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Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”
Radiation Measurements xxx (2015) 1e6
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter” Irina Vlasova a, *, Andrey Shiryaev a, b, Boris Ogorodnikov c, Boris Burakov d, Ekaterina Dolgopolova a, Roman Senin e, Alexey Averin b, Yan Zubavichus e, Stepan Kalmykov a a
Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1 Bld. 3, Moscow 119991, Russia Institute of Physical Chemistry and Electrochemistry RAS, Leninsky Av.,31 Bld. 1, Moscow 119991, Russia Karpov Physical Chemistry Institute, Vorontsovo Pole Str. 10, Moscow 105064, Russia d Khlopin Radium Institute, 2nd Murinsky Av. 28, St. Petersburg 194021, Russia e Kurchatov Institute, Akademika Kurchatova 1, Moscow 123098, Russia b c
h i g h l i g h t s We discovered the homogeneity of a-activity of Chernobyl lava on the scale of tens microns. We proved that brown type of Chernobyl lava has higher a-activity than black type. Disintegration of the brown lava heap at the level 0.0 m was shown.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2014 Received in revised form 29 April 2015 Accepted 11 June 2015 Available online xxx
Homogeneity of the radioactivity distribution, composition and structure of radioactive phases were investigated for several samples of Chernobyl fuel-containing materials (Chernobyl “lava”) collected in different parts of the destroyed reactor building. The spatial distribution of the a- and of the total activity is homogeneous on the scale of tens microns. XAFS results suggest that the major fraction of U is dissolved in a glassy matrix. At the same time, SEM-EDX and X-ray tomography reveal a high concentration of inclusions and bubbles in the lava samples. The process of lava disintegration continues with different rates depending on environmental conditions around a particular lava stream in the reactor building. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chernobyl “lava” Radioactive particles High-uranium zircon Dendrites of UO2þx with Zr Etched alpha tracks in CR-39 Imaging Plate radiography
1. Introduction The major part (about 95%) of radioactivity of the former 4th Unit of the Chernobyl Nuclear Power Plant (NPP) is located inside the confinement building “Shelter”, as assumed by some researchers [Arutyunyan et al., 2010], and it is mainly concentrated in lava-like fuel-containing materials (“lava”). The lava was produced by interaction of hot UO2 fuel first with zircalloy cladding and then with concrete, metal and other components of the reactor building.
* Corresponding author. E-mail address: [email protected] (I. Vlasova).
The lava spread in horizontal and vertical directions, forming highly radioactive glassy solids which penetrated in many premises below the former reactor shaft. The lava consists of a U-containing silicate glass matrix with inclusions of high-uranium zircon crystals, molten stainless steel particles, uranium oxide dendrites and grains, and particles of ZreUeO phases [Burakov et al., 1994, 2010; € ml et al., 2013]. The lava is differentiated in two Borovoi, 2006; Po main phases: a) brown lava with higher U concentration and more abundant inclusions, and b) black lava [data of V.G. Khlopin Radium Institute, Trotabas et al., 1993]. The lava matrices are weathered both by self-irradiation and interaction with air and water; mechanical stresses also play a role. Surface disintegration of the lava, oxidation of U(IV) to U(VI), formation of secondary uranium
http://dx.doi.org/10.1016/j.radmeas.2015.06.005 1350-4487/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
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I. Vlasova et al. / Radiation Measurements xxx (2015) 1e6
minerals have been previously studied [Burakov et al., 1997a]. In some parts of the “Shelter” the amount of radioactive aerosols increases with time as a result of dust remobilization due to the drying of concrete walls and mechanical disintegration of the lava streams. Prediction of the lava destruction rate is often based on the assumption that radioactivity of the lava is mainly related to UOx inclusions. The size and composition of the radioactive aerosol particles inside the “Shelter” is the subject of a long-term study [Bogatov et al., 1991; Ogorodnikov et al., 2008]. The current work summarizes some new results on the present (as of year 2014) state of the lava samples and of aerosols collected inside the “Shelter” building. 1.1. Sampling and experimental All the samples were collected from the confinement building “Shelter” of Chernobyl NPP. 1) Two bulk lava fragments were mechanically detached in 1990 from two different types of lava matrix: a fragment of the black lava (approx. 3 1.5 1.5 mm3, Fig. 1a) was collected from the lava stream “Elephant Foot”, level þ6.0 m [Burakov et al., 1997b]; a fragment of the brown lava (3 2 mm2, Fig. 1d) was collected from the steam-discharge corridor at a level þ6.0 m. Both fragments were mounted into an acrylic resin and polished. 2) Individual particles were collected in 2013e2014 on a planar cuvette placed on the floor 0.50 m in front of a lava heap (level 0.0 m, the first floor of the Bubbler Tank, exposure for 6 months [Ogorodnikov et al., 2013]). These particles are of particular interest, since their detachment from the lava heap appears to be spontaneous. Note that the particular lava heap is mechanically heterogeneous: the internal part is highly porous (pumice-like or granulated) since it was formed when hot lava stream contacted water in the Bubbler Tank, whereas the outer shell is
glassy due to rapid quenching (Borovoi et al., 1991). The glassy shell was partly broken by researchers. It is impossible to say with confidence whether the studied particles come from the heap's shell or from its interior. 3) Aerosol particles were collected in November 2011 at the distance of 20e30 cm from the same lava heap as in the previous case (level 0.0 m, the first floor of the Bubbler Tank) using a pack of three Petryanov filters with different particulate retention sizes mounted on the nose of the air blower Н810 RadeCo operating for 2 h at a pump rate 100 dm3/min. The distribution of a-emitting radionuclides in the lava fragments and aerosol filters was assessed by a-track analysis using CR39 track detector (TASTRAK, Bristol, UK) and optical microscope Olympus BX-51 with ImageScopeM software. To obtain maximum information from the a-track analysis of different samples the exposition time was varied between 10 s and 3 min. Homogeneities of a-activity distribution in the lava fragments were proved by measuring the number of a-tracks per constant area of 150 150 mm2 under 100 magnification (Fig. 1f). a-tracks in the brown lava were measured after a 30 s exposure; in the black lava, after 2 min of exposure. CR-39 detectors were etched in 6.25 M NaOH with 1% ethanol at 70 C during 6 h. Individual aerosol particles were localized by radiography with Imaging Plates (Cyclone Plus Storage Phosphor System, PerkinElmer; their suitability for investigation of 137Cs was recently shown in Korobova et al., 2014). Gamma-activity of the samples was determined using g-spectroscopy (HPGe detector, CANBERRA, Genie-2000 software). SEM-EDX was performed using JEOL JSM-6380 LA with a JED 2300 analyser. Raman spectroscopy (Senterra by Bruker, excitation wavelengths 532 and 785 nm, laser spot size 2e10 mm) and infrared mapping (aperture size 100 mm, reflection geometry) provided information on structure of the lava matrix and inclusions. X-ray tomography and Extended X-Ray Absorption Fine Structure (EXAFS) were performed at the Synchrotron Center at Kurchatov Institute.
Fig. 1. Optical images of the black (a) and brown (d) lava fragments in acrylic mount; a-track radiography images of the black (b) and brown (e) lava surfaces (exposure e 2 min). (c) etomographic slice corresponding to the black lava surface (resolution 10 mm/pixel); (f) e grid for measuring of the number of alpha tracks per 150 150 mm2 (brown lava; exposure time e 30 s, magnification 100). Scale bars (a, b, c) e 500 mm; (d, e) e 1 mm; (f) e 200 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
I. Vlasova et al. / Radiation Measurements xxx (2015) 1e6
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Fig. 2. Imaging Plate radiography (exposure 11 min) of folded paper sheet containing particles from the floor of the first Bubbler Tank. Scale bar e 1 cm.
2. Results Fig. 4. Secondary U minerals (NaeSeUeO) on the surface of fragments of porous lava.
2.1. Black lava from the “Elephant foot” (level þ6.0 m) and brown lava from steam-discharge corridor (level þ6.0 m) According to Raman spectroscopy and X-ray microdiffraction the glassy matrix of the black lava is rather depolymerized silicate glass with dominating Q2 silicate units (Q2 > Q3 Q1). Raman spectra of different spots of the glass matrix are virtually identical; the infrared map is also rather featureless (with the exception of obvious zircon inclusions). Optical examination of both the polished lava surfaces reveals several inclusions represented by dendrites of UO2þx with a Zr admixture or (U, Zr)O2, crystals of highuranium zircon, (Zr,U)SiO4, and FeeCreNi spherical droplets (see also Anderson et al., 1992). The presence of strong peak at 1150 cm-1 in Raman spectra of UO2þx inclusions suggests that the deviation from ideal stoichiometry is minor (x < 0.07, Manara and Renker, 2003). Zircon grains are crystalline (no signs of amorphization in Raman spectra) and often consist of several growth zones with variable degrees of perfection. Limited morphological information from optical microscopy of the polished inclusions indicate that at least some of the zircon inclusions represent intergrowths of several crystals (Anderson et al., 1992; Geisler et al., 2005), which partly explains the heterogeneous chemical composition. The zircons are not strongly elongated, suggesting a moderate crystallization rate. In contrast, the urania inclusions often possess a dendritic morphology, indicating rapid diffusion-limited growth.
The marked difference in morphology of these two phases reflects differences in their crystallization temperatures. Whereas the urania dendrites grew rapidly at high temperatures from a supersaturated melt, zircons began crystallization later with growth during slow cooling of the lava. This scenario is supported by occasional observation of UO2 inclusions in zircons. EXAFS analysis of the U and Zr atomic environment in the black lava fragment indicates that both elements are largely present in an amorphous or strongly distorted environment, although a weak second coordination sphere corresponding to UeU scattering in UO2 is observed. According to XAFS results, Zr is mostly present in silicate glass in a 6-coordinated environment (e.g., Farges and Rossano, 2000). At the same time, X-Ray tomography of the black lava fragment performed at various spatial resolutions (2.4 and 11 mm/pixel) reveals the presence of numerous dense inclusions and bubbles with a broad size distribution in the lava volume. Their number density appears to exceed that of inclusions exposed on the polished surface, which likely reflects partial loss of particles during the polishing (Fig. 1c). Optical and SEM examination of the brown lava fragment proved its extreme heterogeneity with much more abundant inclusions than in the black lava fragment. The presence of abundant microinclusions (not necessarily radioactive) in the
Table 1 Gamma-activity of the samples from the “Shelter”, Bq/sample. Sample
Date of measurement Cs-137, Bq/sample Am-241, Bq/sample Eu-154, Bq/sample
29 Dec. 2014 Black lava fragment, 3.0 1.5 1.5 mm3, level þ6.0 m, “Elephant foot” Brown lava fragment, 2.5 2.0 mm2, level þ6.0 m, steam-discharge corridor 15 Apr 2015 Individual particles from the cuvette (3/4 part of the sample disc), level 0.0 m 23 May 2014 Individual particles on the sheet of paper, (1/4 part of the sample disc), level 0.0 m 26 June 2014 Aerosol particles in the filter pack, level 0.0 m 25 Nov. 2011 A quarter of a blue filter (collected on 18-11-2011), level 0.0 m 09 March 2013
68310 132000 1100 542 81 5.3
± ± ± ± ± ±
500 500 20 11 5 0.2
7030 1120 123 47 7.1 0.18
± ± ± ± ± ±
7 5 15 6 0.9 0.04
875 ± 18 1050 ± 10 21 ± 5 6.6 ± 0.7 Not detected Not detected
Fig. 3. (a, b) e particles from disintegrating lava from the first Bubbler Tank with distinct UO2þx inclusions; (c) e alpha-track radiography image of the whole lava fragment shown on Fig. 3b (exposure e 2 h). Scale bar (c) e 200 mm.
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
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Fig. 5. (On the top) e three layers of a filter pack with aerosol particles from the 1st Floor of the Bubbler Tank: (see text for description); (on the bottom) e Imaging Plate radiography of 3 filters, exposure time: the 1st filter (blue) e 15 min; the 2nd and the 3rd e 24.5 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Alpha track radiography of radioactive aerosol particles from the 1st Floor of the Bubbler Tank (level 0.0 m), exposure time e 43 h. Scale bars: (a) e 200 mm; (b) e 100 mm.
Fig. 7. SEM images of radioactive aerosol particles: (a) e back-scattered electron image of individual UeO particles e white points; (b) secondary electron image of the compact aggregate of UeO particles.
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
I. Vlasova et al. / Radiation Measurements xxx (2015) 1e6
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Fig. 8. Secondary electron image (on the left) and elemental mapping of a single UeFeeO particle: U e in the center; Fe e on the right.
lava volume was noted earlier (e.g., Savonenkov et al., 1991). In spite of the clearly observed non-uniform distribution of the U-bearing particles in two lava fragments, the total radioactivity (measured by Imaging Plate) along with surface a-activity are evenly distributed on the scale of tens of microns (Fig. 1b, e). The number of a-tracks per square per time (cm2 s1) measured in the black and brown lava is 2.5$103 (±3s) and 8.4$103 (±3s), respectively. The apparent contradiction of the radiography, suggesting a homogeneous distribution of radionuclides, and the obvious presence of numerous inclusions (optical microscopy, SEM, X-ray tomography) can be partly resolved by the limited spatial resolution of track radiography (~10 mm) and the loss of spatial resolution in digital radiography due to the strong contribution of high energy band g-radiation from the specimen volume. A consistent explanation of the XAFS results for uranium (weakness of the UeU second sphere) would be more complicated since urania inclusions appear to be abundant. According to Raman spectra the crystalline quality of the larger (>10 mm) inclusions is rather high. This might be explained by a fortuitous selection of the volume sampled by XAFS, or by differences in crystalline perfection (and stoichiometry?) of small and large UO2 inclusions.
size of the second filter (rose, FPA-70-0.20) is similar to the first one. The last filter (white, AFA RSP-20) traps the smallest particles (Fig. 5, top). The area of each filter is 20 cm2, the flow velocity was equal to 0.85 m/s. Imaging Plate radiography shows that the major fraction of the total activity is concentrated on the first filter (Fig. 5, bottom). The other two filters also contain radioactive aerosol particles but of much lower total activity. Note that some radioactive particles (possibly large ones) were only weakly retained by the filter material and could have been easily detached from it. gspectroscopy results of the three filters with aerosol particles are presented in Table 1. Alpha track radiography reveals particles with alpha-emitting nuclides (Fig. 6). According to SEM-EDX data they are represented by aggregates of UeO containing particles about 0.2e0.4 mm in size (Fig. 7). Extensive investigations (Burakov et al., 1994) show that virtually all urania inclusions from Chernobyl lava contain at least a minor Zr admixture. Absence of Zr in the studied aerosol particles suggests that they might represent dispersed fuel dust widely spread inside the “Shelter”; morphology of some of the aerosols confirms this hypothesis. Very rarely larger U-containing particles with a complex composition are found, see example in Fig. 8 (UeFeeO particle).
2.2. Individual lava particles, the 1st floor of the Bubbler Tank (level 0.0 m)
3. Conclusions
The radioactive particles were located in the collection filters using Imaging Plate (Fig. 2). The gamma-spectroscopy data for two parts of the sample disc shown in Fig. 2 are presented in the Table 1. SEM-EDX of selected individual particles reveals a morphology resembling chips of porous glass (pumice-like). Numerous UeZreO dendrite-shaped inclusions (Fig. 3a, b) with different U/Zr ratio are also observed. The alpha-radioactivity of all investigated lava fine fragments (particles) is distributed homogeneously (Fig. 3c). SEM shows that agglomerates of needle-like secondary crystals are frequently present on surface of the aerosol particles (Fig. 4). Often they are observed in former bubbles, but at present it is unclear whether the bubbles were indeed important for the crystals' formation or just facilitated their preservation. EDX analysis indicates that these secondary crystals contain NaeUeSeO (no Zr) and likely belong to water-soluble uranyl sulphates (hydrates?) (e.g., Burakov et al., 1997a). 2.3. Aerosol particles collected with an air blower on the Petryanov filters, the 1st floor of the Bubbler Tank (level 0.0 m) Radioactive aerosol particles were located on individual filters using Imaging Plate radiography and subsequently extracted by filter dissolution in acetone. The frontal filter (blue colour, FPA-700.12) traps particles with aerodynamic radii 1 mm; the retention
The matrices of Chernobyl lava are metastable and disintegrate due to mechanical stresses, interaction with water, self-irradiation, etc. The destruction rate appears to be related to the lava composition and location in the building. As a by-product of the lava-water interaction, formation of water soluble U-bearing phases takes place. The glass matrix of Chernobyl lava can be considered as a rather depolymerized silicate-based glass. Uranium and zirconium are largely dissolved in the glass, though inclusions of UeZreO phases are not uncommon. In spite of an extremely heterogeneous mineral composition of the Chernobyl lava, its radioactivity is distributed homogeneously on a scale of 5e10 microns. This means that a relatively homogeneous common source of highly radioactive silicate melt of Chernobyl lava existed for at least several days as an anoxic closed system. During this time it had separated into two layers: brown lava (relatively thin layer at the bottom) and black lava (main volume). According to a-track analysis data the total a-activity of the brown lava is approximately 3 times higher than the a-activity of the black lava. This correlates with the higher U content of the brown lava. As a result of some yet unclear reason (melting or mechanical destruction of concrete in contact with the melt's crust) the lava spread into premises of 4th Unit, located below former shaft of the reactor. A fraction of liquid brown lava has fallen into water forming porous material (porous lava) that is similar to natural volcanic pumice-stone.
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
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Acknowledgements The authors are very grateful to V. Zirlin and L. Nikolaeva from the V.G. Khlopin Radium Institute for sample collecting and treatment under the extreme conditions of the Chernobyl “Shelter”. The work was partly supported by RSCF grant 14-13-00615 (AAS). References Anderson, E.B., Borovoi, A.A., Burakov, B.E., et al., 1992. Artificial products of nuclear fuel e constructional materials interaction that formed as a result of Chernobyl NPP accident. Sov. Radiochem. (5), 144e155 (In Russian). Arutyunyan, R.V., Bolshov, L.A., Borovoi, A.A., Velikhov, E.P., Klyuchnikov, A.A., 2010. Nuclear Fuel in the «Shelter» Encasement of the Chernobyl NPP. Nauka, Moscow, p. 240. Bogatov, S.A., Lebedeva, L.I., Levina, L.A., et al., 1991. Physical-chemical Properties of Radioactive Aerosols inside the “Sarkophagus” Building. Kurchatov Institute preprint. IAE-5435/3. Мoscow, 24 с. Borovoi, A., 2006. Database for the Model of Lava Generation and Spreading. Tech. Rep. Project ‘‘Chess’’, 2916. Borovoi, A.A., Galkin, B.Ya, Drapchinskii, L.V., et al., 1991. Neogenic products of fuel e constructional materials interaction at the 4th block of Chernobyl NPP. Sov. Radiochem. No. 4, 177e196 (In Russian). Burakov, B.E., Anderson, E.B., Galkin, B.Y., Pazukhin, E.M., Shabalev, S.I., 1994. Study of chernobyl “Hot” particles and fuel containing masses: implications for reconstructing the initial phase of accident. Radiochim. Acta 65, 199e202. Burakov, B.E., Ojovan, M.I., Lee, W.E., 2010. Crystalline Materials for Actinide Immobilisation, Materials for Engineering, vol. 1. Imperial College Press. Burakov, B.E., Strykanova, E.E., Anderson, E.B., 1997a. Secondary uranium minerals
on the surface of Chernobyl “lava”. Mat. Res. Soc. Symp. Proc. 465, 1309. Burakov, B.E., Anderson, E.B., Shabalev, S.I., Strykanova, E.E., Ushakov, S.V., Trotabas, M., Blanc, J.Y., Winter, P., Duco, J., 1997b. In: Gray, W.J., Triay, I.R. (Eds.), Scientific Basis for Nuclear Waste Management XX, 65, Mat. Res. Soc., Warrendale, Pennsylvania, 1297. Farges, F., Rossano, S., 2000. Water in Zr-bearing synthetic and natural glasses. Eur. J. Mineralogy 12 (6), 1093e1107. €ml, P., 2005. A Raman spectroGeisler, T., Burakov, B.E., Zirlin, V., Nikolaeva, L., Po scopic study of high-uranium zircon from the Chernobyl “lava”. Eur. J. Mineral. 17, 883e894. Korobova, E.M., Dogadkin, N.N., Shiryaev, A.A., Kolotov, V.P., Kononkova, N.N., Turkov, V.A., 2014. A study of Cs-137 spatial distribution in soil thin sections by digital autoradiography and microscopy. J. Geochem. Explor. 142, 94e100. Manara, D., Renker, B., 2003. J. Nucl. Mater. 321, 233e237. Ogorodnikov, B.I., Khan, V.E., Kovalchuk, V.P., 2013. Aerosols as witnesses of destruction of lava-liked fuel containing materials in the object “UKRITTYA”. Problems of nuclear power plants' safety and of Chornobyl/Chornobyl, vol. 20. Institute for Safety Problems of Nuclear Power Plants. NAS of Ukraine, pp. 94e106. Ogorodnikov, B.I., Pazukhin, E.M., Klyuchnikov, A.A., 2008. Radioactive aerosols of the “Shelter”, 1986d2006”, 1991 Chornobyl 455. €ml, P., Burakov, B., Geisler, T., et al., 2013. Micro-analytical uranium isotope and Po chemical investigations of zircon crystals from the Chernobyl “lava” and their nuclear fuel inclusions. J. Nucl. Mat. 439, 51. Savonenkov, V.G., Krivokhatskii, A.S., Dubasov, YuV., Pazukhin, E.M., 1991. Radioactive artificial new formations from the former 4th block of Chernobyl NPP. Sov. Radiochem. (4), 140e149 (In Russian). Trotabas, M., Blanc, J.-Y., Burakov, B., Anderson, E., Duco, J., 1993. Examination of Chernobyl Samples Impact on the Accident Scenario Understanding. Report DMT/92/309, SETIC/LECR-92/36; Report IPSN/93/02; Report RI-1e63/92.
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter” Irina Vlasova a, *, Andrey Shiryaev a, b, Boris Ogorodnikov c, Boris Burakov d, Ekaterina Dolgopolova a, Roman Senin e, Alexey Averin b, Yan Zubavichus e, Stepan Kalmykov a a
Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1 Bld. 3, Moscow 119991, Russia Institute of Physical Chemistry and Electrochemistry RAS, Leninsky Av.,31 Bld. 1, Moscow 119991, Russia Karpov Physical Chemistry Institute, Vorontsovo Pole Str. 10, Moscow 105064, Russia d Khlopin Radium Institute, 2nd Murinsky Av. 28, St. Petersburg 194021, Russia e Kurchatov Institute, Akademika Kurchatova 1, Moscow 123098, Russia b c
h i g h l i g h t s We discovered the homogeneity of a-activity of Chernobyl lava on the scale of tens microns. We proved that brown type of Chernobyl lava has higher a-activity than black type. Disintegration of the brown lava heap at the level 0.0 m was shown.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2014 Received in revised form 29 April 2015 Accepted 11 June 2015 Available online xxx
Homogeneity of the radioactivity distribution, composition and structure of radioactive phases were investigated for several samples of Chernobyl fuel-containing materials (Chernobyl “lava”) collected in different parts of the destroyed reactor building. The spatial distribution of the a- and of the total activity is homogeneous on the scale of tens microns. XAFS results suggest that the major fraction of U is dissolved in a glassy matrix. At the same time, SEM-EDX and X-ray tomography reveal a high concentration of inclusions and bubbles in the lava samples. The process of lava disintegration continues with different rates depending on environmental conditions around a particular lava stream in the reactor building. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chernobyl “lava” Radioactive particles High-uranium zircon Dendrites of UO2þx with Zr Etched alpha tracks in CR-39 Imaging Plate radiography
1. Introduction The major part (about 95%) of radioactivity of the former 4th Unit of the Chernobyl Nuclear Power Plant (NPP) is located inside the confinement building “Shelter”, as assumed by some researchers [Arutyunyan et al., 2010], and it is mainly concentrated in lava-like fuel-containing materials (“lava”). The lava was produced by interaction of hot UO2 fuel first with zircalloy cladding and then with concrete, metal and other components of the reactor building.
* Corresponding author. E-mail address: [email protected] (I. Vlasova).
The lava spread in horizontal and vertical directions, forming highly radioactive glassy solids which penetrated in many premises below the former reactor shaft. The lava consists of a U-containing silicate glass matrix with inclusions of high-uranium zircon crystals, molten stainless steel particles, uranium oxide dendrites and grains, and particles of ZreUeO phases [Burakov et al., 1994, 2010; € ml et al., 2013]. The lava is differentiated in two Borovoi, 2006; Po main phases: a) brown lava with higher U concentration and more abundant inclusions, and b) black lava [data of V.G. Khlopin Radium Institute, Trotabas et al., 1993]. The lava matrices are weathered both by self-irradiation and interaction with air and water; mechanical stresses also play a role. Surface disintegration of the lava, oxidation of U(IV) to U(VI), formation of secondary uranium
http://dx.doi.org/10.1016/j.radmeas.2015.06.005 1350-4487/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
2
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minerals have been previously studied [Burakov et al., 1997a]. In some parts of the “Shelter” the amount of radioactive aerosols increases with time as a result of dust remobilization due to the drying of concrete walls and mechanical disintegration of the lava streams. Prediction of the lava destruction rate is often based on the assumption that radioactivity of the lava is mainly related to UOx inclusions. The size and composition of the radioactive aerosol particles inside the “Shelter” is the subject of a long-term study [Bogatov et al., 1991; Ogorodnikov et al., 2008]. The current work summarizes some new results on the present (as of year 2014) state of the lava samples and of aerosols collected inside the “Shelter” building. 1.1. Sampling and experimental All the samples were collected from the confinement building “Shelter” of Chernobyl NPP. 1) Two bulk lava fragments were mechanically detached in 1990 from two different types of lava matrix: a fragment of the black lava (approx. 3 1.5 1.5 mm3, Fig. 1a) was collected from the lava stream “Elephant Foot”, level þ6.0 m [Burakov et al., 1997b]; a fragment of the brown lava (3 2 mm2, Fig. 1d) was collected from the steam-discharge corridor at a level þ6.0 m. Both fragments were mounted into an acrylic resin and polished. 2) Individual particles were collected in 2013e2014 on a planar cuvette placed on the floor 0.50 m in front of a lava heap (level 0.0 m, the first floor of the Bubbler Tank, exposure for 6 months [Ogorodnikov et al., 2013]). These particles are of particular interest, since their detachment from the lava heap appears to be spontaneous. Note that the particular lava heap is mechanically heterogeneous: the internal part is highly porous (pumice-like or granulated) since it was formed when hot lava stream contacted water in the Bubbler Tank, whereas the outer shell is
glassy due to rapid quenching (Borovoi et al., 1991). The glassy shell was partly broken by researchers. It is impossible to say with confidence whether the studied particles come from the heap's shell or from its interior. 3) Aerosol particles were collected in November 2011 at the distance of 20e30 cm from the same lava heap as in the previous case (level 0.0 m, the first floor of the Bubbler Tank) using a pack of three Petryanov filters with different particulate retention sizes mounted on the nose of the air blower Н810 RadeCo operating for 2 h at a pump rate 100 dm3/min. The distribution of a-emitting radionuclides in the lava fragments and aerosol filters was assessed by a-track analysis using CR39 track detector (TASTRAK, Bristol, UK) and optical microscope Olympus BX-51 with ImageScopeM software. To obtain maximum information from the a-track analysis of different samples the exposition time was varied between 10 s and 3 min. Homogeneities of a-activity distribution in the lava fragments were proved by measuring the number of a-tracks per constant area of 150 150 mm2 under 100 magnification (Fig. 1f). a-tracks in the brown lava were measured after a 30 s exposure; in the black lava, after 2 min of exposure. CR-39 detectors were etched in 6.25 M NaOH with 1% ethanol at 70 C during 6 h. Individual aerosol particles were localized by radiography with Imaging Plates (Cyclone Plus Storage Phosphor System, PerkinElmer; their suitability for investigation of 137Cs was recently shown in Korobova et al., 2014). Gamma-activity of the samples was determined using g-spectroscopy (HPGe detector, CANBERRA, Genie-2000 software). SEM-EDX was performed using JEOL JSM-6380 LA with a JED 2300 analyser. Raman spectroscopy (Senterra by Bruker, excitation wavelengths 532 and 785 nm, laser spot size 2e10 mm) and infrared mapping (aperture size 100 mm, reflection geometry) provided information on structure of the lava matrix and inclusions. X-ray tomography and Extended X-Ray Absorption Fine Structure (EXAFS) were performed at the Synchrotron Center at Kurchatov Institute.
Fig. 1. Optical images of the black (a) and brown (d) lava fragments in acrylic mount; a-track radiography images of the black (b) and brown (e) lava surfaces (exposure e 2 min). (c) etomographic slice corresponding to the black lava surface (resolution 10 mm/pixel); (f) e grid for measuring of the number of alpha tracks per 150 150 mm2 (brown lava; exposure time e 30 s, magnification 100). Scale bars (a, b, c) e 500 mm; (d, e) e 1 mm; (f) e 200 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. Imaging Plate radiography (exposure 11 min) of folded paper sheet containing particles from the floor of the first Bubbler Tank. Scale bar e 1 cm.
2. Results Fig. 4. Secondary U minerals (NaeSeUeO) on the surface of fragments of porous lava.
2.1. Black lava from the “Elephant foot” (level þ6.0 m) and brown lava from steam-discharge corridor (level þ6.0 m) According to Raman spectroscopy and X-ray microdiffraction the glassy matrix of the black lava is rather depolymerized silicate glass with dominating Q2 silicate units (Q2 > Q3 Q1). Raman spectra of different spots of the glass matrix are virtually identical; the infrared map is also rather featureless (with the exception of obvious zircon inclusions). Optical examination of both the polished lava surfaces reveals several inclusions represented by dendrites of UO2þx with a Zr admixture or (U, Zr)O2, crystals of highuranium zircon, (Zr,U)SiO4, and FeeCreNi spherical droplets (see also Anderson et al., 1992). The presence of strong peak at 1150 cm-1 in Raman spectra of UO2þx inclusions suggests that the deviation from ideal stoichiometry is minor (x < 0.07, Manara and Renker, 2003). Zircon grains are crystalline (no signs of amorphization in Raman spectra) and often consist of several growth zones with variable degrees of perfection. Limited morphological information from optical microscopy of the polished inclusions indicate that at least some of the zircon inclusions represent intergrowths of several crystals (Anderson et al., 1992; Geisler et al., 2005), which partly explains the heterogeneous chemical composition. The zircons are not strongly elongated, suggesting a moderate crystallization rate. In contrast, the urania inclusions often possess a dendritic morphology, indicating rapid diffusion-limited growth.
The marked difference in morphology of these two phases reflects differences in their crystallization temperatures. Whereas the urania dendrites grew rapidly at high temperatures from a supersaturated melt, zircons began crystallization later with growth during slow cooling of the lava. This scenario is supported by occasional observation of UO2 inclusions in zircons. EXAFS analysis of the U and Zr atomic environment in the black lava fragment indicates that both elements are largely present in an amorphous or strongly distorted environment, although a weak second coordination sphere corresponding to UeU scattering in UO2 is observed. According to XAFS results, Zr is mostly present in silicate glass in a 6-coordinated environment (e.g., Farges and Rossano, 2000). At the same time, X-Ray tomography of the black lava fragment performed at various spatial resolutions (2.4 and 11 mm/pixel) reveals the presence of numerous dense inclusions and bubbles with a broad size distribution in the lava volume. Their number density appears to exceed that of inclusions exposed on the polished surface, which likely reflects partial loss of particles during the polishing (Fig. 1c). Optical and SEM examination of the brown lava fragment proved its extreme heterogeneity with much more abundant inclusions than in the black lava fragment. The presence of abundant microinclusions (not necessarily radioactive) in the
Table 1 Gamma-activity of the samples from the “Shelter”, Bq/sample. Sample
Date of measurement Cs-137, Bq/sample Am-241, Bq/sample Eu-154, Bq/sample
29 Dec. 2014 Black lava fragment, 3.0 1.5 1.5 mm3, level þ6.0 m, “Elephant foot” Brown lava fragment, 2.5 2.0 mm2, level þ6.0 m, steam-discharge corridor 15 Apr 2015 Individual particles from the cuvette (3/4 part of the sample disc), level 0.0 m 23 May 2014 Individual particles on the sheet of paper, (1/4 part of the sample disc), level 0.0 m 26 June 2014 Aerosol particles in the filter pack, level 0.0 m 25 Nov. 2011 A quarter of a blue filter (collected on 18-11-2011), level 0.0 m 09 March 2013
68310 132000 1100 542 81 5.3
± ± ± ± ± ±
500 500 20 11 5 0.2
7030 1120 123 47 7.1 0.18
± ± ± ± ± ±
7 5 15 6 0.9 0.04
875 ± 18 1050 ± 10 21 ± 5 6.6 ± 0.7 Not detected Not detected
Fig. 3. (a, b) e particles from disintegrating lava from the first Bubbler Tank with distinct UO2þx inclusions; (c) e alpha-track radiography image of the whole lava fragment shown on Fig. 3b (exposure e 2 h). Scale bar (c) e 200 mm.
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Fig. 5. (On the top) e three layers of a filter pack with aerosol particles from the 1st Floor of the Bubbler Tank: (see text for description); (on the bottom) e Imaging Plate radiography of 3 filters, exposure time: the 1st filter (blue) e 15 min; the 2nd and the 3rd e 24.5 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Alpha track radiography of radioactive aerosol particles from the 1st Floor of the Bubbler Tank (level 0.0 m), exposure time e 43 h. Scale bars: (a) e 200 mm; (b) e 100 mm.
Fig. 7. SEM images of radioactive aerosol particles: (a) e back-scattered electron image of individual UeO particles e white points; (b) secondary electron image of the compact aggregate of UeO particles.
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Fig. 8. Secondary electron image (on the left) and elemental mapping of a single UeFeeO particle: U e in the center; Fe e on the right.
lava volume was noted earlier (e.g., Savonenkov et al., 1991). In spite of the clearly observed non-uniform distribution of the U-bearing particles in two lava fragments, the total radioactivity (measured by Imaging Plate) along with surface a-activity are evenly distributed on the scale of tens of microns (Fig. 1b, e). The number of a-tracks per square per time (cm2 s1) measured in the black and brown lava is 2.5$103 (±3s) and 8.4$103 (±3s), respectively. The apparent contradiction of the radiography, suggesting a homogeneous distribution of radionuclides, and the obvious presence of numerous inclusions (optical microscopy, SEM, X-ray tomography) can be partly resolved by the limited spatial resolution of track radiography (~10 mm) and the loss of spatial resolution in digital radiography due to the strong contribution of high energy band g-radiation from the specimen volume. A consistent explanation of the XAFS results for uranium (weakness of the UeU second sphere) would be more complicated since urania inclusions appear to be abundant. According to Raman spectra the crystalline quality of the larger (>10 mm) inclusions is rather high. This might be explained by a fortuitous selection of the volume sampled by XAFS, or by differences in crystalline perfection (and stoichiometry?) of small and large UO2 inclusions.
size of the second filter (rose, FPA-70-0.20) is similar to the first one. The last filter (white, AFA RSP-20) traps the smallest particles (Fig. 5, top). The area of each filter is 20 cm2, the flow velocity was equal to 0.85 m/s. Imaging Plate radiography shows that the major fraction of the total activity is concentrated on the first filter (Fig. 5, bottom). The other two filters also contain radioactive aerosol particles but of much lower total activity. Note that some radioactive particles (possibly large ones) were only weakly retained by the filter material and could have been easily detached from it. gspectroscopy results of the three filters with aerosol particles are presented in Table 1. Alpha track radiography reveals particles with alpha-emitting nuclides (Fig. 6). According to SEM-EDX data they are represented by aggregates of UeO containing particles about 0.2e0.4 mm in size (Fig. 7). Extensive investigations (Burakov et al., 1994) show that virtually all urania inclusions from Chernobyl lava contain at least a minor Zr admixture. Absence of Zr in the studied aerosol particles suggests that they might represent dispersed fuel dust widely spread inside the “Shelter”; morphology of some of the aerosols confirms this hypothesis. Very rarely larger U-containing particles with a complex composition are found, see example in Fig. 8 (UeFeeO particle).
2.2. Individual lava particles, the 1st floor of the Bubbler Tank (level 0.0 m)
3. Conclusions
The radioactive particles were located in the collection filters using Imaging Plate (Fig. 2). The gamma-spectroscopy data for two parts of the sample disc shown in Fig. 2 are presented in the Table 1. SEM-EDX of selected individual particles reveals a morphology resembling chips of porous glass (pumice-like). Numerous UeZreO dendrite-shaped inclusions (Fig. 3a, b) with different U/Zr ratio are also observed. The alpha-radioactivity of all investigated lava fine fragments (particles) is distributed homogeneously (Fig. 3c). SEM shows that agglomerates of needle-like secondary crystals are frequently present on surface of the aerosol particles (Fig. 4). Often they are observed in former bubbles, but at present it is unclear whether the bubbles were indeed important for the crystals' formation or just facilitated their preservation. EDX analysis indicates that these secondary crystals contain NaeUeSeO (no Zr) and likely belong to water-soluble uranyl sulphates (hydrates?) (e.g., Burakov et al., 1997a). 2.3. Aerosol particles collected with an air blower on the Petryanov filters, the 1st floor of the Bubbler Tank (level 0.0 m) Radioactive aerosol particles were located on individual filters using Imaging Plate radiography and subsequently extracted by filter dissolution in acetone. The frontal filter (blue colour, FPA-700.12) traps particles with aerodynamic radii 1 mm; the retention
The matrices of Chernobyl lava are metastable and disintegrate due to mechanical stresses, interaction with water, self-irradiation, etc. The destruction rate appears to be related to the lava composition and location in the building. As a by-product of the lava-water interaction, formation of water soluble U-bearing phases takes place. The glass matrix of Chernobyl lava can be considered as a rather depolymerized silicate-based glass. Uranium and zirconium are largely dissolved in the glass, though inclusions of UeZreO phases are not uncommon. In spite of an extremely heterogeneous mineral composition of the Chernobyl lava, its radioactivity is distributed homogeneously on a scale of 5e10 microns. This means that a relatively homogeneous common source of highly radioactive silicate melt of Chernobyl lava existed for at least several days as an anoxic closed system. During this time it had separated into two layers: brown lava (relatively thin layer at the bottom) and black lava (main volume). According to a-track analysis data the total a-activity of the brown lava is approximately 3 times higher than the a-activity of the black lava. This correlates with the higher U content of the brown lava. As a result of some yet unclear reason (melting or mechanical destruction of concrete in contact with the melt's crust) the lava spread into premises of 4th Unit, located below former shaft of the reactor. A fraction of liquid brown lava has fallen into water forming porous material (porous lava) that is similar to natural volcanic pumice-stone.
Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005
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Acknowledgements The authors are very grateful to V. Zirlin and L. Nikolaeva from the V.G. Khlopin Radium Institute for sample collecting and treatment under the extreme conditions of the Chernobyl “Shelter”. The work was partly supported by RSCF grant 14-13-00615 (AAS). References Anderson, E.B., Borovoi, A.A., Burakov, B.E., et al., 1992. Artificial products of nuclear fuel e constructional materials interaction that formed as a result of Chernobyl NPP accident. Sov. Radiochem. (5), 144e155 (In Russian). Arutyunyan, R.V., Bolshov, L.A., Borovoi, A.A., Velikhov, E.P., Klyuchnikov, A.A., 2010. Nuclear Fuel in the «Shelter» Encasement of the Chernobyl NPP. Nauka, Moscow, p. 240. Bogatov, S.A., Lebedeva, L.I., Levina, L.A., et al., 1991. Physical-chemical Properties of Radioactive Aerosols inside the “Sarkophagus” Building. Kurchatov Institute preprint. IAE-5435/3. Мoscow, 24 с. Borovoi, A., 2006. Database for the Model of Lava Generation and Spreading. Tech. Rep. Project ‘‘Chess’’, 2916. Borovoi, A.A., Galkin, B.Ya, Drapchinskii, L.V., et al., 1991. Neogenic products of fuel e constructional materials interaction at the 4th block of Chernobyl NPP. Sov. Radiochem. No. 4, 177e196 (In Russian). Burakov, B.E., Anderson, E.B., Galkin, B.Y., Pazukhin, E.M., Shabalev, S.I., 1994. Study of chernobyl “Hot” particles and fuel containing masses: implications for reconstructing the initial phase of accident. Radiochim. Acta 65, 199e202. Burakov, B.E., Ojovan, M.I., Lee, W.E., 2010. Crystalline Materials for Actinide Immobilisation, Materials for Engineering, vol. 1. Imperial College Press. Burakov, B.E., Strykanova, E.E., Anderson, E.B., 1997a. Secondary uranium minerals
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Please cite this article in press as: Vlasova, I., et al., Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.06.005