MCCI on LCS concrete with and without rebars

Transcription

1 1/10 MCCI on LCS concrete with and without rebars J. J. Foit Karlsruhe Institute of Technology, Karlsruhe (DE) Summary Even though extensive research has been undertaken over several years in the area of core-concrete interaction, several subjects need further investigations. One of the still unresolved issues is the long-term interaction of a melt with a reinforced concrete. The knowledge of the distribution of the heat flux to the concrete in the lateral and axial directions during the long-term 2-dimensional concrete erosion is important for evaluation of the consequences of a severe reactor accident. The large-scale MOCKA (KIT, Karlsruhe) experiments study the interaction of a simulant oxide (Al 2 O 3, ZrO 2, CaO) and metal melt (Fe) in a stratified configuration with different types of concrete. The CaO admixture lowers the solidus temperature and the viscosity of the oxide melt. The resulting solidus temperature of approximately 1360 C is sufficiently low to prevent a formation of an initial crust at the oxide/concrete interface which was observed in the CCI experiments (ANL, USA).The oxide/concrete interface contact temperature in MOCKA test was estimated to be 1400 C. To allow for a long-term interaction, internal heating was provided by alternating additions of thermite and Zr metal to the upper oxide layer of the stratified melt. Current tests in MOCKA (KIT, Germany) program are focused on the erosion behaviour of the LCS concrete. To this purpose, a series of MOCKA experiments have been conducted with and without rebars. All performed experiments have shown a pronounced lateral concrete ablation. A INTRODUCTION An important issue concerns the distribution of the heat flux to the concrete in the lateral and axial directions during the long-term 2-dimensional concrete erosion by a prototypic core melt. The knowledge of this partition is important in the evaluation of the consequences of a severe reactor accident (Ref. [1]). Axial erosion can penetrate the reactor basemat, while lateral erosion can destroy containment structures. The composition of the concrete seems to influence the interaction of melt and concrete. Concrete is a complex mixture of cement, water and aggregates which for most plants consists of variable proportions of silica (SiO 2 ) and limestone (CaCO 3 ). Depending on the SiO 2 /CaCO 3 ratio the concrete used in nuclear power plants can be divided into three categories, namely siliceous, limestone/common sand (LCS) and pure limestone. For a concrete with limestone aggregates, lime burning, i. e. the thermal decomposition of CaCO 3 to CaO and CO 2 at approx. 900 C, may increase the amount of released gases by a factor 3 in comparison with siliceous concrete. The BETA test series (Ref. [2]) provided valuable data on two-dimensional metallic core concrete interaction under dry conditions. Experiments with prototypic oxide corium melts were performed only in one-dimensional crucibles (Ref. [3], [4]). The CCI tests (Ref. [5]) investigated the long-term interaction of a heated (direct electrical heating) core oxide melt within a rectangular limestone/common sand (LCS) and siliceous concrete

2 2/10 crucibles. The initial crusts, which formed at the oxide/concrete interface in all experiments, were not appreciably heated by the used heating system. A local crust failure in the CCI-1 and CCI-3 tests with siliceous concrete lead to an irregular cavity shape (Ref. [6]). There is an evidence that the crust failure mode depends on the type of concrete. The high gas release rates from the decomposing LCS concrete can help to destabilise these initial crusts as observe in the CCI-2 experiment. However, the extrapolation of the obtained results to reactor conditions is uncertain. The experiment COMET-L2 (Ref. [7], [8]) was designed to investigate a long-term MCCI of metallic corium in cylindrical siliceous concrete cavity under dry conditions with decay heat simulation of intermediate power, and subsequently at reduced power. In the first section of the paper some important aspects of Zr oxidation process during MCCI are presented. The second part is devoted to the description of the MOCKA experiments. Recent experimental findings on erosion behaviour of a LCS concrete without rebars and with 12 wt.% rebars are presented and discussed in the following two subsections. B. BEHAVIOUR OF ZR DURING MCCI The experimental findings of the SURC-4 test (Ref. [9]) have led to investigations of special aspects of the Zr-SiO 2 condensed phase reaction within the framework of BETA-II tests (Ref. [10]). In three experiments involving silicate concrete crucibles a melt with a 300 kg steel (Fe, Cr, and Ni) together with 80 kg of Zircaloy-4 and, initially, 500 kg of oxide at an initial temperature of about 2173 K was used. The 21 wt.% of Zr corresponds to a typical PWR scenario. The experimental results have shown the dominance of Zr oxidation during concrete attack by the reduction of SiO 2 to elemental Si and subsequent Si oxidation by H 2 O and CO 2 released from the concrete. The oxidation reactions under consideration together with their reaction enthalpies are given by: Zr + SiO 2 ZrO 2 + Si [J/g Zr] for T m 2200 K (1) Zr + 2H 2 O ZrO 2 + 2H (T-2000) [J/gZr], T=T m (2) where T m [K] is the melt temperature Zr + 2CO 2 ZrO 2 + 2CO [J/g Zr] (3) Si + 2H 2 O SiO 2 + 2H [J/g Si] (4) Si + 2CO 2 SiO 2 + 2CO [J/g Si] (5) A complete reduction of SiO 2, H 2 O, and CO 2 is assumed. A comparison with equilibrium calculations (Ref. [11]) shows that the complete reduction model is based on a valid assumption in this temperature range. In BETA V 5.2 experiment the zirconium oxidation has been completed within only 130 s (Figure 1). At the same time, approximately 5-7 wt. % Si has been built up.

3 3/10 Figure 1: Time dependent composition of oxide and metal melts in BETA V 5.2 C. MCCI ON LCS CONCRETE The former MCCI experiments used different electrical heating techniques to simulate the internal decay heat to allow studies of long-term interaction. Either induction heating or direct electrical heating was used, therefore, the investigation of the concrete erosion process was restricted to concrete without rebars. The recently developed method of using the heat of chemical reactions, i. e. the Zr oxidation and alumina-thermic reactions as a source of internal heat has allowed to carry out an experimental program to study the interaction of the melt with a reinforced concrete. More recent tests in the MOCKA facility are focused on assessing the influence of a typical 12 wt.% reinforcement in a LCS concrete on the erosion behaviour. In all MOCKA experiments concrete crucibles with an inner diameter of 25 cm are used. The initial melt consists of 42 kg Fe together with 4 kg Zr, overlaid by 68 kg oxide melt (initially 56 wt.% Al 2 O 3, 44 wt.% CaO). The collapsed height of the metal melt is about 13 cm and that of the oxide melt 50 cm. The initial melt temperature at start of interaction is approximately 1900 C. The CaO admixture lowers the solidus temperature and the viscosity of the oxide melt. The resulting solidus temperature of approx C is sufficiently low to prevent a formation of an initial crust at the oxide/concrete which was observed in the CCI experiments (Ref. [12]).The oxide/concrete interface contact temperature in MOCKA test was estimated to be 1400 C. After the completion of the thermite burn and the end of the oxidation reactions of the initially added Zr, i. e. after approx. 90 s after thermite ignition a total mass of 300 kg thermite and 105 kg Zr was added within approximately 40 minutes to the melt in the experiments under consideration. The heat generated by the thermite reaction and the exothermal oxidation reaction of Zr is mainly deposited in the oxide phase Due to density-driven phase segregation the metal melt at the bottom of the crucible is fed by the enthalpy of the steel which is generated in the oxide phase by the thermite reaction of the added thermite. Approximately 75 % of the heating power is deposited in the oxide phase and 25 % in the metal melt. In this way a rather prototypic heating of both melt phases can be

4 4/10 achieved. The power input was estimated to be 454 kw to the oxide melt and 142 kw to the metal melt. C1. MCCI ON CONCRETE WITHOUT REBARS An important finding from the MOCKA tests with LCS concrete is that the cavity erosion behaviour is different in comparison to CCI 2 (Ref. [12]). MOCKA tests with and without rebar in the LCS concrete exhibited a highly pronounced lateral ablation. The lateral/axial ratios of the ablation depth for the MOCKA tests are approximately 3 in contrast to 1 estimated for CCI 2 experiment. The spatial heat flux distribution at the melt-concrete interface during melt concrete interaction with siliceous concrete in MOCKA experiments was found to depend on the presence of rebar in the concrete. MOCKA tests on siliceous concrete without rebar have shown more pronounced downward erosion by the metal melt whereas in all tests with reinforced concrete an almost isotropic ablation behaviour has been observed (Ref. [13]). The post-test section of the MOCKA 6.3 concrete crucible without rebar is shown in the following figure, Figure 2. The final maximum downward erosion was 50 mm, the lateral erosion by the metal melt was 120 mm and the final ablation by the overlying oxide exceeded 120 mm (Figures 2 and 3). Figure 2: Section of the MOCKA 6.3 concrete crucible with an indication of the initial size of the crucible. The orange line indicates the initial height (13 cm) of the metal melt and the red line marks the outer surface of the LCS cylindrical crucible.

5 5/10 The efficient heat transfer from the metal melt leads to a fast decrease in temperature (Figure 4), therefore, the concrete ablation will be influenced by crust formation processes. The progression of the concrete erosion found in MOCKA 6.3 test is depicted in Figure 3. During the early phase of the interaction, the observed low erosion rate of 0.7 mm/min is a consequence of the formation of a stable crust at the metal/concrete interface. This low concrete erosion rate (Figure 3, Figure 6) gave rise to a considerable heat-up of a rather thick layer of concrete (Ref. [14]) behind the slowly moving melt front. Decomposition of concrete during heat-up starts with evaporation of physically bound water around 100 C. Dehydration of chemically bound water occurs up to 550 C. Decarbonation of CaCO 3 from the cement and carbonate aggregates occurs from 700 to 900 C. Liquid phases start to form between C. The subsequent increase of the temperature and of the mass of the metal melt due to the continuous addition of thermite caused a melting of the crust at about 1700 s and, consequently, a start of a somewhat faster progression into the thermally damaged concrete (Figure 3, Figure 6). A maximum lateral progression of approx. 6 mm/min of the metal melt front was estimated. Similar behaviour, i. e. a fast removal of a thermally destructed concrete was also observed in experiments which were performed within the SURC, ACE and CCI programs (Ref. [6], [12], [15], [16], [17], [18]) as well as in other MOCKA tests (Ref. [13]). Figure 3: Erosion depth as a function of time in the MOCKA 6.3 test.

6 6/10 Figure 4: Melt temperatures at different positions as a function of time in the MOCKA 6.3 test. C2. MCCI ON CONCRETE WITH REBARS The post-test cavity erosion profile of the MOCKA 7.1 test with reinforced concrete (Figure 5) shows a downward erosion of 50 mm and a maximum sideward ablation of more than 120 mm by the metal melt and oxide melt. The progression of the concrete erosion found in MOCKA 7.1 test is depicted in Figure 6. The estimated erosion rates are much the same as those obtained in MOCKA 6.3 test without rebas (Figure 3). In both experiments approximately 46 % of the simulated decay energy in the oxide melt and 36 % of the heating energy in the metal phase was converted into decomposition of the concrete. The same conversion rate was estimated for the CCI 2 experiment. In MOCKA tests with siliceous concrete the conversion rate in the oxide melt was as low as 31 % whereas in the metal melt the same rate as for the LCS concrete was achieved. The estimated mean lateral heat flux from the oxide melt was 390 kw/m 2. For MOCKA tests with siliceous concrete, the heat fluxes were as low as 220 kw/m 2. The cavity erosion behaviour in MOCKA 7.1 (Figure 6) closely resembles that of MOCKA 6.3 (Figure 3). The melt temperatures which were obtained by immersion lances as well as by tungsten-rhenium thermocouples that were embedded at various positions in the concrete are shown in Figure 4 and Figure 7 for MOCKA 6.3 and MOCKA 7.1, respectively. The rebars in the concrete elevates the decomposition temperature up to the melting temperature of the reinforcing steel. This should result in higher melt pool temperatures than during MCCI with concrete without reinforcement. After a fast initial decrease in melt temperature (Figure 4, Figure 7) the temperature of the oxide melt cluster about 1550 C in MOCKA 7.1 (Figure 7). Surprisingly, the same long-term temperature was also found in MOCKA 6.3 (Figure 4).These findings rise an issue concerning the meltconcrete interface temperature. In the CCI 2 experiment (Ref. [12]) the melt temperature declined to the same level (1550 C). The rising edges of the oxide melt temperature oscillation correspond with addition of thermite and Zr to the oxide. After a fast sideward

7 7/10 concrete erosion of 40 mm within 400 s in MOCKA 7.1 (Figure 6), the metal melt temperature near to the concrete surface has decreased below the solidus temperature of 1526 C (Figure 7) leading to a formation of a crust and, consequently, to the observed low erosion rates (0.7 mm/min) (Figure 3, Figure 6). The subsequent increase in the steel melt temperature above its liquidus temperature at about 1600 s caused a melting of the crust and, consequently, an increase of the concrete ablation rate (Figure 3, Figure 6). Figure 5: Section of the MOCKA 7.1 concrete crucible with an indication of the initial size of the crucible. The orange line indicates the initial height (13 cm) of the metal melt and the red line marks the outer surface of the LCS cylindrical crucible.

8 8/10 Figure 6: Erosion depth as a function of time in the MOCKA 7.1 test. Figure 7: Melt temperatures at different positions as a function of time in the MOCKA 7.1 test.

9 9/10 D CONCLUSIONS The goal of the current large-scale MOCKA tests is to gain insights about a 3-dim concrete erosion process by an oxide and metal melt in a stratified configuration in cylindrical LCS concrete crucibles without and with 12 wt.% rebars. To allow for longer-term MCCI a new method of melt heating was implemented to simulate the decay heating. Dedicated exothermal chemical reactions are used to achieve a prototypic heating of both melt phases. The reinforcing steel in the concrete elevates the decomposition temperature. However, the same long-term temperature was also found in tests with LCS concrete without rebar. In addition, the reinforcing steel, which is melted by the laterally progressing oxide melt, is incorporated into the metal pool by a density-driven segregation. In contrast to the fairly isotropic concrete ablation in the CCI 2 experiment a highly pronounced lateral ablation was obtained in all MOCKA tests with and without rebar in the LCS concrete Lateral/axial ratios of the ablation depth for the MOCKA tests are approximately 3 (approximately 1 in CCI 2). The knowledge of relation between the axial and lateral basemat erosion is important in evaluation of the consequences of a severe reactor accident. However, the extrapolation of the obtained results to reactor conditions is difficult. References [1] J. J. Foit, Modeling oxidic molten core-concrete interaction in WECHSL, Eng. and Design 170, pp (1997). [2] H. Alsmeyer, BETA experiments in verification of the WECHSL code: experimental results on the melt-concrete interaction, Nucl. Eng. and Design 103, pp (1987). [3] E. R. Copus, et al., Core-Concrete Interactions Using Molten UO 2, With Zirconium on a Basaltic Basemat The SURC-2 Experiment, NUREG/CR-5564 (1990). [4] D. H. Thomson et al., Thermal-hydraulic aspects of the large-scale integral MCCI tests in the ACE Program, Second OECD (NEA) CSNI specialist meeting on molten core debris-concrete interactions, Karlsruhe, Proceedings edited by H. Alsmeyer, KfK 5108, pp , NEA/CSNI/R(92)10, 1-3 April [5] M. T. Farmer et al., The result of the CCI-2 reactor material experiment investigating 2- D core-concrete interaction and debris coolability, Proceedings of the 11th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH-11), Avignon, F, CD-ROM Paper 245, October 2-6, [6] M. T. Farmer et al., The Results of the CCI-3 Reactor Material Experiment Investigating 2-D Core-Concrete Interaction and Debris Coolability with a Siliceous Concrete Crucible, International Congress on Advances in Nuclear Power Plants (ICAPP '06), Reno, Nev., Proceedings on CD-ROM, pp (Paper 6164), June 4-8, [7] H. Alsmeyer et al., The COMET-L1 experiment on long-term concrete erosion and surface flooding, Proceedings of the 11th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH-11), Avignon, F, CD-ROM Paper 087, October 2-6, [8] G. Sdouz et al., The COMET-L2 experiment on long-term MCCl with steel melt, Wissenschaftliche Berichte, FZKA-7214, SAM-LACOMERA-D15 (2006). [9] E. R. Copus et al., SURC-4 experiment on core-concrete interactions, Sandia National Laboratories NUREG/CR-4994, SAND (1989).

10 10/10 [10] H. Alsmeyer et al., BETA experiments on melt-concrete interaction: the role of zirconium and the potential sump water contact during core melt-down accidents, Nucl. Eng. and Des. 154, pp (1995). [11] J. J. Foit, L. D. Howe, B. D. Turland, Thermal hydraulic codes, Report-Directoriate- General XII Science, Research and Development EUR (1995). [12] M. T. Farmer, S. Lomperski, D. J. Kilsdonk, and R. W. Aeschlimann, OECD MCCI Project 2-D core concrete interaction (CCI) tests: Final Report, OECD/MCCI TR05 (2005). [13] J. J. Foit et al., Experiments on MCCI with oxide and steel, Annals of Nuclear Energy 74, pp (2014). [14] J. J. Foit, T. Cron, B. Fluhrer, Interaction of a metal and oxide melt with reinforced concrete in MOCKA experiments, 15th International Topical Meeting on Nuclear Reactor Thermalhydraulics (NURETH 2013), Pisa, I, Proceedings on USB-Stick Paper NURETH15-539, May 12-17, [15] J. J. Foit, Overview and history of core concrete interaction issues (international review), International MCCI Seminar, Cadarache, F, Proceedings OECD-MCCI on CD- ROM October 10-11, [16] E. R. Copus, R. E. Blose, J. E. Brockmann, R. B. Simpson, D. A. Lucero, Core- Concrete Interactions Using Molten Urania With Zirconium on a Limestone Concrete Basemat, The SURC-1 Experiment, Sandia National Laboratories NUREG/CR-5443 (1992). [17] E. R. Copus, R. E. Blose, J. E. Brockmann, R. B. Simpson and D. A. Lucero, Core- Concrete Interactions using Molten UO 2 with Zirconium on a Basaltic Basemat: The SURC-2 Experiment, Sandia National Laboratories NUREG/CR-5564, SAND (1990). [18] D. H. Thompson, M. T. Farmer, J. K. Fink, D. R. Armstrong, B. W. Spencer, Compilation, analysis and interaction of ACE Phase C and MACE experimental data, Argonne National Laboratory, Chicago, IL, USA, ACEX TR-C-14 (1997).