h dose rate for a 10×10 mm² sample The gamma radioactivity was over 97% due to the 134Cs and predominantly 137Cs isotopes Previous analysis (Bottomley et al., 2014) showed that an roughly cm² sample of the upper vertical line above the FPT3 degrading bundle would have after cutting (thicker outer layers may be broken off during cutting) an adherent deposit with the following approximate composition: Fission Products (natural isotopes excluded - except Sb): i) Major: Cs ~50 μg, Te ~75 μg, Mo ~20 μg (+50% nat), Rb ~5 μg, ii) Minor: Tc-99: ~1.5 μg, Sb ~ 0.4 μg, Ag ~0.3 μg; Zr ~0.3 μg, Fissile material: U ~4 μg, Control material B (from B4C rod): 200 μg Substrate materials: eg.: Fe, Cr, Ni, Zn, Cu Nevertheless, ICP-MS analysis of the deposits themselves and the aerosols collected on the main outlet filters demonstrated that many other fission products were present and could also revaporise under these conditions Substrate material (Fe, Cr, Ni) and thermocouples materials (W, Re) as well as fuel traces (U, Pu) and B absorber were also present in the deposit and on the filters Pb traces detected were impurities from the revaporisation shielding The results are reported in (Bottomley et al., 2014) 2.3 Experimental procedure The most important parameters of the experiments are the samples used, the furnace/sample temperatures; the process gas that is fed into the furnace and the time-frame in which a TEM-grid was inserted for aerosol sampling The information regarding process gas, temperature of furnace and sample and the period that a TEM-grid remained inside the furnace is shown for each experiment in Fig and Fig The TEM-grids were also labelled according to the experiment and the position in that experiment, so 5-2 is the second TEM-grid in experiment The choice of the heating profile and the carrier gas was based on previous experiments (Knebel et al., 2014b) and the results of the 137Cs kinetics measured by gamma spectroscopy As these experiments were the first tests after an upgrade of the facility, the same mixtures of the process gases air, steam and hydrogen were used However, in the present study the focus is based on the sampled aerosols The data collected with gamma spectroscopy regarding the Cs kinetics will be analysed and discussed in a future study As mentioned in Section 2.1.1., the process gas and thus also the sample never reach the temperature to which the furnace is programmed There is an offset that is at its maximum of up to 180 K (experiment 6), depending on the furnaces temperature, the gas flow and mixture used If not described separately, all temperatures in the following are the sample temperature 2.3.1 Experiment 1: TMI-2 powder in air atmosphere After a number of preparatory tests, experiment used a sample from the TMI-2 core consisting of 1.2 g fine powder produced during sample cutting in the initial TMI-2 fuel exploration programme The sample was first heated under nitrogen as inert gas (1 l/ min) to 515 K by 10 K/min, then the process gas was switched to air (1 l/min) and the sample remained at 573 K for 10 until the next heating phase with K/min began This continued until the maximum temperature of 1171 K was reached with a dwelling time of 60 Afterwards the cooling phase started with 10 K/min (see Fig 3) Three TEM-grids were present using the initial sampling method and retrieved the next morning 2.3.2 Experiment 2: TMI-2 powder in hydrogen atmosphere The same sample from experiment was reused as the online gamma measurements showed no loss of 137Cs from the sample under air Also, as shown in Fig 3, the same heating profile was used but the atmosphere after the initial heat up (10 K/min) to 530 K was hydrogen (1 l/min) Three TEM-grids were used to collect the aerosols (exposed for the whole experiment with the initial sampling method), but only two of them were retrieved successfully 2.3.3 Experiment Phébus sample in alternating air and hydrogen atmospheres In experiment a Phébus sample was used and was the first using the improved sampling method which allows the TEM-grids to be exchanged while the experiment is running This process took ~5 minutes, later it was possible to perform this in about As in previous experiments (Knebel et al., 2014b) the revaporisation of 137Cs did not start before reaching a furnace temperature of 773 K, the heating profile was changed, as shown Fig The sample was heated under air (2 l/min) by 10 K/min up to 680 K and left for 30 at this temperature, during which the first TEM-grid (3-1) was replaced The following heating phase was at K/min up to 827 K followed by a dwell phase of 60 during which the atmosphere was changed to hydrogen (2 l/min) for 20 min, simulating a short hydrogen surge in a severe accident scenario After this hydrogen phase the process gas was switched back to air (2 l/min) and the TEM-grid (3-2) was replaced and the third heating phase up to 1165 K at K/min started At temperature of 71 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al Fig Temperature profile of furnace and sample and process gas as used in experiments 1, and The sampling period of each TEM grid is represented by the dashed black horizontal lines The process gas is shown by the background colour as described at the top of this figure ~1050 K the gamma detectors showed 137Cs revaporising at a high rate therefore the TEM-grid (3-3) was replaced again and afterwards the atmosphere was changed to hydrogen (2 l/min) After reaching maximum temperature of 1165 K, the furnace rested there for 60 and then the cooling phase at 10 K/min was initiated After changing the process gas from hydrogen to nitrogen (8 l/ min) the TEM-grid (3–4) was changed and the system was left overnight to cool to room temperature under nitrogen The final TEMgrid (3–5) was retrieved the next morning 2.3.4 Experiment 4: Phébus sample in alternating steam and hydrogen atmosphere For experiment another sample from Phébus was treated with the same temperature profile as the previous experiment, as shown in Fig The atmosphere was steam (60 g/h) and nitrogen traces (0.5 l/min) or hydrogen (2 l/min) The steam atmosphere required small amounts of nitrogen in the mixing system to remain stable and prevent condensation before the furnace TEM-grid 4-1 was taken out at the beginning of the first dwell phase at 523 K and 4-2 was introduced at 608 K that was left inside during the following steam phase The temperature difference between taking 4-1 out and introducing 4-2 was due to the amount of time that was needed to change the samples As the 137Cs showed a less volatile behaviour compared to the previous experiment the hydrogen phase was brought forward to 772 K with the TEM-grid 4-3 being introduced beforehand Subsequently the process gas was changed to steam and later back to hydrogen Most of the 137Cs had revaporised at this time and the hydrogen phase was interrupted by a short nitrogen phase to change the grid to 4-4 which remained in place until the beginning of the final cooling using nitrogen The cooling grid 4–5 was then retrieved the next morning 2.3.5 Experiment 5: Phébus sample in alternating air and hydrogen atmosphere In experiment the sample originated from a slightly higher position in the Phébus upper vertical line where the deposit showed a higher (predominantly > 97%) Cs radioactivity This sample was treated with either air or hydrogen (2 l/min each) in an alternating pattern and with the same heating profile as the previous two experiments (see Fig 4) Grid 5-1 was introduced shortly before 72 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al Fig Temperature profile of furnace and sample and process gas as used in experiments to The sampling period of each TEM grid is represented by the dashed black horizontal lines The process gas is shown by the background colour as described at the top of the previous figure reaching the first dwelling phase As the sample showed no 137Cs release at ~800 K furnace temperature, as with every previous experiment (Knebel et al., 2014b) with oxidising atmosphere, the planned hydrogen phase was delayed and the grid left inside until the first significant activity loss was observed around 994 K, at which point grid 5-1 was taken out The following grid 5-2 was exposed until the beginning of the dwell phase at 1133 K and replaced by 5-3 Due to the low 137Cs revaporisation rate, compared to previous experiments, the phase at maximum temperature was prolonged to 120 before cooling The sample reached a maximum temperature of 1145 K during that dwell phase Grid 5-3 was retrieved the next morning 2.3.6 Experiment 6: Phébus sample in steam atmosphere For experiment a second upper vertical line Phébus sample was used as in experiment 5, but this time an experiment with pure steam was performed, as can be seen in Fig The sample was heated in nitrogen at 10 K/min to 562 K followed by a 20 dwell phase during which the first grid 6-1 was replaced by 6-2 and the atmosphere was switched to steam (60 g/h steam with nitrogen traces (0.5 l/min) as carrier gas) Thereafter, the temperature was raised by K/min for 50 followed by a 10 dwell phase during which the TEM-grid was replaced This was pursued until the maximum temperature of 1090 K was reached and the final dwell phase of 60 started The last TEM-grid 6–7 was retrieved shortly after the cooling phase started and afterwards the gas was switched to nitrogen 2.3.7 Experiment 7: Phébus sample in alternating steam and hydrogen atmosphere For experiment the Phébus upper vertical line sample showed a behaviour similar to that of the sample in experiment and with previous experiments compared to experiment Again alternating atmospheres were used during the experiment The previous heating sequence was used, as shown in Fig The short plateaus every 50 were again used to exchange the TEM-grids, culminating in the use of grids (7-1 to 7-6) The process gas was changed at the start of each dwell phase from 60 g/h steam (+0.5 l/min nitrogen) for 10 to l/min hydrogen 73 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al 2.3.8 Experiment & 9: Cs2MoO4 powder in air and hydrogen atmosphere Calculations performed by (Kissane & Drosik, 2006) indicate Cs2MoO4 as the most likely source for the vapour phases carrying caesium This led to the decision to test Cs2MoO4 powder (Goodfellow, 99.9% purity, particle size: 75 μm) and collect the aerosols The experiments were performed with the same heating profile as in experiment (Phébus - steam) & (Phébus - alternating steam & H2) Starting with inert nitrogen atmosphere the process gas was switched during the first dwell phase to air for experiment and to hydrogen for experiment respectively To lower the number of TEM-grids that had to be examined only grids were used in each experiment for 673–873 K, 873–1073 K and 1073–1273 K temperature regimes 2.4 Post-test analysis Most of these examinations were performed using a Field Emission (Schottky type) Transmission Electron Microscope (200 kV JEOL JEM2100F, JEOL Ltd., Tokyo, Japan) equipped with liquid nitrogen cooled (Si(Li)) 30 mm2 energy dispersive X-ray spectrometer (Thermo Noran System 7, Madison, WI, USA) A first short examination was performed to get an overview of the grids and already revealed a significant number of aerosols deposited on their surface For experiments 7–9, grids were used that did not have any carbon coating and so did not permit the examination with a TEM Hence these samples were examined with a Field Emission Scanning Electron Microscope (Zeiss Sigma HD VP, Carl Zeiss NTS, Cambridge, UK) equipped with two 60 mm2 (SDD) energy dispersive x-ray spectrometers (Thermo Noran) The digital micrographs were then analysed using the ImageJ software For micrographs showing a large number of particles the automatic particle analysis tool was used to calculate the Feret´s diameter (FD) which is the diameter of the smallest possible circle covering the whole projected surface area of the particle Results Representative micrographs will be shown of the main particle species on the grid along with size distribution plots EDS measurements were performed with TEM or SEM examination The proximity of the characteristic X-ray peaks of molybdenum, sulphur and lead can cause difficulties Lead can often be recognized by the higher energy L lines Previous analyses had shown fission product molybdenum is present in the deposits whereas sulphur was absent (Bottomley et al., 2014) Lead is an inevitable impurity from the large amount of lead shielding in the glove box Due to the relatively large number of samples that were examined with TEM & SEM only the micrographs of the Cs-containing aerosols are shown in Fig and the according FD distribution in Fig Additionally, the elemental compositions and size characteristics of the aerosols that were found during TEM and SEM examination are shown in Table 3.1 Experiment TMI-2 powder in air atmosphere During experiment all three grids were exposed in the filter during the whole time and the population of nanoparticles was large enough to calculate the size distribution for each grid The micrographs of grid 1-1 revealed cubic crystals with about 100 nm edge length and a species of nanoparticles with a bimodal distribution centring at ~6 nm and ~21 nm Elemental analysis revealed the cubic crystals to be of sodium chloride but was not conclusive for the smaller particles, showing traces of silver and molybdenum The next grid, 1–2, showed no significant number of coarse particles but a high density of nanoparticles with a nuclei mode ~11 nm No conclusive EDS could be performed on those particles Grid 1–3 showed a particle density that was lower than 1–2 but considerably higher than 1-1 with a nuclei mode of ~6 nm Furthermore, SEM analysis showed uranium-zirconium particles with ~3 μm diameter and particles consisting of iron-based alloys in wide range of sizes and shapes This confirms the analyses in the initial investigation of the full molten core; it is surprising that they still show some volatility The variation in size (modes varying from nm to 21 nm) shows that the aerosol capture varied with the exact positioning of the grid, namely the grids closer to the entrance of the filter stage showed smaller particles than the ones behind (downstream) In this respect, the improved design with fixed position is expected to have reduced this variation As the caesium activity of the sample did not decrease during the experiment, it was not a surprise to observe any nanoparticles containing Cs 3.2 Experiment 2: TMI-2 powder in hydrogen atmosphere Two out of three grids were retrieved successfully after the experiment Both show spherical nanoparticles with a size distribution mode at 24 nm for grid 2-1 and 10 nm for grid 2-2 Also bigger particles of varying size (100 – 1000 nm) and consisting of steel or uranium-zirconium mixtures were found Thus, similar size ranges and compositions were found under the air and hydrogen atmospheres Even so, the activity measurements showed a loss of about 50% of the 137Cs, although the elemental analysis was not able to identify nanoparticles containing any caesium 3.3 Experiment 3: Phébus sample in alternating air and hydrogen gas Only the grids 3-2, 3-3 and 3–4 showed a sufficient number of nanoparticles to evaluate the size distribution Grid 3-1 (293–680 K under air) showed only a small number of particles with ~14 nm in diameter and some larger (200 nm) structures that were of low contrast and thus most likely to be of material with a low atomic number Grid 3-2 (exposed from 682 to 827 K under air then H2) showed a bimodal distribution of small spherical particles with a peak at 12 nm and a population of larger, elliptical particles with 74 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al Fig Micrographs with Cs-containing particles (left is 20 k and right is 200 k magnification) First row is grid from experiment 4, second row grid from experiment 5, third row grid from experiment and last row grid from experiment Note the many uniformly-sized particles at lower magnification that may be multiphase agglomerates at high magnification 75 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al Fig particle size (FD) Distribution of the micrographs for Cs-containing particles shown in Fig Top left is from grid of experiment (mode: 32 nm), top right grid from experiment (modes nm and 22 nm), bottom left grid from experiment (modes: nm and 22 nm) and bottom right grid from experiment (modes: nm and 52 nm) Feret´s diameter of 30–60 nm Grid 3-3 (828 to 1063 K under air) was densely populated with a species of very small particles with a peak in their size distribution at ~4 nm The next grid, 3–4 (used during final heat to 1165 K, dwelling and subsequent cooling to 923 K under H2), showed larger structures that appeared to be agglomerates of 3–5 nm sized spherical nuclei that are attached to each other by a second phase Due to their size, it was not possible to analyse the elemental composition of the small nuclei But due to the fact that they were sampled during a period of high Cs release, it is very likely that they contain Cs The second phase attaching the small nuclei is represented by a brighter colour and thus consists of elements with a lower atomic number than the small nuclei The size distribution showed a broad spectrum of 6–16 nm Feret´s diameter peaking at ~10 nm Traces of molybdenum, silicon and lead were found in these agglomerates The last grid in this experiment was 3–5 (887 K to 293 K under N2) which showed relatively few particles on the micrograph, most notably were large structures in the size range of a few hundred nanometres EDS analysis indicated large amounts of silicon and some molybdenum in the particles of this grid 3.4 Experiment 4: Phébus sample in alternating steam and hydrogen atmosphere During experiment 4, five TEM-grids were used, from which size distributions could be derived Grid 4-1 (293–523 K under N2) had only a species of irregular shaped agglomerates (10–100 nm in diameter), consisting mainly of iron and some traces of phosphorus Grid 4-2 (608–750 K under steam) showed very similar large agglomerates consisting of iron, chromium, molybdenum and nickel These may be rust particles in the deposit or from corrosion of the sample holder Some of the agglomerates were silicon and silver-based The second type of spherical particles was considerably smaller The size distribution showed a first mode at nm, a second at around 30 nm and a third at 100–150 nm The next grid 4-3 (772–1029 K under H2 then steam), showed firstly a few large, cubic particles with an edge length of 100–150 nm that consisted of pure rhenium or rhenium with traces of manganese; secondly a dense layer of smaller spheroid particles with a broad size distribution centring at 32 nm Feret´s diameter (see Fig top row & Fig top left) EDS showed traces of silicon and molybdenum in those particles There were also a few darker spherical particles of about nm in diameter, embedded in the larger particles Under SEM two cubic caesium-containing particles were observed, very similar in 76 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al Table In this table the main results for the EDS analysis are shown for each grid in the experiments to The conditions during the sampling period are shown as well as the results of the elemental analysis and the modes of the ferrets diameter distribution The Temperature represents the actual sample temperature Exp No Sample origin TMI-powder TMI-powder Phébus FPT3 V11 Phébus FPT3 V11 Phébus FPT3 V15 Phébus FPT3 V15 Phébus FPT3 V15 Cs2MoO4 Cs2MoO4 a Grid No 2 5 3 3 Temperature Duration Process a [K] max [K] [min] gas 300 300 300 300 300 293 682 828 923 887 293 608 772 1052 293 450 994 293 355 562 639 723 812 910 995 610 666 751 842 932 1025 523 769 966 615 788 977 1171 1171 1171 1173 1173 680 827 1063 1165 293 523 750 1029 1165 1044 994 1133 1145 562 639 723 812 910 995 1090 660 751 842 932 1025 1129 769 966 1152 788 977 1164 500 500 500 500 500 61 92 160 110 air air air N2 N2 air air/H2 air H2 N2 N2 steam H2/steam H2 N2 air air H2/air N2 steam steam steam steam steam steam H2/steam H2/steam H2/steam H2/steam H2/steam H2/steam air air air H2 H2 H2 36 79 194 138 308 62 30 60 60 60 63 59 127 47 60 60 60 60 120 120 120 120 120 120 120 Elements observed FD modes observed [nm] Na, Cl, Ag, 8/21 11 23 – 12/40 10 – – 5/30/125 32 15 15/34 6/20 – 10 – – – 4/22 6/50 – – – – – – – – – – – – – – U, Zr, Fe U, Zr U, Zr Mo, Pb, Si Mo, Si Fe, P Fe, Cr, Mo, Ni, Si, Ag Re, Cs, Mn, Mo, Si Pb, Na, Cl, F, Te Cs, Re Pb, Te, Mo Ag, Mo, Cl, F, Ca, Na, K Mo, Cl, K, Na Cs, Re Na, Cl Cs, F, Cl, Mg, P Cs If a grid was exposed to two different process gases (N2, H2, air and steam) in an experiment, then it was consecutively size and appearance to the rhenium particles observed also with TEM This coincides with the gamma measurements during the experiments that showed a loss of ~35% in 137Cs activity during the exposure time of this grid The next grid, 4-4 (1052–1165 K under H2) showed a large number of agglomerates with an irregular shape and an even size distribution between ~4–20 nm and a semi distinctive peak at 15 nm can be observed Nearly all had lead as their main component sometimes with fluorine, sodium, tellurium and/or chlorine Several of these particle are attached to larger spherical objects (40–100 nm diameter) consisting mainly of sodium and fluorine The last grid 4–5 (1044-293 K under N2) shows spherical nanoparticles of about 15 nm in diameter and a second species of ellipsoidal shape with ~35 nm diameter Only the later particles could be analysed with EDS These all had tellurium as their main component with some traces of molybdenum The traces of silicon and fluorine found in the analyses may result from the vacuum grease used to seal the sections of the revaporisation line Na and Cl are considered to be impurities; their origin is not certain 3.5 Experiment 5: Phébus sample in alternating air and hydrogen atmosphere In experiment only TEM-grids were used to collect particles The reason for this was to provide a check on the data reliability The first grid, 5-1 (450–994 K under air), was inside the experimental line for more than h and thus the longest time of all For the micrographs see Fig second row from top and for the FD distribution see Fig top right It showed three particle types, one being spheroid with Feret´s diameter of 40–60 nm Higher magnification reveals those to be agglomerates consisting of small spherical nuclei of about nm diameter EDS indicated rhenium and caesium as the main components with traces of potassium, chromium and chlorine A second type of spherical particles was observed A third type appeared to be condensed light elements on the spherical particles mentioned before In the size distribution the second type had a peak at nm while the third species showed a peak at 20 nm Grid 5-2 (994–1133 K under air) showed a surface densely covered with spheroid- or ellipsoid-shaped particles with very low 77 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al contrast varying between 5–25 nm As seen in grid 5-1, some small, spherical nuclei of 2–5 nm in diameter embedded in these particles were observed EDS was not conclusive as the first species of particles (40–60 nm) evaporated before a sufficient measurement time had passed and the second type was too small Grid 5-3 (1145-293 K under H2 then air) was covered with rather large structures that varied in shape (between sphere and cube), size (20 to 100 nm Feret´s diameter) and contrast Higher magnification revealed they were embedded with small nuclei, EDS showed they contained lead, tellurium and molybdenum Those nuclei were held together by a phase with lighter elements like silicon, fluorine, sodium and oxygen, forming larger structures These structures were accompanied by another species of roughly spherical particles of circa 10 nm diameter that had too low a contrast for examination at high magnification 3.6 Experiment 6: Phébus sample in steam atmosphere During experiment seven TEM-grids were used to collect aerosols, grid 6-4 & 6-5 showed bimodal distributions (see Fig bottom row) with a smaller mode at < 10 nm and the second mode at 20–60 nm The first grid was nearly empty Grid 6-2 (562– 639 K under steam) was mostly covered in chain-like agglomerates with a length of ~200–600 nm It contained high amounts of silver with traces of chlorine, fluorine and molybdenum At other positions fluorine, potassium, calcium, sodium and molybdenum were found The chain structure may have been formed by either a 2-stage condensation process producing long filaments (1-D condensation process) followed by spherical (3-D) condensation process at sites along the filaments Alternatively, if fibres from the filter were detaching, then this could also produce this result As this was only observed here then this suggests that these are artefacts produced from the filter filaments (containing quartz - SiO2, Ca and Ba compounds); nevertheless, the EDS analysis shows the presence of genuine fission products (silver, molybdenum) in these condensates Grid 6-3 (639–723 K under steam) was densely covered by three phases of particles Firstly, a light grey phase that forms the outer spheroid shape with a Feret´s diameter of 200– 600 nm Embedded in this are irregularly shaped particles with a clear layer pattern inside and a rather constant diameter of ~100– 150 nm EDS shows them to probably consist of molybdenum In some cases, a third phase is present which is roughly cubic shaped and contains chlorine, potassium and sodium Grids 6-4 (723–812 K under steam) & 6-5 (812–910 K under steam) have a very similar appearance with two particle types present, one being very small with peaks in the size distribution at 4–6 nm and the second species of spherical-shaped agglomerates with a diameter mode of ~20–30 nm In grid 6-5 there are agglomerates of 2–3 of those particles present, giving a second peak in the size distribution at ~50 nm EDS shows the larger particle species on both grids to consist of caesium and rhenium At high magnification those particles consist of several ~2 nm sized spheres Grids 6-4 and 6-5 show Cs-Re particles forming both the primary aerosol particles but also agglomerating at higher temperatures with other condensing volatiles absent Under steam, hydroscopic Cs hydroxides may be formed that would stick together and assist agglomeration Grid 6-6 (910–995 K under steam) had some small particles, similar to the nm spheres on the previous two grids were observed Grid 6–7 (995–1090 K under steam) had particles that where either of similar shape as previously observed steel particles (found in grid 8-2) or cubic and consisted of NaCl 3.7 Experiment 7: Phébus sample in alternating steam and hydrogen atmosphere For experiment a new batch of TEM-grids was introduced into the glove box and unfortunately they did not have the carbon layer between the copper grids This meant the examination was restricted to SEM & EDS The most notable results were found on the edge of the copper grids 7-3 (751–842 K under H2 then steam), 7-4 (842–932 K under H2 then steam) and 7-5 (932–1025 K under H2 then steam), as shown in Fig 7, 500–1000 nm sized and irregular shaped particles were found with a high caesium content Several other elements were found on the grids (F, Cl, Mg, P), except on grid 7-1 which was empty Those elements most likely represent impurities from the experimental set-up 3.8 Experiment & 9: Cs2MoO4 powder in air and hydrogen atmosphere The sample was Cs2MoO4 powder in experiments (under air) & (under H2) using just grids The grids in experiment were used at a temperature of 523–769 K, 769–966 K and 966–1152 K In experiment the grids were used at 615–788 K, 788–977 K and 977–1164 K Only SEM and EDS was available for the analysis No molybdenum was found on any of the grids, but grid 9-3 (977– 1164 K under H2 then steam) showed several agglomerate-like structures with high caesium content An example is shown in Fig Taking into account the usual background of the EDS signal it appears that these agglomerates consist only of caesium (copper is from the substrate) This suggests that the Cs2MoO4 has been partially reduced under H2 to MoO3 and Cs (with the Cs depositing) although the steam at the highest temperatures would be expected to allow Cs2MoO4 to volatilise (Grégoire & Haste, 2013) & (Grégoire et al., 2015) noted that they obtained both Cs and Mo–containing particles depositing out at low temperatures from a CsOH, MoO3, I2 mixtures under inert Ar/H2 mixture They were able to note from Raman spectroscopy on other lower temperature zones the presence of polymoybdates (eg Cs2Mo3O7) in the deposits For the Phébus sample possibly the Cs deposition on alumina plates occurred during the reducing phase and this was not completely reversed under steam 78 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al Fig Micrograph taken by SEM from the surface of grid 7–4 (842–932 K under H2 then steam) The white circles mark the positions of particles containing caesium The diagram in the bottom shows the EDS analysis of the second particle from top Discussion 4.1 Test conditions and sampling method Experiments & that used powder demonstrate that the initial sampling method works and a gas flow of l/min is low enough to avoid most particle carry-over as very few large particles were observed on the grids The aerosol sampling at different temperatures shows the diverse particle species found at the different grids/temperatures/atmospheres examined in one experiment All tests showed that very few particles deposited on the grids below roughly 500 K Furthermore, when looking just at Cs, four grids (4-3, 6-4, 6-5 and 7-4) that showed Cs were present during a temperature range of ~720–1000 K This corresponds well with the Cs gamma data, showing revaporisation from ~750 K onwards The very uniform and fine size of the particles (< 10 nm) indicates a very rapid and efficient vapour condensation by the high flows of cold nitrogen in the diluter The lack of Cs activity in the diluter (from post-test scanning) also indicates that most particles condensed from the vapour phase have remained airborne in the very turbulent N2 flow through the porous wall of the diluter Thus most particles were collected on the filter or the sampling grids The larger particles observed on the grids were mainly agglomerates, therefore condensation on the airborne particles continued after the initial rapid condensation to form multi-element/phase particles But it has to be noted, that several of the elements found by the EDS analyses were unexpected and most likely pollutants: Si, Ca & Ba probably come from the quartz filter; Si and F from the vacuum grease for sealing the revaporisation line; Na, Cl perhaps comes from sample handling and Pb from hot cell contamination Fig SEM micrograph of grid 9-3 (1073–1273 K under alternating steam & H2) The area marked in white showed high amounts of caesium in the EDS 79 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al 4.2 Particle size and composition The results of the TEM and EDS examination showed that most particles are in the nanoparticle size range and consist only of a few different elements For the experiments with Phébus samples these are the initial aerosols produced by nucleation and are containing the most volatile elements such as Cs often with Re, but also Mo and Te (Re comes from the Phébus FP test thermocouple sheaths) A diversity of particle shape, size and elemental composition shows how widely the aerosols can differ by just raising the temperature by a 100 K between each grid Clearly at higher temperatures there are more elements resulting in condensation of the primary aerosols and creation of agglomerates For example, there was no significant revaporisation below 720 K, but thereafter Cs (with Re) was present with an ever-increasing number of elements until ~1173 K The distribution of the particle size between the grids of one experiment does not show any pattern that would allow to define a clear correlation between particle size and temperature Also the influence of the gas atmosphere on the particle size shows no clear connection This has to be analysed in further test, in which the sample are changed more often during the phase that now showed significant particle numbers and with fixed temperatures and only changes in the process gas The experiments & in which powder from the TMI core was used showed a large variation of particles Uranium and metallic particles with a size in the range of nm to 3000 nm were observed With the initial sampling method the grids were used over the whole duration of the experiments and could not be exchanged In combination with the sampling consisting of powder, it is not conclusive which particles were deposited by vaporisation and subsequent condensation and which were dragged by the gas flow The updated sampling method was designed and constructed to eliminate this by enabling a temperature dependent collection of aerosols 4.3 Cs-containing particles The examination of aerosols with traces of the fission product caesium is of special interest Cs seems to revaporise under all atmospheres whether reducing (H2), lightly reducing (steam) or very oxidising (air) However, comparison of grids in experiment showed that Cs also may be stickier in steam atmosphere and could form larger agglomerates in steam without other elements The grids 5-1 (450–994 K in air), 6-4 (723–812 K in steam) & 6-5 (812–910 K in steam) showed very similar results All grids are populated by a species of caesium and rhenium bearing particles with their size varying from single particles (~20 nm Feret´s diameter) or agglomerates of two or three particles (40–60 nm Feret´s diameter) Higher magnifications show that these particles in fact consist of many small spherical nuclei with ~2 nm diameter In all three cases they were accompanied by 4–6 nm sized spherical particles, possibly the same as the small nuclei forming the caesium-rhenium particles This leads to the conclusion that the primary (initial) Cs and Re-containing aerosol particles are ~2 nm size, formed at the lowest revaporisation temperatures At higher revaporisation temperatures they can form larger 4–6 nm particles of the same composition (or 2–3 particle agglomerates), simply due to the higher vapour concentration in the gas phase The much larger particles probably form at higher temperatures when more elements are evaporising and so condensing in greater mass in the diluter Cs-Re particles form first by nucleation and provide nuclei for the other elements or compounds (such as Te) to condense If these particles are gelatinous then this will increase the agglomeration rate and form the larger ~20 nm-sized particles observed At the highest temperatures the ~20 nm agglomerates may also have a long enough cooling phase to cause them to create new agglomerates of 2–3 particles (i.e 40–60 nm dia.) or cause other elements to condense and so grow further The Cs/Re aerosols are the main aerosol species that form when Phébus fission product deposits are revaporised Re was initially metallic as it is present as sheaths of the ultrasonic thermocouples of the Phébus FP bundle (Krischer & Rubinstein, 1992) Re was also found in large quantities of the deposits in the vertical line of FPT3 (as well as earlier tests) In the steam–rich FPT0 test the vertical-line deposits Re was found with In and it was presumed that Re was no longer metallic (Bottomley, Glatz, Stalios, Sätmark & Walker, 2000) Cs is already known from earlier work using radiotracers that it will revaporise and condense out alone without the presence of Re Cs in these air/steam atmospheres was expected to form oxides or hydroxides (Anderson et al., 2000) As during experiment in air no loss of Cs activity in the sample was observed it was not a surprise that the grids 1-1, 1–2 and 1–3 showed no Cs particles During experiment in hydrogen a significant amount of the Cs activity (50%) vaporised, but no particles were observed on the grids Nevertheless, compared to the Phébus samples the amount of Cs was rather low in these samples and thus showed no significant signal peak in the EDS The loss of Cs activity only in reducing condition can be explained with the history of the TMI samples The TMI core was exposed to oxidising conditions and high temperatures, thus the Cs that volatilise under such conditions already did so during the accident (Akers, 1992) 4.4 Other fission products observed As stated in Section 4.3, Re is present in the Phebus deposits used in this study Thermodynamic equilibrium calculations were performed for the fission product compounds Phébus deposits for FPT0 to FPT3 by (Girault & Payot, 2013) This showed that CsReO4 was the most stable fission product containing Cs & Re, particularly for FPT0 when there was no boron competing for the Cs Nevertheless it was still present under FPT3 with B4C absorber present CsReO4 appears to be the most likely form of the deposits on the vertical line Although only indirect evidence, CsMoO4 therefore appears to be the most likely form of the Re-Cs mixed aerosol particles detected in the filter cartridge The amount of Re as thermocouple sheaths is considerable (eg there was ~260 g Re in the FPT3 Phébus bundle (Payot et al., 2011)) Re because of the high mass present in whatever form is likely to be in saturated conditions and to condense rapidly It would therefore act as a primary seeding element, Clearly the primary Re or Cs containing condensed particles can act as nuclei, in sizes 80 Journal of Aerosol Science 106 (2017) 68–82 K Knebel et al from a few to 10–20 nm for Te or Mo vapours to condense out on, forming a mixed fine aerosol that could further agglomerate or deposit on a surface These would still be capable of revaporising at a later stage if a hot steam surge occurred in a reactor coolant system (RCS) (Sehgal, 2012) The form of the revaporising species would depend upon the atmosphere and flow conditions Ag was also detected in grid 4-2 (Phébus sample at 608–750 K under steam, experiment 4), this is possibly from the small amounts of Ag fission product analysed in the Phébus FP deposits (Haste, Payot, & Bottomley, 2013) Ag metal is known as a volatile element from various works (Hagen & Hofmann, 1987) to be ejected early upon rod failure with other volatiles and would condense rapidly to act as a primary seeding element along with Cs or Cs-containing compounds for aerosol formation Te is also a very volatile fission product and which rapidly condenses in cooler regions (Gregoire et al., 2008) This was observed at high temperatures and during cooling in experiment where a Phébus sample was exposed to steam and then H2, (grid 4–5 at 1044 to 293 K under N2); small agglomerates (15–30 nm) of Te with Mo were found Probably a Mo oxide was formed earlier under air, the Mo oxide and the Te (as element or oxide) has condensed together to form the agglomerates A Te-Mo oxide is also possible In addition, Mo (or its oxide) was also observed as an initial seeding component in experiments and Experiment revaporised a Phébus PF fission product deposit, and Mo was found on grid 6-3 (639–723 K under steam) in large (200–400 nm), multiphase agglomerates Calculations and data (Gouello et al., 2013) show that Mo oxides (eg MoO3) can react with Cs oxides to form stable molybdates In the FP simulant Cs2MoO4 powder experiments (air) & (H2), the chemical composition, size and shape of the particles carrying Cs are still unknown Only grid 9-3 (977–1164 K) in H2 showed traces of caesium, while grid 8-3 (966–1152 K) in air showed none This suggests that Cs2MoO4 only emits a caesium vapour phase at temperatures above 1152 K in air but in hydrogen already from 977–1164 K It is expected from SOPHAEROS modelling by (Gouello et al., 2013) that Cs2MoO4 would volatilise at around 1173 K, which was not reached in the experiments due to the temperature offset between furnace and sample Conclusions To enhance the current knowledge on fission product chemistry in severe accident conditions an existing experimental device investigating fission product volatility was upgraded with an aerosol sampling stage An advanced sampling method was designed and used for various experiments Heating samples containing fission products deposits up to ~1173 K using different process gases caused revaporisation of the deposited species During those experiments TEM-grids were used to collect the aerosols, which were formed by condensation of the revaporised vapours TEM, SEM and EDS results show that from ~450 K onward, various species of nanoparticles were formed in the cooled exhaust gas which varied in size and elemental composition depending on the process gases used and the temperature in the furnace Typical fission products Cs, Mo, Te and Re were observed regularly during the EDS analysis Some unexpected elements could be attributed to contamination from the filter or prior handling of the samples A clear pattern was found for the behaviour of fission product deposits from the Phébus FPT3 experiments in oxidising atmosphere where TEM-grids were used in the range of 450 K to 910 K and showed an initial species of nm-sized spherical caesiumrhenium particles that coagulated to spherical, about 20 nm-sized particles This leads to the conclusion that the smaller particles are the primary particles that form immediately after condensation by nucleation which then subsequently attach to each other to form the larger agglomerates Those primary particles are generally bigger when produced at higher temperatures, which can be explained by higher vapour concentrations and slightly longer time cooling down below their condensation temperature At the higher temperature the larger agglomerates also tend to agglomerate further by forming groups of particles containing two or three primary particles The formation of caesium-rhenium particles show that rhenium also rapidly condenses and is a primary seeding element Although Re is an experimental artefact from the Phébus tests, its behaviour would be expected to simulate Ru fission product and Ag absorber in a severe nuclear reactor accident and so understanding its behaviour will assist the analysis of complex aerosols and their formation Even so thermodynamic calculations suggested the formation of caesium molybdate during this study no particles were observed containing both In combination with the observed Cs-Re particles this leads to the conclusion that if a significant amount of Re is 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