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Steel oxidation phenomena during Molten Corium siliceous Concrete Interaction (MCCI)
Journal of Alloys and Compounds 622 (2015) 1005–1012
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Steel oxidation phenomena during Molten Corium siliceous Concrete Interaction (MCCI) Mathieu Sanchez-Brusset a,⇑, Pascal Piluso a, Marianne Balat-Pichelin b, Paul David Bottomley c, Thierry Wiss c a b c
CEA/DEN/Cadarache, SMTA/LPMA, 13108 St. Paul lez-Durance, France PROMES-CNRS Laboratory, 7 rue du four solaire, 66120 Font-Romeu Odeillo, France European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, German
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Article history: Received 12 September 2014 Received in revised form 24 October 2014 Accepted 6 November 2014 Available online 13 November 2014 Keywords: Corium Corium concrete interaction Oxidation Scanning electron microscopy, SEM
a b s t r a c t The VULCANO facility at CEA Cadarache is a Molten Corium Concrete Interaction (MCCI) installation for testing material reactions representative of the late stages of a nuclear reactor severe accident. The objectives of the VBS-U3 test were to study ablation phenomena and oxidation of the metallic phase when two liquid phases are present: oxide phase and metallic phase (steel). In this paper we describe the materials post-test analysis of the VULCANO VBS-U3 test performed at the Institute for Transuranium Elements in Karlsruhe (JRC-ITU) with the focus on the metallic phase oxidation of the corium. Post-test analyses show that the remaining metallic phase of the corium is under two forms: drops discontinuously dispersed in the oxide phase forming an emulsion and a continuous metallic ingot clearly separated from the oxide phase. In average, taking into account or not the metallic phase dispersed in the oxide phase, between 60% and 70% of the steel has been oxidized. The size of the drops and their proportion in the oxide phase is depending on their distance from horizontal and vertical walls of the concrete test section. Oxidation mechanisms are mainly depending on two parameters: nature of the metallic interface and localization in the test section. Calculations at thermodynamic equilibrium show that the only product from steel oxidation is (Fe,Cr)3O4, Cr2O3 is never formed. Moreover taking into account the two gaseous species coming from the concrete (CO2 and H2O), considered up to now as being the only sources of oxidation of the metallic phase, the steel oxidation proportion is estimated at only 14% at thermodynamic equilibrium meaning that complementary phenomena are involved during MCCI. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction During a severe nuclear accident, the core melting produces a complex mixture called corium constituted by the materials from the vessel, mainly nuclear fuel and structural materials. The components of the corium can initially be divided into two immiscible liquid phases: an oxide and a metallic one. If the failure of the vessel occurs, corium could interact with the basemat concrete and could finally lead to the failure of the last containment barrier. Because of its different physical properties, the presence of molten steel in the corium changes the concrete ablation rate and the geometry. So the mechanisms of molten steel oxidation during Molten Corium Concrete Interaction (MCCI) have to be well understood to estimate the concrete’s lifetime. Several experimental projects have been conducted to study MCCI with an oxide/metal corium, using
⇑ Corresponding author. http://dx.doi.org/10.1016/j.jallcom.2014.11.041 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
prototypical materials (VULCANO/HECLA [1], COTELS [2]) or simulant materials (COMET [3], MOCKA [4]). Peehs and Hassman studied the oxidation of the metallic phase by gases [5], and concluded that zirconium, chromium and iron are successively oxidized. Nickel cannot be oxidized and only iron monoxide can be formed because of the partial pressure of oxygen in the corium. Kinetics studies of corium oxidation from the atmosphere led by Sulatsky have demonstrated that melt oxidation is controlled by oxygen diffusion [6]. Oxidation of liquid steel is study in the metallurgical field and is mainly focused on decarburization and slag behaviour for temperatures around the steel melting point [7,8]. At CEA Cadarache, three tests using prototypic oxide/metal corium have been performed in the VULCANO facility [9]. According to the general understanding of main MCCI phenomena, only gases from the ablated concrete (H2O and CO2) are considered as sources of oxidation of the metallic phase. Nevertheless, recent VULCANO experiments indicate these gases alone cannot explain the observed steel oxidation proportion. Thus, other sources of oxidation as
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corium oxide, concrete oxide or air have to be considered as possible sources of oxidation. In this paper, a deep analysis of VULCANO VBS-U3 – an MCCI test with oxide and metallic phases – is presented. Post-test materials analyses are described and the results are compared with thermodynamic equilibrium calculations. Differences between post-test material analyses and calculations at thermodynamic equilibrium are identified and some mechanisms of metallic phase oxidation are described. 2. Materials and methods 2.1. VULCANO VBS-U3 test MCCI experiments with prototypical materials are performed in the VULCANO facility at CEA-Cadarache. For the MCCI test with oxide and metallic phases, a plasma arc furnace produces the oxide prototypical corium and three induction furnaces produce the molten steel (Fig. 1). Both liquid phases are transferred simultaneously into a concrete test section. During corium–concrete interaction, the decay heat (provided by fission product in the reactor case) is simulated by induction heating of the oxide phase. The concrete test section is semi-cylindrical with a height of 25 cm and a diameter of 30 cm. 129 thermocouples are implanted in the concrete to follow the concrete ablation. This study is focused on the VBS-U3 test [9], where 36 kg oxide and 15.3 kg steel have been poured in the test section. Initial composition of concrete, corium oxide and steel are summarized in Table 1. The initial temperatures of the oxide and metallic melts during the transfer into the concrete test section are respectively 2000 °C and 1700 °C. The concrete is a siliceous concrete representative of a French nuclear power plant. The experimental compositions are given in Table 1. Induction heating is maintained during 3h30 with a total electric power between 34 and 38 kW. 2.2. Sampling and post-test analyses After the test, 7 samples have been collected in the test section (see Fig. 2). Sample A (representative of the top of the melt in contact with air) and sample B (representative of the centre of the corium near the metallic phases) are collected in the oxidic corium; samples G and F in the metallic phase and samples C–E are representative of the corium–concrete interface. The metallic phase is divided into two parts: one continuous metallic phase (the blue1 part in Fig. 2) separated from the oxide phase (the red part in Fig. 2), and a second one dispersed in the corium oxide in the form of microscopic droplets (see following analysis). The analyses have been performed at the Institute for Transuranium Elements (JRC/ITU), Karlsruhe (in Germany). The microscope used is a SEM Vega Tescan TS5130LSH equipped with an Oxford EDX system with a detection limit of 0.1% at best. The INCA software has been used for the spectrum analyses and image analyses performed using the software ImageJ. The solid phase identified and presented in this report are the closest stoichiometric phases experimentally measured, for instance: FeO1.49 is assumed to be as Fe2O3. For post-test analyses, the samples were sectioned using a diamond disc, cleaned by an ultrasound bath using water and alcohol, then embedded in epoxy resin and polished. The polishing has been done with successive clothes impregnated with diamond particles of 9 lm, 6 lm and 3 lm, before a final washing. 2.3. Thermodynamic study VULCANO VBS-U3 experimental results are compared to the results obtained assuming thermodynamic equilibrium during MCCI. A Gibbs energy minimizer – GEMINI-2 – coupled with the nuclear thermodynamic database Nuclea-10 [10] allows calculation of the phases at thermodynamic equilibrium. For calculating multiphase equilibria in multicomponent system, analytical description of the Gibbs energies of all possible substances and solution phases is performed as a function of temperature and composition compared to a given reference state. The excess of multicomponent solutions is modelled from the binary ones and may include ternary contributions and higher-order ones using the Calphad approach [11]. Two kinds of calculations have been performed. Firstly, VULCANO VBS-U3 initial compositions of oxide and steel are used. Siliceous concrete is then added from 5 to 95 wt.% to the initial corium melt to simulate the concrete ablation by the corium and enrichment in concrete elements of the melt. For a given enrichment in concrete, the temperature is varied to estimate existing phases at thermodynamic equilibrium. Secondly, compositions of corium, steel and two kind of concrete representative of a nuclear power plant (Siliceous concrete as VBS-U3 and Limestone/Common Sand (LCS) concrete) are used with the following conditions: 70 wt.% oxide corium (80 wt.% UO2 and 20 wt.% ZrO2)–30 wt.% stainless steel. 1 For interpretation of colour in Figs. 2 and 6, the reader is referred to the web version of this article.
Fig. 1. VULCANO facility.
3. Results 3.1. Metallic samples After the test, 4.3 kg of metallic phases, separated from the oxide phase, have been collected from the test section, which represents 28% of the initial steel (without taking into account the metallic drops in the oxide phase). Two metallic samples have been analysed. Samples G and F were collected in the continuous metallic phase. The samples were homogeneous and their compositions are given in Table 2. Consequently the continuous metallic phase (found at the end of the test) is heavily chromium depleted by around 90 wt.%.
3.2. Morphology of oxidized top surface layer of corium ingot The microstructure of the oxide sample collected in the top of the melt is given in Fig. 3 (sample A). With the image analyses, using the software ImageJ, three phases (metal, corium and concrete oxides) can be extracted as shown in red in Fig. 3b–d respectively. The metallic phase is under the form of drops in the oxide part and of a bigger metallic block. Diameter of the drops is in between 5 and 300 lm. The cross-sectional surface of the metallic block is estimated to be about 7.3 mm2 (4% of the total surface of the sample) and the total metallic phase represents about 6% of the total sample surface. Porosity represents around 30% of the surface sample. Analysing in more details the remaining metallic phase, it appears that chromium and nickel concentration is depending on the metallic drop size (Fig. 4). - Drops smaller than 15 lm are completely nickel depleted. - Drops bigger than 40 lm are depleted in nickel by between 0% and 15%. - Drops bigger than 100 lm are depleted in chromium by around 60%. - Between 15 and 100 lm, chromium depletion ranges between 30% and 80%. By analysing the oxide phases resulting from the oxidation of the metallic phase, two kinds of behaviour have been identified: at the gas bubble–metal interface, three oxide layers are formed with a thickness between 20 and 30 lm (Fig. 5a and b) being: an iron oxide layer Fe2O3 (hematite), a iron–chromium oxide layer (Fe,Cr)3O4 in contact with metal and an internal oxidation layer constituted by (Fe,Cr)3O4 respectively while at the melt-steel
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UO2 56
ZrO2 39
SiO2 3.6
AISI 304 wt.%
Fe 71.5
Cr 19.3
Ni 9.2
Siliceous concrete wt.%
SiO2 64.7
CaO 16.4
Al2O3 5.1
Fe2O3 0
CaO 1.4
Fe2O3 1.5
CO2 9.2
H2O 3.1
Table 2 Composition of metallic samples F and G. Sample
Fe (wt.%)
Cr (wt.%)
Ni (wt.%)
Sample G Sample F
87 88
2 1
11 11
95 wt.%. Except a few isolated cases, no drop oxidation has been observed (Fig. 9). 3.5. Thermodynamic study: VULCANO and reactor case
Fig. 2. Schematic view of the test section – localization of samples for SEM–EDX analyses.
interface, only one oxide layer of Cr2O3 is formed with a thickness between 1 and 5 lm (Fig. 5c and d). 3.3. Morphology of oxidized central part of corium ingot The microstructure of the oxide sample collected in the centre of the melt is given in Fig. 6 (sample B). The phases (metal and corium oxides) are extracted (in red in Fig. 6b and c respectively). This sample is mainly constituted by corium oxide (U,Zr)O2. The total metallic phase represents about 3% of the total surface of the sample. Porosity is around 15% of the surface sample. The metallic phase is in the form of drops with a diameter between 5 and 100 lm. Analysing the oxide phases formed from the oxidation of the metallic drops, three kinds of oxidation behaviour have been identified, leading in all cases to the formation of (Fe,Cr)3O4 (Fig. 7): (1) Internal oxidation (in the bulk of the drop). (2) Continuous oxidation with formation of an external homogeneous layer. (3) Local oxidation at the surface of the drops. 3.4. Analyses of the corium–concrete interface The microstructures of the samples collected in the corium– concrete (samples C–E) are given in Fig. 8. Samples C and D have similar microstructure. They are constituted of a concrete oxide matrix containing corium oxide (U,Zr)O2, and a metallic phase. Sample E is slightly different with a lower proportion of corium oxide and a higher porosity. For samples C and E, the matrix has micro-porosity around 20% (surface area). For these three samples, the metallic phase is only observed in the form of droplets (up to 20 lm) and represents between 1% and 1.6% of the surface area. These metallic drops are chromium and nickel depleted by around
3.5.1. Study of VBS-U3 at thermodynamic equilibrium Experimental results of the VBS-U3 test were compared with calculations at the thermodynamic equilibrium, performed between 1800 and 3000 K in the same conditions as VULCANO experiment with an enrichment in concrete of 18 wt.% (Fig. 10) using GEMINI-2 and NUCLEA-10 thermodynamic database. For temperature above 2300 K, two liquid phases are present: a metallic one and an oxide one. Below this temperature, solid (U,Zr)O2 and (U,Zr)SiO4 are formed. Steel oxidation leads to the formation of solid (Fe,Cr)3O4 below 2100 K. The remaining metallic phase represents 21 wt.%, so steel oxidation proportion with 18 wt.% of ablated concrete is evaluated between 20% and 11% according to the range of temperature during MCCI in Fig. 10. The extent of oxidation at thermodynamic equilibrium with enrichment in siliceous concrete was then studied at two temperatures; 2100 K and 2500 K, representing partially liquid and fully liquid conditions respectively. Results are shown in Fig. 11. Total oxidation of stainless steel by concrete gases is observed with an enrichment of 65 wt.% concrete at 2100 K and of 60 wt.% concrete at 2500 K. In both cases H2O and CO2 produced in the concrete ablation are sufficient to oxidize stainless steel. In the case of VULCANO VBS-U3 test, 60–70 wt.% stainless steel oxidation proportion has been observed experimentally, meaning at 2100 K, 55 wt.% (95.8 kg) concrete enrichment would have been necessary, whereas only 18% of concrete enrichment have been observed. 3.5.2. Study of the reactor case at thermodynamic equilibrium Two families of concrete, siliceous and Limestone/Common Sand (LCS), representative of nuclear power plants were taken to perform calculations at thermodynamic equilibrium. For a given concrete enrichment, phases forming during MCCI are studied. For LCS and siliceous concretes, solid corium phases such as (U,Zr)O2 and spinel (Fe,Cr)3O4 are formed. The zircon (U,Zr)SiO4 phase is formed for siliceous concrete; In all cases, the oxidation of steel leads to the formation of spinel (Fe,Cr)3O4. At fixed temperatures, 2100 and 2500 K, concrete is added to a corium composition of (U,Zr)O2 (80 wt.%) – stainless steel (20 wt.%) from 0% to 100% in order to simulate the ablation rate during MCCI (Fig. 12). Results show the amount of oxidized steel versus the mass of concrete. Reaching a certain enrichment of concrete in the melt (Table 3), complete oxidation of steel is possible with siliceous and LCS concrete, assuming there are no rebars, as e.g.
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Fig. 3. (a) Sample A, (b) metallic phase, (c) concrete oxide, and (d) corium oxide.
Fig. 4. Nickel and chromium depletion compared to initial composition versus drop size distribution.
in Fukushima Daichi pedestal concrete. For instance, a 50 wt.% enrichment in LCS concrete is enough to oxidize totally the metallic phase, assuming thermodynamic equilibrium at 2500 K. For the same mass of concrete, the proportion of steel oxidation is higher for limestone concrete because of the amount of gases which is higher (the molar ratio is 1.75). For 18 wt% of siliceous concrete enrichment for the reactor case, the proportion of steel oxidation is estimated to be in between 13% and 20%, results close to the VULCANO test VBS-U3 assuming thermodynamic equilibrium. Nevertheless, VULCANO experimental results show a steel oxidation proportion much higher (60–70%) for this concrete ablation enrichment. 4. Discussion During the VULCANO VBS-U3 MCCI test, the siliceous concrete test section has been ablated until an 18 wt.% average concrete enrichment of the final melt (oxide corium and metallic phase). At thermodynamic equilibrium with 18 wt% of siliceous concrete at 2100 K, the proportion of steel oxidation reaches 14%. Experimentally, after the dismounting of the concrete test section, the non-oxidized metallic phase, separated from the oxide phase,
weighs 4.3 kg, meaning 72% oxidation of the steel, to which dispersed metallic drops in the oxide phase must be added leading to 60–70% oxidation. If CO2 and H2O gases from the ablated concrete were considered as the only sources of oxidation of the metallic phase during MCCI, equilibrium thermodynamic calculations at equilibrium show too large a difference compared with the experimental results. This indicates that, another source than H2O and CO2 gases must be taken into account for the oxidation phenomena during MCCI. This had already been observed in previous test VBS-U1 with limestone-rich concrete [12]. For calculation at thermodynamic equilibrium, (Fe,Cr)3O4 is found to be the only product of steel oxidation whereas post-test analyses show the existence of three oxide products: (Fe,Cr)3O4, Fe2O3 and Cr2O3. Experimentally, it is observed that the nature of the oxide product formed from the steel is linked to the nature of the interface between the steel and the surrounding environment. Furthermore, depletion in Ni and Cr elements is observed in the steel. Up to 90% depletion in Cr and 100% depletion in Ni have been identified at certain locations. A relation between the steel drop size and Ni/Cr depletion has also been shown. This substantial depletion in Ni and Cr of stainless steel is specific to MCCI system whereas for classical oxidation processes in industry the chromium depletion is only observed in the metallic zone immediately below as a result of the atmosphere allowing the chromia layer to grow preferentially [13,14]. At the oxide corium melt–steel interface, Cr2O3 or (Fe,Cr)3O4 layers are formed with a thickness between 5 and 10 lm. When Cr2O3 layer is complete, oxidation seems to be practically stopped since no thicker oxide layer were observed in the sample. However its highly protective nature in air (i.e. slow oxygen diffusion through the Cr2O3 layer) is well-known. Several (Fe,Cr)3O4 precipitates of about 5 lm size have been observed inside of some steel drops in contact with oxide corium. Up to now, the reason for the internal oxidation forming these precipitates inside the bulk of the drop is not well understood. This is thought to be linked to the local variation in oxygen potential in the different drops: thus some drops reach high enough potential inside to initiate precipitation of Cr2O3 as the most stable oxide.
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Fig. 5. (a) And (b) oxidation layers at the metal–gas interface; (c) and (d) oxidation layer at the metal–melt interface.
Fig. 6. (a) Sample B, (b) metallic phase, and (c) corium oxide.
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Fig. 7. (a) Internal oxidation, (b) continuous oxidation with formation of a homogeneous layer, (c) local oxidation at the surface of the drops, and (d) zoom of figure (c).
Fig. 8. (a) Sample C, (b) sample D, and (c) sample E.
At the gas (bubble)–steel (drop) interface, different oxide layers are observed: either two continuous oxide layers – (Fe,Cr)3O4 and Fe2O3 – or a discontinuous oxide layer (Fe,Cr)3O4. For the double oxide structure, the outer oxide layer is hematite Fe2O3 while the inner oxide layer is a spinel (Fe,Cr)3O4.
Similar arrangement of inner and outer oxide layers have been observed previously for the oxidation of 304L exposed in wet atmosphere [15]. By contrast stainless steel oxidation in dry O2-containing atmosphere generally result in a mixed (Fe,Cr) oxide spinel layer above a pure Cr2O3 layer at least up to temperatures up
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Fig. 9. Metallic drops without oxidation.
Table 3 Required enrichment of concrete to totally oxidize the metallic phase at 2100 and 2500 K. Temperature (K)
Siliceous concrete (wt.%)
LCS concrete (wt.%)
2100 2500
65 55
60 50
has been observed in a number of high alloy steels such as 304L (18Cr–8Ni) and 353MA (25Cr–20Ni) [15,18,19]. In this case, chromium is considered to have evaporated in the form of Cr(VI) oxy-hydroxides following the chemical reaction: Fig. 10. Phases formed at thermodynamic equilibrium with 18 wt.% of concrete.
1=2 Cr2 O3 ðsÞ þ 3=4 O2 ðgÞ þ H2 OðgÞ ! CrO2 ðOHÞ2 ðgÞ There is a very high evaporation rate of chromium oxy-hydroxides at temperatures above only 600 °C [15]. There is the possibility of both mechanisms operating at higher temperatures above 1000 °C. Thus it would seem likely that the Cr depletion observed in the post-test analyses could go to completion through chromium oxyhydroxide volatilisation. This could explain the complete Cr depletion observed in the outer oxide layers surrounding the metallic phases in the VULCANO samples. 5. Conclusion
Fig. 11. Oxidation of steel versus mass of siliceous concrete.
In order to study oxidation of the metallic phase during MCCI phenomena, post-test analyses of the VULCANO VBS-U3 corium have been performed. After the test, the remaining metallic phases were present in two forms: - A discontinuous phase in the form of drops, dispersed in the oxide melt. - A continuous phase separated from the oxide melt.
Fig. 12. Oxidation of steel depending on the mass of concrete in the melt.
to 900 °C; above this temperature evaporation of the volatile CrO3 species begins to diminish the chromia protective thickness [16]. Also high oxygen partial pressures assist this volatilisation [17]. Chromium depletion in the outer layer in presence of steam vapour
Experimentally, between 60% and 70% of the metal is oxidized whereas at thermodynamic equilibrium – taking into account only concrete decomposition gases as oxygen sources – oxidation of around 15 wt.% steel oxidation is expected. At thermodynamic equilibrium, between 45 and 55 wt.% of siliceous concrete was calculated to be necessary to oxidize 70% of steel, whereas 18 wt.% siliceous concrete was sufficient experimentally. Consequently, other sources of oxidation, such as gases from the non-ablated concrete, the oxide phase or oxygen from the air, are necessary to explain the extent of oxidation of the metallic phase during the MCCI processes. Another important point of concerns is the nature of the oxide formed: at thermodynamic equilibrium, only the spinel (Fe,Cr)3O4
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should be formed whereas it has been found experimentally that (Fe,Cr)3O4 and Cr2O3 have been formed. At the interface between the metallic drops and the oxide melt, Cr2O3 or (Fe,Cr)3O4 are formed. Moreover, in certain cases, internal oxidation of the metallic is seen as oxides in the bulk of the drops. At the interface between the metallic drops and gas: an iron oxide layer Fe2O3 (hematite), an iron–chromium oxide layer (Fe,Cr)3O4 in contact with metal and an internal oxidation layer constituted by (Fe,Cr)3 O4. Formation of hematite could result from the chromium evaporation under volatile chromium forms from the mixed (Fe,Cr)2O3 formed in the first stage of the oxidation. The chromium oxy-hydroxides formed with ambient water vapour could be the most active mechanism to explain the observed depletion in chromium and the formation of hematite in the outer oxide layer. Further investigations will be needed to verify this oxidation mechanism and when it occurs during MCCI as well as the various factors, both thermodynamic and kinetic involved. It will also require more study to establish how widespread the chromium depletion is in the large corium mass. Finally this result indicates how volatile species can affect an element’s chemistry and increase the potential late stage releases of a severe accident. Acknowledgments The authors are very grateful to the JRC Institute for Transuranium Elements (ITU) and student exchange programme in the frame of the European project SARNET EURATOM FP7, grant n° 231747. This work is a part of the PhD of M. Sanchez-Brusset, co-financed by CEA and EDF and supported by the University of Perpignan Via Domitia and Marianne Balat-Pichelin at PROMES-CNRS laboratory. Acknowledgements are also extended for the VULCANO tests co-financed by CEA, EDF, IRSN and GDF Suez as well as by the SARNET FP7 grant. The PLINIUS team is joined to the acknowledgements for their work and effort. References [1] C. Journeau, J.M. Bonnet, E. Boccacio, P. Piluso, J. Monerris, M. Breton, G. Fritz, T. Sevon, P.M. Pankakovski, S. Holmstrom, European experiments on 2-D molten core concrete interaction: HECLA and VULCANO, Nucl. Technol. 170 (2010) 189–200.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Steel oxidation phenomena during Molten Corium siliceous Concrete Interaction (MCCI) Mathieu Sanchez-Brusset a,⇑, Pascal Piluso a, Marianne Balat-Pichelin b, Paul David Bottomley c, Thierry Wiss c a b c
CEA/DEN/Cadarache, SMTA/LPMA, 13108 St. Paul lez-Durance, France PROMES-CNRS Laboratory, 7 rue du four solaire, 66120 Font-Romeu Odeillo, France European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, German
a r t i c l e
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Article history: Received 12 September 2014 Received in revised form 24 October 2014 Accepted 6 November 2014 Available online 13 November 2014 Keywords: Corium Corium concrete interaction Oxidation Scanning electron microscopy, SEM
a b s t r a c t The VULCANO facility at CEA Cadarache is a Molten Corium Concrete Interaction (MCCI) installation for testing material reactions representative of the late stages of a nuclear reactor severe accident. The objectives of the VBS-U3 test were to study ablation phenomena and oxidation of the metallic phase when two liquid phases are present: oxide phase and metallic phase (steel). In this paper we describe the materials post-test analysis of the VULCANO VBS-U3 test performed at the Institute for Transuranium Elements in Karlsruhe (JRC-ITU) with the focus on the metallic phase oxidation of the corium. Post-test analyses show that the remaining metallic phase of the corium is under two forms: drops discontinuously dispersed in the oxide phase forming an emulsion and a continuous metallic ingot clearly separated from the oxide phase. In average, taking into account or not the metallic phase dispersed in the oxide phase, between 60% and 70% of the steel has been oxidized. The size of the drops and their proportion in the oxide phase is depending on their distance from horizontal and vertical walls of the concrete test section. Oxidation mechanisms are mainly depending on two parameters: nature of the metallic interface and localization in the test section. Calculations at thermodynamic equilibrium show that the only product from steel oxidation is (Fe,Cr)3O4, Cr2O3 is never formed. Moreover taking into account the two gaseous species coming from the concrete (CO2 and H2O), considered up to now as being the only sources of oxidation of the metallic phase, the steel oxidation proportion is estimated at only 14% at thermodynamic equilibrium meaning that complementary phenomena are involved during MCCI. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction During a severe nuclear accident, the core melting produces a complex mixture called corium constituted by the materials from the vessel, mainly nuclear fuel and structural materials. The components of the corium can initially be divided into two immiscible liquid phases: an oxide and a metallic one. If the failure of the vessel occurs, corium could interact with the basemat concrete and could finally lead to the failure of the last containment barrier. Because of its different physical properties, the presence of molten steel in the corium changes the concrete ablation rate and the geometry. So the mechanisms of molten steel oxidation during Molten Corium Concrete Interaction (MCCI) have to be well understood to estimate the concrete’s lifetime. Several experimental projects have been conducted to study MCCI with an oxide/metal corium, using
⇑ Corresponding author. http://dx.doi.org/10.1016/j.jallcom.2014.11.041 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
prototypical materials (VULCANO/HECLA [1], COTELS [2]) or simulant materials (COMET [3], MOCKA [4]). Peehs and Hassman studied the oxidation of the metallic phase by gases [5], and concluded that zirconium, chromium and iron are successively oxidized. Nickel cannot be oxidized and only iron monoxide can be formed because of the partial pressure of oxygen in the corium. Kinetics studies of corium oxidation from the atmosphere led by Sulatsky have demonstrated that melt oxidation is controlled by oxygen diffusion [6]. Oxidation of liquid steel is study in the metallurgical field and is mainly focused on decarburization and slag behaviour for temperatures around the steel melting point [7,8]. At CEA Cadarache, three tests using prototypic oxide/metal corium have been performed in the VULCANO facility [9]. According to the general understanding of main MCCI phenomena, only gases from the ablated concrete (H2O and CO2) are considered as sources of oxidation of the metallic phase. Nevertheless, recent VULCANO experiments indicate these gases alone cannot explain the observed steel oxidation proportion. Thus, other sources of oxidation as
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corium oxide, concrete oxide or air have to be considered as possible sources of oxidation. In this paper, a deep analysis of VULCANO VBS-U3 – an MCCI test with oxide and metallic phases – is presented. Post-test materials analyses are described and the results are compared with thermodynamic equilibrium calculations. Differences between post-test material analyses and calculations at thermodynamic equilibrium are identified and some mechanisms of metallic phase oxidation are described. 2. Materials and methods 2.1. VULCANO VBS-U3 test MCCI experiments with prototypical materials are performed in the VULCANO facility at CEA-Cadarache. For the MCCI test with oxide and metallic phases, a plasma arc furnace produces the oxide prototypical corium and three induction furnaces produce the molten steel (Fig. 1). Both liquid phases are transferred simultaneously into a concrete test section. During corium–concrete interaction, the decay heat (provided by fission product in the reactor case) is simulated by induction heating of the oxide phase. The concrete test section is semi-cylindrical with a height of 25 cm and a diameter of 30 cm. 129 thermocouples are implanted in the concrete to follow the concrete ablation. This study is focused on the VBS-U3 test [9], where 36 kg oxide and 15.3 kg steel have been poured in the test section. Initial composition of concrete, corium oxide and steel are summarized in Table 1. The initial temperatures of the oxide and metallic melts during the transfer into the concrete test section are respectively 2000 °C and 1700 °C. The concrete is a siliceous concrete representative of a French nuclear power plant. The experimental compositions are given in Table 1. Induction heating is maintained during 3h30 with a total electric power between 34 and 38 kW. 2.2. Sampling and post-test analyses After the test, 7 samples have been collected in the test section (see Fig. 2). Sample A (representative of the top of the melt in contact with air) and sample B (representative of the centre of the corium near the metallic phases) are collected in the oxidic corium; samples G and F in the metallic phase and samples C–E are representative of the corium–concrete interface. The metallic phase is divided into two parts: one continuous metallic phase (the blue1 part in Fig. 2) separated from the oxide phase (the red part in Fig. 2), and a second one dispersed in the corium oxide in the form of microscopic droplets (see following analysis). The analyses have been performed at the Institute for Transuranium Elements (JRC/ITU), Karlsruhe (in Germany). The microscope used is a SEM Vega Tescan TS5130LSH equipped with an Oxford EDX system with a detection limit of 0.1% at best. The INCA software has been used for the spectrum analyses and image analyses performed using the software ImageJ. The solid phase identified and presented in this report are the closest stoichiometric phases experimentally measured, for instance: FeO1.49 is assumed to be as Fe2O3. For post-test analyses, the samples were sectioned using a diamond disc, cleaned by an ultrasound bath using water and alcohol, then embedded in epoxy resin and polished. The polishing has been done with successive clothes impregnated with diamond particles of 9 lm, 6 lm and 3 lm, before a final washing. 2.3. Thermodynamic study VULCANO VBS-U3 experimental results are compared to the results obtained assuming thermodynamic equilibrium during MCCI. A Gibbs energy minimizer – GEMINI-2 – coupled with the nuclear thermodynamic database Nuclea-10 [10] allows calculation of the phases at thermodynamic equilibrium. For calculating multiphase equilibria in multicomponent system, analytical description of the Gibbs energies of all possible substances and solution phases is performed as a function of temperature and composition compared to a given reference state. The excess of multicomponent solutions is modelled from the binary ones and may include ternary contributions and higher-order ones using the Calphad approach [11]. Two kinds of calculations have been performed. Firstly, VULCANO VBS-U3 initial compositions of oxide and steel are used. Siliceous concrete is then added from 5 to 95 wt.% to the initial corium melt to simulate the concrete ablation by the corium and enrichment in concrete elements of the melt. For a given enrichment in concrete, the temperature is varied to estimate existing phases at thermodynamic equilibrium. Secondly, compositions of corium, steel and two kind of concrete representative of a nuclear power plant (Siliceous concrete as VBS-U3 and Limestone/Common Sand (LCS) concrete) are used with the following conditions: 70 wt.% oxide corium (80 wt.% UO2 and 20 wt.% ZrO2)–30 wt.% stainless steel. 1 For interpretation of colour in Figs. 2 and 6, the reader is referred to the web version of this article.
Fig. 1. VULCANO facility.
3. Results 3.1. Metallic samples After the test, 4.3 kg of metallic phases, separated from the oxide phase, have been collected from the test section, which represents 28% of the initial steel (without taking into account the metallic drops in the oxide phase). Two metallic samples have been analysed. Samples G and F were collected in the continuous metallic phase. The samples were homogeneous and their compositions are given in Table 2. Consequently the continuous metallic phase (found at the end of the test) is heavily chromium depleted by around 90 wt.%.
3.2. Morphology of oxidized top surface layer of corium ingot The microstructure of the oxide sample collected in the top of the melt is given in Fig. 3 (sample A). With the image analyses, using the software ImageJ, three phases (metal, corium and concrete oxides) can be extracted as shown in red in Fig. 3b–d respectively. The metallic phase is under the form of drops in the oxide part and of a bigger metallic block. Diameter of the drops is in between 5 and 300 lm. The cross-sectional surface of the metallic block is estimated to be about 7.3 mm2 (4% of the total surface of the sample) and the total metallic phase represents about 6% of the total sample surface. Porosity represents around 30% of the surface sample. Analysing in more details the remaining metallic phase, it appears that chromium and nickel concentration is depending on the metallic drop size (Fig. 4). - Drops smaller than 15 lm are completely nickel depleted. - Drops bigger than 40 lm are depleted in nickel by between 0% and 15%. - Drops bigger than 100 lm are depleted in chromium by around 60%. - Between 15 and 100 lm, chromium depletion ranges between 30% and 80%. By analysing the oxide phases resulting from the oxidation of the metallic phase, two kinds of behaviour have been identified: at the gas bubble–metal interface, three oxide layers are formed with a thickness between 20 and 30 lm (Fig. 5a and b) being: an iron oxide layer Fe2O3 (hematite), a iron–chromium oxide layer (Fe,Cr)3O4 in contact with metal and an internal oxidation layer constituted by (Fe,Cr)3O4 respectively while at the melt-steel
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UO2 56
ZrO2 39
SiO2 3.6
AISI 304 wt.%
Fe 71.5
Cr 19.3
Ni 9.2
Siliceous concrete wt.%
SiO2 64.7
CaO 16.4
Al2O3 5.1
Fe2O3 0
CaO 1.4
Fe2O3 1.5
CO2 9.2
H2O 3.1
Table 2 Composition of metallic samples F and G. Sample
Fe (wt.%)
Cr (wt.%)
Ni (wt.%)
Sample G Sample F
87 88
2 1
11 11
95 wt.%. Except a few isolated cases, no drop oxidation has been observed (Fig. 9). 3.5. Thermodynamic study: VULCANO and reactor case
Fig. 2. Schematic view of the test section – localization of samples for SEM–EDX analyses.
interface, only one oxide layer of Cr2O3 is formed with a thickness between 1 and 5 lm (Fig. 5c and d). 3.3. Morphology of oxidized central part of corium ingot The microstructure of the oxide sample collected in the centre of the melt is given in Fig. 6 (sample B). The phases (metal and corium oxides) are extracted (in red in Fig. 6b and c respectively). This sample is mainly constituted by corium oxide (U,Zr)O2. The total metallic phase represents about 3% of the total surface of the sample. Porosity is around 15% of the surface sample. The metallic phase is in the form of drops with a diameter between 5 and 100 lm. Analysing the oxide phases formed from the oxidation of the metallic drops, three kinds of oxidation behaviour have been identified, leading in all cases to the formation of (Fe,Cr)3O4 (Fig. 7): (1) Internal oxidation (in the bulk of the drop). (2) Continuous oxidation with formation of an external homogeneous layer. (3) Local oxidation at the surface of the drops. 3.4. Analyses of the corium–concrete interface The microstructures of the samples collected in the corium– concrete (samples C–E) are given in Fig. 8. Samples C and D have similar microstructure. They are constituted of a concrete oxide matrix containing corium oxide (U,Zr)O2, and a metallic phase. Sample E is slightly different with a lower proportion of corium oxide and a higher porosity. For samples C and E, the matrix has micro-porosity around 20% (surface area). For these three samples, the metallic phase is only observed in the form of droplets (up to 20 lm) and represents between 1% and 1.6% of the surface area. These metallic drops are chromium and nickel depleted by around
3.5.1. Study of VBS-U3 at thermodynamic equilibrium Experimental results of the VBS-U3 test were compared with calculations at the thermodynamic equilibrium, performed between 1800 and 3000 K in the same conditions as VULCANO experiment with an enrichment in concrete of 18 wt.% (Fig. 10) using GEMINI-2 and NUCLEA-10 thermodynamic database. For temperature above 2300 K, two liquid phases are present: a metallic one and an oxide one. Below this temperature, solid (U,Zr)O2 and (U,Zr)SiO4 are formed. Steel oxidation leads to the formation of solid (Fe,Cr)3O4 below 2100 K. The remaining metallic phase represents 21 wt.%, so steel oxidation proportion with 18 wt.% of ablated concrete is evaluated between 20% and 11% according to the range of temperature during MCCI in Fig. 10. The extent of oxidation at thermodynamic equilibrium with enrichment in siliceous concrete was then studied at two temperatures; 2100 K and 2500 K, representing partially liquid and fully liquid conditions respectively. Results are shown in Fig. 11. Total oxidation of stainless steel by concrete gases is observed with an enrichment of 65 wt.% concrete at 2100 K and of 60 wt.% concrete at 2500 K. In both cases H2O and CO2 produced in the concrete ablation are sufficient to oxidize stainless steel. In the case of VULCANO VBS-U3 test, 60–70 wt.% stainless steel oxidation proportion has been observed experimentally, meaning at 2100 K, 55 wt.% (95.8 kg) concrete enrichment would have been necessary, whereas only 18% of concrete enrichment have been observed. 3.5.2. Study of the reactor case at thermodynamic equilibrium Two families of concrete, siliceous and Limestone/Common Sand (LCS), representative of nuclear power plants were taken to perform calculations at thermodynamic equilibrium. For a given concrete enrichment, phases forming during MCCI are studied. For LCS and siliceous concretes, solid corium phases such as (U,Zr)O2 and spinel (Fe,Cr)3O4 are formed. The zircon (U,Zr)SiO4 phase is formed for siliceous concrete; In all cases, the oxidation of steel leads to the formation of spinel (Fe,Cr)3O4. At fixed temperatures, 2100 and 2500 K, concrete is added to a corium composition of (U,Zr)O2 (80 wt.%) – stainless steel (20 wt.%) from 0% to 100% in order to simulate the ablation rate during MCCI (Fig. 12). Results show the amount of oxidized steel versus the mass of concrete. Reaching a certain enrichment of concrete in the melt (Table 3), complete oxidation of steel is possible with siliceous and LCS concrete, assuming there are no rebars, as e.g.
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Fig. 3. (a) Sample A, (b) metallic phase, (c) concrete oxide, and (d) corium oxide.
Fig. 4. Nickel and chromium depletion compared to initial composition versus drop size distribution.
in Fukushima Daichi pedestal concrete. For instance, a 50 wt.% enrichment in LCS concrete is enough to oxidize totally the metallic phase, assuming thermodynamic equilibrium at 2500 K. For the same mass of concrete, the proportion of steel oxidation is higher for limestone concrete because of the amount of gases which is higher (the molar ratio is 1.75). For 18 wt% of siliceous concrete enrichment for the reactor case, the proportion of steel oxidation is estimated to be in between 13% and 20%, results close to the VULCANO test VBS-U3 assuming thermodynamic equilibrium. Nevertheless, VULCANO experimental results show a steel oxidation proportion much higher (60–70%) for this concrete ablation enrichment. 4. Discussion During the VULCANO VBS-U3 MCCI test, the siliceous concrete test section has been ablated until an 18 wt.% average concrete enrichment of the final melt (oxide corium and metallic phase). At thermodynamic equilibrium with 18 wt% of siliceous concrete at 2100 K, the proportion of steel oxidation reaches 14%. Experimentally, after the dismounting of the concrete test section, the non-oxidized metallic phase, separated from the oxide phase,
weighs 4.3 kg, meaning 72% oxidation of the steel, to which dispersed metallic drops in the oxide phase must be added leading to 60–70% oxidation. If CO2 and H2O gases from the ablated concrete were considered as the only sources of oxidation of the metallic phase during MCCI, equilibrium thermodynamic calculations at equilibrium show too large a difference compared with the experimental results. This indicates that, another source than H2O and CO2 gases must be taken into account for the oxidation phenomena during MCCI. This had already been observed in previous test VBS-U1 with limestone-rich concrete [12]. For calculation at thermodynamic equilibrium, (Fe,Cr)3O4 is found to be the only product of steel oxidation whereas post-test analyses show the existence of three oxide products: (Fe,Cr)3O4, Fe2O3 and Cr2O3. Experimentally, it is observed that the nature of the oxide product formed from the steel is linked to the nature of the interface between the steel and the surrounding environment. Furthermore, depletion in Ni and Cr elements is observed in the steel. Up to 90% depletion in Cr and 100% depletion in Ni have been identified at certain locations. A relation between the steel drop size and Ni/Cr depletion has also been shown. This substantial depletion in Ni and Cr of stainless steel is specific to MCCI system whereas for classical oxidation processes in industry the chromium depletion is only observed in the metallic zone immediately below as a result of the atmosphere allowing the chromia layer to grow preferentially [13,14]. At the oxide corium melt–steel interface, Cr2O3 or (Fe,Cr)3O4 layers are formed with a thickness between 5 and 10 lm. When Cr2O3 layer is complete, oxidation seems to be practically stopped since no thicker oxide layer were observed in the sample. However its highly protective nature in air (i.e. slow oxygen diffusion through the Cr2O3 layer) is well-known. Several (Fe,Cr)3O4 precipitates of about 5 lm size have been observed inside of some steel drops in contact with oxide corium. Up to now, the reason for the internal oxidation forming these precipitates inside the bulk of the drop is not well understood. This is thought to be linked to the local variation in oxygen potential in the different drops: thus some drops reach high enough potential inside to initiate precipitation of Cr2O3 as the most stable oxide.
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Fig. 5. (a) And (b) oxidation layers at the metal–gas interface; (c) and (d) oxidation layer at the metal–melt interface.
Fig. 6. (a) Sample B, (b) metallic phase, and (c) corium oxide.
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Fig. 7. (a) Internal oxidation, (b) continuous oxidation with formation of a homogeneous layer, (c) local oxidation at the surface of the drops, and (d) zoom of figure (c).
Fig. 8. (a) Sample C, (b) sample D, and (c) sample E.
At the gas (bubble)–steel (drop) interface, different oxide layers are observed: either two continuous oxide layers – (Fe,Cr)3O4 and Fe2O3 – or a discontinuous oxide layer (Fe,Cr)3O4. For the double oxide structure, the outer oxide layer is hematite Fe2O3 while the inner oxide layer is a spinel (Fe,Cr)3O4.
Similar arrangement of inner and outer oxide layers have been observed previously for the oxidation of 304L exposed in wet atmosphere [15]. By contrast stainless steel oxidation in dry O2-containing atmosphere generally result in a mixed (Fe,Cr) oxide spinel layer above a pure Cr2O3 layer at least up to temperatures up
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Fig. 9. Metallic drops without oxidation.
Table 3 Required enrichment of concrete to totally oxidize the metallic phase at 2100 and 2500 K. Temperature (K)
Siliceous concrete (wt.%)
LCS concrete (wt.%)
2100 2500
65 55
60 50
has been observed in a number of high alloy steels such as 304L (18Cr–8Ni) and 353MA (25Cr–20Ni) [15,18,19]. In this case, chromium is considered to have evaporated in the form of Cr(VI) oxy-hydroxides following the chemical reaction: Fig. 10. Phases formed at thermodynamic equilibrium with 18 wt.% of concrete.
1=2 Cr2 O3 ðsÞ þ 3=4 O2 ðgÞ þ H2 OðgÞ ! CrO2 ðOHÞ2 ðgÞ There is a very high evaporation rate of chromium oxy-hydroxides at temperatures above only 600 °C [15]. There is the possibility of both mechanisms operating at higher temperatures above 1000 °C. Thus it would seem likely that the Cr depletion observed in the post-test analyses could go to completion through chromium oxyhydroxide volatilisation. This could explain the complete Cr depletion observed in the outer oxide layers surrounding the metallic phases in the VULCANO samples. 5. Conclusion
Fig. 11. Oxidation of steel versus mass of siliceous concrete.
In order to study oxidation of the metallic phase during MCCI phenomena, post-test analyses of the VULCANO VBS-U3 corium have been performed. After the test, the remaining metallic phases were present in two forms: - A discontinuous phase in the form of drops, dispersed in the oxide melt. - A continuous phase separated from the oxide melt.
Fig. 12. Oxidation of steel depending on the mass of concrete in the melt.
to 900 °C; above this temperature evaporation of the volatile CrO3 species begins to diminish the chromia protective thickness [16]. Also high oxygen partial pressures assist this volatilisation [17]. Chromium depletion in the outer layer in presence of steam vapour
Experimentally, between 60% and 70% of the metal is oxidized whereas at thermodynamic equilibrium – taking into account only concrete decomposition gases as oxygen sources – oxidation of around 15 wt.% steel oxidation is expected. At thermodynamic equilibrium, between 45 and 55 wt.% of siliceous concrete was calculated to be necessary to oxidize 70% of steel, whereas 18 wt.% siliceous concrete was sufficient experimentally. Consequently, other sources of oxidation, such as gases from the non-ablated concrete, the oxide phase or oxygen from the air, are necessary to explain the extent of oxidation of the metallic phase during the MCCI processes. Another important point of concerns is the nature of the oxide formed: at thermodynamic equilibrium, only the spinel (Fe,Cr)3O4
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should be formed whereas it has been found experimentally that (Fe,Cr)3O4 and Cr2O3 have been formed. At the interface between the metallic drops and the oxide melt, Cr2O3 or (Fe,Cr)3O4 are formed. Moreover, in certain cases, internal oxidation of the metallic is seen as oxides in the bulk of the drops. At the interface between the metallic drops and gas: an iron oxide layer Fe2O3 (hematite), an iron–chromium oxide layer (Fe,Cr)3O4 in contact with metal and an internal oxidation layer constituted by (Fe,Cr)3 O4. Formation of hematite could result from the chromium evaporation under volatile chromium forms from the mixed (Fe,Cr)2O3 formed in the first stage of the oxidation. The chromium oxy-hydroxides formed with ambient water vapour could be the most active mechanism to explain the observed depletion in chromium and the formation of hematite in the outer oxide layer. Further investigations will be needed to verify this oxidation mechanism and when it occurs during MCCI as well as the various factors, both thermodynamic and kinetic involved. It will also require more study to establish how widespread the chromium depletion is in the large corium mass. Finally this result indicates how volatile species can affect an element’s chemistry and increase the potential late stage releases of a severe accident. Acknowledgments The authors are very grateful to the JRC Institute for Transuranium Elements (ITU) and student exchange programme in the frame of the European project SARNET EURATOM FP7, grant n° 231747. This work is a part of the PhD of M. Sanchez-Brusset, co-financed by CEA and EDF and supported by the University of Perpignan Via Domitia and Marianne Balat-Pichelin at PROMES-CNRS laboratory. Acknowledgements are also extended for the VULCANO tests co-financed by CEA, EDF, IRSN and GDF Suez as well as by the SARNET FP7 grant. The PLINIUS team is joined to the acknowledgements for their work and effort. References [1] C. Journeau, J.M. Bonnet, E. Boccacio, P. Piluso, J. Monerris, M. Breton, G. Fritz, T. Sevon, P.M. Pankakovski, S. Holmstrom, European experiments on 2-D molten core concrete interaction: HECLA and VULCANO, Nucl. Technol. 170 (2010) 189–200.
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