The interactions of molten core with different types of concretes in EPR severe accident

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1 Journal of Sustainable Cement-Based Materials ISSN: (Print) (Online) Journal homepage: The interactions of molten core with different types of concretes in EPR severe accident Yingjun Yu, Jinyang Jiang, Wei Sun, Zuquan Jin & Qiaofen Zhang To cite this article: Yingjun Yu, Jinyang Jiang, Wei Sun, Zuquan Jin & Qiaofen Zhang (2015) The interactions of molten core with different types of concretes in EPR severe accident, Journal of Sustainable Cement-Based Materials, 4:1, 44-53, DOI: / To link to this article: Published online: 10 Apr Submit your article to this journal Article views: 15 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at Download by: [McMaster University] Date: 16 April 2016, At: 03:33

2 Journal of Sustainable Cement-Based Materials, 2015 Vol. 4, No. 1, 44 53, The interactions of molten core with different types of concretes in EPR severe accident Yingjun Yu a, Jinyang Jiang b *, Wei Sun b, Zuquan Jin c and Qiaofen Zhang d a School of Material Science and Engineering, Southeast University, Nanjing, P.R. China; b Faculty of School of Material Science and Engineering, Southeast University, Nanjing, P.R. China; c Faculty of Department of Structural Engineering, Qingdao Technological University, Qingdao, P.R. China; d Faculty of China Construction Second Engineering Bureau Ltd., Nanjing, P.R. China (Received 19 April 2014; accepted 12 September 2014) Sacrificial concrete is very important in the advanced European pressurized water reactor (EPR). To compare the behaviors of different kinds of sacrificial concretes in the EPR reactor pit, several sacrificial concretes were adopted in the calculation of molten core concrete interaction molten core-concrete interaction (MCCI), i.e. the FeSi concrete, the siliceous concrete, and the limestone concrete. Using a computer code MELCOR, the main parameters, i.e. concrete ablation depth and rate, hydrogen generation mass and rate and temperature in a typical small break loss of coolant severe accident in EPR were calculated. The results indicate that the ablation rate decreased in the order of the FeSi concrete, siliceous concrete and limestone concrete. Meanwhile, the generated hydrogen mass of the FeSi concrete is 13% less than that of the siliceous concrete. In addition, as the basemat of reactor pit is melt through, the temperature of melt pool is the lowest for the limestone concrete and the temperature of siliceous concrete is slightly higher than that of FeSi concrete. Keywords: molten core concrete interaction (MCCI); EPR; sacrificial concrete 1. Introduction In a nuclear reactor severe accident, molten core will penetrate the reactor pressure vessel (RPV) and spread over the concrete basemat which is the ultimate barrier for most current nuclear power plants, as shown in Figure 1(a).[1] This causes a molten core concrete interaction (MCCI) which will lead to failure of containment and release of radioactive materials to the environment.[2] To enclose melt and guarantee long-term heat removal so as to preserve the containment integrity, a core catcher device is thus implemented in advanced nuclear power plants, such as the European Pressurized Water Reactor (EPR),[3] as shown in Figure 1(b). In particular, as a part of the core catcher, the sacrificial concrete, which is designed to react with the molten core to adjust the properties of the melt so as to sustain the integrity of containment, is very important. In practice, siliceous and limestone concretes are commonly used to fulfill such purpose. Moreover, in the EPR reactor pit, a special FeSi concrete cast from both hematite (Fe 2 O 3 ) and siliceous aggregates has been developed. The FeSi concrete shows advantages in achieving an intermediate retention of the melt released from the RPV, including [4]: (1) the accumulation of core melt *Corresponding author. jinyangjiang@163.com 2015 Taylor & Francis

3 Journal of Sustainable Cement-Based Materials 45 Figure 1. (a) Schematic of the Chinese 1000MWe NPP [1] and (b) main components of the EPR melt stabilization concept.[7] inventory; (2) the oxidation of chemically aggressive Zr mainly by Fe 2 O 3 to avoid the thermo-chemical attack of protective ZrO 2 in spreading area; (3) the conditioning of melt prior to spreading. Researchers [5 7] found that as the most abundant ingredients in concretes, aggregates have an important role in many aspects on the behavior of the concrete including the mechanical properties and the interactions with molten core. In contrast to extensive studies on the interaction of molten core with siliceous and limestone concretes, [2,3,7 10] scarce studies are reported on the performance of the special FeSi concrete during MCCI. Researchers [2,8] conducted simulation tests to investigate the interactions of melt with different kinds of hematite-containing concretes for few hundred seconds. In a long-term test VULCANO VB-U7 presented by Sevon et al. [10] the simulated melt was totally

4 46 Y. Yu et al. oxidic, which is in fact composed of both metal and oxide. As a result, rare experimental information is available about the behavior of FeSi concrete during MCCI. In this paper, we intend to present some results about the behaviour of the special FeSi concrete interacting with molten core during MCCI, while siliceous and limestone concretes are adopted as well for the sake of comparison. With respect to the extreme complexity of the simulated MCCI experiments and the high requirement of MCCI simulation test, we choose to simulate MCCI by the computer code MELCOR. The interaction induced by a typical small break loss of coolant severe accident sequence (SBLOCA) of the 1000 MW EPR nuclear power plant was assumed. Since there are mainly three factors that may lead to failure of the containment: overpressure, hydrogen explosion or melt through of the basemat concrete driven by the high temperature of the melt and decay heat generated by the fission products.[2,10] we calculated various parameters, including concrete ablation depth and rate, hydrogen generation mass and rate and temperature. 2. Numerical approach 2.1. Computer code MELCOR We choose to simulate the MCCI in EPR reator pit by the computer code MEL- COR which consists of a series of modules because of its systems-level approach, detailed codes and fast-running. One of the modules, i.e. CAV package, is based on the code CROCON which can be applied in a wide range of severe accidents for reactor plants. The code CROCON, which was validated by the simulation tests SURC, BETA, SWISS and ACE, can predict main phenomena during MCCI, including heat transfer, concrete ablation, cavity shape change, temperature and gas generation.[11] In terms of EPR reactor pit, the code CROCON can meet the basic requirements, though modeling of the Fe O system is not so sufficient as some specific codes, like COSACO,[4]. There are many models that can be selected by the users in the code MELCOR. A modified version of the Kutateladze correlation is used for heat transfer between the melt and concrete.[7] Based on the density of the melt, there are three models to select for the melt configuration: enforced mixing, enforced Figure 2. [13] (a) Cavity contents and boundary conditions and (b) cavity geometry.

5 Journal of Sustainable Cement-Based Materials 47 stratification and mechanistic mixing. In addition, the thermal resistance between the concrete and the melt has two models:slag model and gas model. Moreover, the interface temperature between the melt and concrete is defined as the solidus temperature of the melt which can not be changed.[11] The shape of concrete cavity is described by a series of so-called body points lying in vertical cross-section of the concrete surface. To maintain the stability of numerical calculation, the CAS- CET model [12] written by ACUREX/ Aerotherm Corporation is implemented. As illustrated in Figure 2(b),[13] the ablated point is rezoned onto a series of guiding lines called rays. The point on the rays parallel to the axis keeps removing Figure 3. Geometry of the reactor pit. vertically downward to ensure that the flat bottom remains flat Initial condition of calculation A typical SBLOCA of the 1000 MW EPR nuclear power plant is assumed in this study. Before the accident, the EPR operates at the full power with the thermal power of 3426 MW and the average coolant temperature of 293 C. Upon a stable calculation, i.e. 550 to 0 s, the break initiates. No security intervention is applied during the accident process The main inputs The geometry of EPR reactor pit is indicated in Figure 3. The basemat thickness is 0.45 m and the inner radius is m. The height and the thickness of the side wall are 4.95 and 0.55 m, respectively. Main parameters of the three kinds of concretes are listed in Table 1. In addition, main models selected for calculation are defaults, as shown in Table Results 3.1. Concrete ablation As shown in Figure 4(a), the core melt is poured into the pit at 4360 s. Meanwhile, the concrete starts to ablate, which can be attributed to two factors. On one hand, Table 1. [2] Chemical components, ablation temperature, and density of typical concretes in nuclear power plants. FeSi Siliceous Limestone Mass fraction (%) SiO Fe 2 O CaO MgO Al 2 O CO H 2 O EVAP H 2 O CHEM Total Ablation temperature (K) Density (kg/m 3 )

6 48 Y. Yu et al. Table 2. Models for calculation. Parameters Mixing of the debris Melt/concrete interface Debris-to-surface heat transfer Pool/crust interface temperature Model Enforce mixing Slag at both bottom and radial surfaces of the debris Standard CORCON-Mod3 at bottom, interior interfaces, and radial surfaces of the debris T solidus Figure 4. (a) Concrete axial ablation depth and (b) concrete axial ablation rate. the initial temperature of the molten corium is very high; on the other hand, the heat inside the melt generated by the radioactive decay of the fission products is significant.[2,10] The concrete axial ablation rate calculated by ablation depth per unit time shows a peak-valley tendency which may result from a form re-melt reform process of the crust, as shown in Figure 4(b). Similar to the results presented by Nie and Fischer [4] the axial ablation rate of the FeSi concrete is about m/s at the steady stage. The ablation rate of the siliceous concrete is higher than that of the limestone concrete but lower than that of the FeSi concrete. Thus, the time to melt through the basemat increases from FeSi concrete, siliceous concrete to limestone concrete, as listed in Table 3. The maximum radial ablation depth is m for limestone concrete and about m for the other two types of concrete Hydrogen As shown in Table 4, the masses of hydrogen generated by the interactions of molten core with the FeSi concrete and with the siliceous concrete are quite close, though the free water content in the former one is twice of that in the latter one. As for the limestone concrete, Table 3. Melt through time and maximum radial ablation depth. Concrete FeSi Siliceous Limestone Melt through time (s) ,244 10,665 Maximum radial ablation depth (m)

7 Table 4. Mass of gas. Journal of Sustainable Cement-Based Materials 49 Concrete (kg) FeSi Siliceous Limestone Hydrogen mass Total mass of gas the mass of generated hydrogen is 30% lower when compared with the other two types. The total mass of generated gas decreases from limestone concrete and FeSi concrete to siliceous concrete. In particular, the total mass of generated gas of the FeSi or the siliceous concretes is around 14 21% in comparison to that of the limestone concrete. As shown in Figure 5, similar to the concrete ablation rate, the generation rate of hydrogen changes in a peak-valley way as well. In particular, a slow increase can be detected before 6000 s, while a sharp increase takes place at 6000 s. During s, the values of FeSi and siliceous concretes are steady at around kg/s, while the value of limestone concrete is kg/s. Thereafter, it shows a sharp decrease and increases slowly again. The maximum rate is about kg/s for the FeSi and siliceous concretes and about kg/s for the limestone concrete. The rapid change of hydrogen generation rate is probably resulted by the form re-melt reform process of the crust as well. In comparison, the hydrogen generation rate of limestone concrete is lower than that of the other two concretes, which results in the mass of hydrogen generated by molten core limestone concrete to be the lowest Temperature As shown in Figure 6, the temperature of melt pool increases firstly and then decreases. Before 6000 s, as the high temperature core pouring into the pit, the sharp increase of temperature is induced. During the interaction with cold concrete decomposition products, the temperature of the melt pool decreases. Moreover, in correspondence to the second release of the core at 7500 s, the temperature increases slightly and then decreases again. When the basemat is melt through, the temperature of melt pool for the FeSi, the siliceous and the limestone concrete pit is 2156, 2181 and 2100 K, respectively. Figure 5. Hydrogen generation rate. Figure 6. Temperature of the melt pool.

8 50 Y. Yu et al. 4. Discussion In a severe accident for nuclear power plants, the molten core will penetrate the RPV if it can t be cooled inside the reactor and flow into the concrete cavity under the reactor, which then initiates the MCCI. Early simulation tests indicate that the concrete ablation rate, which characterizes the ratio of the heat flux over the enthalpy to ablate per unit mass of concrete, can be expressed as follows [7]: t ¼ Q (1) qadh where Q is the heat flux to concrete, ρ is the concrete density, ΔH is the decomposition enthalpy per unit mass of concrete, and A is the area of the eroding concrete. For Equation (1), the heat conducted into concrete is ignored with respect to the very poor heat conductivity of concrete.[7] The rate of heat loss to concrete ablation is shown in Figure 7. Due to the endothermic decomposition of calcium carbonate at C, the heat loss for limestone concrete turns out much higher than FeSi and siliceous concretes. A similar conclusion can be drawn for the decomposition enthalpy, as indicated in Figure 8.[7] As a result, the ablation rate of limestone concrete is the lowest. Compared with the siliceous Figure 7. Rate of heat loss to concrete ablation. Figure 8. concretes. [7] Enthalpy of three types of concrete, the ablation rate of FeSi concrete is higher for its lower decomposition enthalpy, though the heat flux is close. After the melt is released into the pit the second time, the significant increase of decay heat then leads to an increase of heat loss to concrete. Thus, the ablation rate at the later stage is larger than that at the early stage. The concrete decomposition products (SiO 2,CO 2,H 2 O, Fe 2 O 3 ) will react with the metal in the melt which will release hydrogen by reaction [7]: Zr þ 2H 2 O! ZrO 2 þ 2H 2 þ 6:3 MJ/kgzr (2) Zr þ 2CO 2! ZrO 2 þ 2CO þ 5:7 MJ/kgzr (3) Zr þ SiO 2! ZrO 2 þ Si þ 1:6 MJ/kgZrðT\1870 CÞ (4) Zr þ 2SiO 2 þ 4:7 MJ/kgZr! ZrO 2 þ 2SiOðgÞðT [ 1870 CÞ (5) 3Zr þ 2Fe 2 O 3! 3ZrO 2 þ 4Fe þ 5:8 MJ/kgZr (6) The free water content in the FeSi concrete is twice of that in the siliceous

9 Journal of Sustainable Cement-Based Materials 51 concrete. Thus, a larger total gas mass is calculated for the FeSi concrete. However, the mass of hydrogen generated by the interaction of molten core with the FeSi concrete is about 13% less than siliceous concrete. In fact, as one of the decomposition products of FeSi concrete, Fe 2 O 3 oxidizes partial Zr, which causes the less mass of hydrogen by reducing the mass of Zr reacting with H 2 O. The mass of hydrogen generated by the limestone concrete is the lowest, though the free water content is the highest. It seems that the lowest mass of H 2 O generated by limestone concrete can account for this. Moreover, CO 2 generated by the decomposition of limestone concrete can oxidize partial Zr. The temperature of melt pool is closely related to the heat and mass change in pit. The heat in pit comes from the decay heat and the chemical reaction heat, which dissipates to concrete and the surface of the melt pool.[7] As seen in Figures 9 and 10, the chemical reaction heat turns out be the lowest, while the relevant heat loss to concrete is the highest due to the higher enthalpy for the limestone concrete. Thus, the corresponding temperature of melt pool is reduced most quickly and the final temperature is the lowest for the limestone concrete. Meanwhile, to oxidize the same amount of Zr, the chemical reaction heat released Figure 9. Heating rate by chemical reaction. Figure 10. Heat loss to concrete ablation. by Fe 2 O 3 is lower than H 2 O, as indicated in Equations (2) and (6). As shown in Figure 9, the chemical reaction heat released by the siliceous concrete is slightly higher than the FeSi concrete. In addition, the effect of added concrete decomposition products on the decreasing of the melt temperature is more significant due to the larger ablated mass for the FeSi concrete. As a result, the temperature of melt pool in the siliceous concrete pit is slightly higher than the FeSi concrete. 5. Conclusion In this paper, in order to study the performance of sacrificial concrete during a severe accident of nuclear power plant, we adopt three types of concretes for comparison, i.e. FeSi concrete, siliceous concrete, and limestone concrete. In particular, by the computer code MEL- COR-based numerical approach, we calculate various parameters which are present in a typical SBLOCA of the 1000 MW EPR, including the concrete ablation depth and rate, the generated hydrogen mass rate and the temperature. Results confirm that the performance of concrete is closely related to its chemical components.[7] Compared to the limestone concrete, the performances of FeSi concrete and siliceous concrete

10 52 Y. Yu et al. are close. Some general conclusions can be drawn as follows: (1) Due to the highest decomposition enthalpy, the ablation rate and the hydrogen generation rate of limestone concrete are both the lowest in spite of the largest heat loss to concrete ablation. In addition, because of the large amount of CO 2 generated by the decomposition of calcium carbonate, the total mass of gas generated from the molten core limestone concrete interaction is five times and seven times of FeSi concrete and siliceous concrete, respectively. (2) For its lower enthalpy, the ablation rate of FeSi concrete is higher than that of siliceous concrete. Meanwhile, the hydrogen mass generated from the molten core- FeSi concrete interaction is 13% less than that from the molten core-siliceous concrete interaction, though the free water content is twice in the FeSi concrete. (3) The heat loss to concrete ablation and the chemical reaction heat are the major factors that influence the temperature of melt. With the highest heat to concrete ablation, the final temperature of melt pool decreases from siliceous concrete, FeSi concrete to limestone concrete. With respect to the design of sacrificial concrete for EPR reactor pit, we suggest that a novel design should be of low contents of H 2 O, CO 2, and small decomposition enthalpy. In that way, the reactor pit can reduce the amount of combustible gas, while the ablation of concrete can be more efficient. Besides that, the designed concrete should contain some Fe 2 O 3 to oxidize the chemically aggressive metal Zr in reactor pit [4]. Acknowledgments Financial supports from Natural Science Foundation of China (No ) and Research subject of Taishan nuclear power are grateful acknowledged. Funding This work was supported by the Natural Science Foundation of China [grant number ]; Research subject of Taishan nuclear power. References [1] Yuan K, Qie WQ, Tong LL, Cao XW. Analysis on containment depressurization under severe accidents for a Chinese 1000 MWe NPP. Prog. Nucl. Energy. 2013;65:8 14. [2] Sevón T, Kinnunen T, Virta J, Holmström S, Kekki T, Lindholm I. HECLA experiments on interaction between metallic melt and hematite-containing concrete. Nucl. Eng. Des. 2010;240: [3] Widmann W, Bürger M, Lohnert G, Alsmeyer H, Tromm W. Experimental and theoretical investigations on the COMET concept for ex-vessel core melt retention. Nucl. Eng. Des. 2006;236: [4] Nie M, Fischer M. Advanced MCCI modeling based on stringent coupling of thermal hydraulics and real solution in COSACO. Paper presented at the Proceeding of ICONE 10; 2002; Arlington, VA. [5] Butler LJ, West JS, Tighe SL. Towards the classification of recycled concrete aggregates: influence of fundamental aggregate properties on recycled concrete performance. J. Sustainable Cement- Based Mater. 2014;3: [6] Shaikh FUA, Nguyen HL. Properties of concrete containing recycled construction and demolition wastes as coarse aggregates. J. Sustainable Cement-Based Mater. 2013;2: [7] Sevon T. Molten core-concrete interactions in nuclear accidents: theory and design of an experimental facility [Master thesis]. Helsinki: Helsinki University of Technology; 2005.

11 Journal of Sustainable Cement-Based Materials 53 [8] Eppinger B, Fellmoser F, Fieg G, Massier H, Stern G. (2000). Experiments on concrete erosion by a Corium melt in the EPR reactor cavity: KAPOOL 6 8. Karlsruhe, Forschungszentrum Karlsruhe. [9] Cranga M, Spindler B, Dufour E, Dimov D, Atkhen K, Foit J, Garcia-Martin M, Sevon T, Schmidt W, Spengler C. Simulation of corium concrete interaction in 2D geometry. Prog. Nucl. Energy. 2010;52: [10] Sevón T, Journeau C, Ferry L. VUL- CANO VB-U7 experiment on interaction between oxidic corium and hematitecontaining concrete. Ann. Nucl. Energy. 2013;59: [11] Bradley D.R., Gardner D.R., Brockmann J.E., Griffith R.O. CORCON-MOD3: An Integrated Computer Model for Analysis of Molten Core-Concrete Interactions. Users Manual, NUREG/CR-5843, SAND , Sandia National Laboratories, Albuquerque, NM (October 1993). [12] Kwong KC, Beck RAS, Derbidge TC. CORCON Program Assistance, FR-79-10/AS. Mountain View (CA): ACUREX Corporation/Aerotherm Aerospace Division; [13] Gauntt R.O., Cole R.K., Erickson C.M., Rodriguez S.B., Gido R.G., Gasser R.D., Young M.F., MELCOR computer code manuals: cavity (CAV) package reference manual. Version 1.8.5, NUNUREG/ CR-6119, SAND /2.

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