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Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP
Annals of Nuclear Energy xxx (xxxx) xxx
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Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP Xuefeng Lyu ⇑, Boxue Wang, Shuai Liu, Xiong Huang, Xiangyuan Meng Beijing Key Laboratory of Passive Nuclear Safety Technology, School of Nuclear Science and Engineering, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 5 August 2019 Received in revised form 10 October 2019 Accepted 14 October 2019 Available online xxxx Keywords: Hydrogen risk Post-inerting Total amount of inert gas
a b s t r a c t To study the hydrogen risk at severe accident of LB-LOCA of cold leg in 1# SG compartment with gravity injection failure and hydrogen igniters without power in AP1000 NPP, we investigate the effect of total amount of inert gas on reducing hydrogen risk during AP1000 post-inerting. The total amount of inert gas is 42 tons, 56 tons and 70 tons respectively. We analyze the average volume fraction of hydrogen, flame acceleration factor and deflagration to detonation transition factor in 1# SG compartment and in the upper space of containment, and the pressure in containment. The results show that, post-inerting can reduce hydrogen risk in containment at severe accident, the more the total amount of inert gas is injected, the lower the possibility of FA and DDT is. Moreover, we must ensure that containment filtration and discharge system is functioning normally at accident conditions to prevent containment from overpressure. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The reactors in China are mainly pressurized water reactor (PWR). Under the condition of Loss of Coolant Accident (LOCA), if the core is not cooled enough, the fuel cladding will react with water or steam to produce a large amount of hydrogen (Sehgal, 2001). Local accumulation of hydrogen in containment may lead to rapid turbulent combustion or explosion. The combustion or explosion of hydrogen not only affects the effective implementation of safety functions of the safety systems in containment, but also may destroy containment and lead to leakage of radioactive materials. Therefore, study of hydrogen risk mitigation measures for PWR nuclear power plants is greatly significant to ensure the safety of nuclear power plants. At present, many scholars have studied hydrogen behavior and hydrogen risk at severe accidents in nuclear power plants. R. Gharari summarized hydrogen generation, explosion and hydrogen risk mitigation measures at light water reactor severe accidents (Gharari et al., 2018). Y. Halouane validated the CFD model of passive hydrogen composite dehydrogenation by using the data measured on the scaling experimental bench (Halouane and Dehbi, 2018). L. L. Tong studied hydrogen generation and hydrogen risk at the accident of passive advanced pressurized water reactor nuclear power plant (Tong, 2016). In the previous work of the ⇑ Corresponding author.
authors, the effectiveness of hydrogen control system in AP1000 containment is analyzed by using three-dimensional fluid dynamics code (Lyu et al., 2018). OECD/NEA has organized a series of International Standard Problem (ISP) tests to study the distribution of hydrogen in containment at severe accidents (Karwat,1993; Malet et al., 2010). Lin Qian and Li Jingxi respectively studied the hydrogen elimination measures of AP1000 nuclear power plant (Qian and Quan-fu, 2012; Jingxi et al., 2012). Huang Xingguan numerically simulated and analyzed hydrogen risk and effect of hydrogen mitigation system in containment of Ling’ao Nuclear Power Plant (Xingguan et al., 2011). Wang Hui analyzed the hydrogen behavior and risk at severe accident of advanced pressurized water reactor nuclear power plant (Hui et al., 2015). Zhou Kefeng simulated the state of reactor core in the process of power failure accident of CPR1000 nuclear power plant, and evaluated the generation, distribution and behavior of hydrogen (Ke-feng et al., 2018). Meilan Chen studied on the mitigation of strategies for combustible gases in NPP, especially in AP1000 and EPR (Chen and Chen, 2017). Watada M and Sumer Sahin respectively studied the effectiveness of igniters and hydrogen combiners under different severe accidents in different nuclear power plants (Watada and Furuta, 1996; Sahin and Sarwar, 2013). Long Chen found that only open the top three ADS couldn’t prevent the generation of hydrogen on AP1000 during LB-LOCA (Chen et al., 2013). Xiaolong Zhang put forward and validated a new method for hydrogen deflagration to detonation transition prediction under severe accidents (Zhang et al., 2019).
E-mail address: [email protected] (X. Lyu). https://doi.org/10.1016/j.anucene.2019.107125 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
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Briefly, the research on hydrogen risk in PWR nuclear power plant is mainly focused on numerical simulate hydrogen behavior and the effect of igniters on mitigating hydrogen risk. In the previous work of the authors, the effects of initial injection time, injection rate, injection point of inert gas on hydrogen risk are studied (Lyu and Chen, 2016; Lyu et al., 2017; Lyu et al., 2019). In this paper, we will study the effect of total amount of inert gas on mitigating hydrogen risk during post-inerting process of nuclear power plant. 2. Containment model and inert gas selection 2.1. AP1000 containment model We simulate a LB-LOCA of cold leg in 1# steam generation (SG) compartment with gravity injection failure in AP1000 nuclear power plant, which is the same as the reference (Lyu et al., 2019). In this hypothetical accident, the equivalent diameter of break is 300 mm, the hydrogen igniters are all out of work, and the passive hydrogen recombiners operate normally, the RPV bottom head remains intact. According to the simulation results in reference (Lyu et al., 2019), the hydrogen generation time is mainly concentrated in 5700 s to 6700 s. Based on the hydrogen and steam source terms, we will simulate the hydrogen behavior in the containment of AP1000 nuclear power plant at severe accident by using the multi-dimensional computational fluid dynamics code Gasflow (Travis et al., 2007). GASFLOW is developed by Fzk in Germany as a best-estimate tool to characterize local phenomena within a flow field, it has been widely applied in the prediction of hydrogen turbulent dispersion and heat transfer inside the containment (Li et al., 2018; Zhang et al., 2017; Zhang et al., 2018). Pal Kostka proved that the modeling of hydrogen distribution in the VVER-440/213 containment has reasonable results and remarkable physical insights (Kostka, 2002). Xiao Jian-jun indicated that the combination of recombiners and igniters is a safe and effective way to reduce the risk of hydrogen combustion (Jian-jun and Zhi-wei, 2006). Zhiqiang Zou analyzed and compared three kinds of different hydrogen mitigation measures in subcompartment (Zou et al., 2013). Yabing Li numerically validated a dynamic film model of GASFLOW-MPI, which can provide reasonable predictions for both heat and mass transfer between film and gas (Li et al., 2019). Zhang developed the advanced LES and DES turbulence models for GASFLOW-MPI (Zhang et al., 2017; Zhang et al., 2018; Zhang et al., 2018) and applied to the well-known benchmark based on THAI facility (Zhang et al., 2019). The results show that new turbulence models could capture the local unsteady turbulence behavior accurately. Zhang also quantified the uncertainties propagated from the boundary conditions, physical models and mesh size in the condensation heat transfer benchmark (Zhang et al., 2018). The diameter of AP1000 steel containment is about 40 m and the height is about 66 m. Based on the containment geometric model of AP1000 nuclear power plant, we establish a containment model in cylindrical coordinate system. As shown in Fig. 1, the model mainly includes two SG compartments, one pressure vessel compartment, one pressurizer compartment, one in-containment refueling water storage tank (IRWST), upper space and so on. 2.2. Simplification and assumption Since the release time of hydrogen is mainly between 5700 s and 6700 s, we begin with the analysis of hydrogen behavior from 5700 s, which is set as zero time of calculation, and the total calculation time is 2000 s. According to the results by the severe accident analysis program, the average temperature in containment
Fig. 1. AP1000 containment model.
is 382 K and the pressure is 0.253 MPa at 5700 s. The volume fractions of steam and air are 0.374 and 0.626 respectively. If hydrogen is ignited at a lower concentration, it is eliminated by a slower diffusion combustion mode, which actually reduces the risk of flame acceleration and deflagration to detonation transition. In order to eliminate this uncertainty and maximize the hydrogen risk, it is conservatively assumed that hydrogen does not diffuse combustion at low concentration in containment. 2.3. Inert gas selection Helium and Neon have good chemical stability, but they are difficult to liquefy during storage, so they are not used in nuclear power plant. Some commonly used fire extinguishing gases, such as Halon 1301, can produce refractory corrosive and radioactive materials, which are also not suitable to be used as inert gases. CO2 and N2 are usually used as inert gases. Table 1 shows the physical properties of CO2 and N2. Liquid injection and gas injection are commonly used inert gas injection modes. The existing gas injection system can completely accomplish the injection of inert gas into containment, and doesn’t affect the normal operation of nuclear power plant. In this paper, CO2 is selected as inert gas, and injected at 300 K and 3 MPa. The injection position is in the upper space of containment.
Table 1 Physical properties of CO2 and N2. Physical properties
unit
CO2
N2
Density (0 °C, 1 bar) Melting Temperature Enthalpy of fusion Saturation temperature (103 bar) Critical temperature Critical pressure Specific heat capacity (25 °C, 1 bar) Thermal conductivity (25 °C, 1 bar) Kinetic viscosity
Kg/m3 °C kJ/kg °C °C bar kJ/(kgK) w/(mK) 105Ns/m2
1.977 56.6 184 78.2 31.1 73.92 0.846 0.016 1.48
1.25 209.8 25.75 195.7 147.16 33.93 1.308 0.026 1.78
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
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In order to study the effect of total amount of inert gas on mitigating hydrogen risk, we assume that inert gas is injected into the containment from 600 s. The total amount of inert gas injected is 42 tons (case1), 56 tons (case2) and 70 tons (case3), respectively. Since hydrogen is released in 1 # SG compartment in this hypothetical accident and more likely to flow upward because of its lower density, the hydrogen risk in 1 # SG compartment and the upper space of containment is relatively high, so we mainly analyze hydrogen distribution and hydrogen risk in 1 # SG compartment and the upper space of containment. 3. Results and discussion 3.1. The volume fraction of hydrogen
Average fraction of hydrogen in 1#SG compartment
Figs. 2 and 3 show the variation of hydrogen average volume fraction with time in 1 # SG compartment and the upper containment space, respectively. As can be seen from Fig. 2, the average
volume fraction of hydrogen in case 1 is significantly higher than that in case 2 and case 3 in 1 # SG compartment. The injection position of CO2 is in the upper space of containment. A large amount of CO2 migrates rapidly to the lower space of containment under the action of gravity and diffusion force, which results in that H2 does not diffuse to the upper space of containment in time, more hydrogen still accumulates in 1 # SG compartment. So, hydrogen volume fraction increases in 1 # SG compartment until it reaches the highest value. Then, with the increase of inert gas in 1 # SG compartment, the average volume fraction of hydrogen decreases gradually. The injection amount in case 1 is less than that of the other two cases, so the average volume fraction of hydrogen in case 1 is higher than that in case 2 and case 3. In Fig. 3, the hydrogen volume fraction in the upper space of containment at different cases is almost the same trend. The average volume fraction of hydrogen in case 1 is higher than that in case 2 and case 3, the reason is that the inert gas injected in case 1 is the least. 3.2. The average pressure in containment The influence of total amount of inert gas on the average pressure in containment during post-inerting is shown in Fig. 4. When the inert gas is injected into the containment at 600 s, the pressure basically shows a positive upward trend. The greater the total amount of CO2, the greater the pressure rise. At the end of the calculation, the average pressure in containment in case 1, case 2 and case 3 is 5.1297 bar, 6.7757 bar and 7.6306 bar respectively, which exceeds the containment design pressure by 4.07 bar, and seriously threatens the integrity of containment. Therefore, in order to mitigate the hydrogen risk in containment, it is necessary to ensure the normal operation of containment filtration and discharge system at severe accident condition.
0.35
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case1 case2 case3
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3.3. Hydrogen risk analysis
0.05
0.00 0
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1000 1200 1400 1600 1800 2000
Flame acceleration is the process that the mixed gas produced by flame combustion expands, and accompanied by strong turbulence generation. It is often used to judge a process from the laminar state to the rapid transformation of turbulent combustion. In
Time(second) Fig. 2. Average volume fraction of hydrogen in 1#Scompartment.
7
Average pressure(bar)
Average fraction of hydorogen in the upper space of containment
0.12
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6
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Time(second) Fig. 3. Average volume fraction of hydrogen in the upper space of containment.
0
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800 1000 1200 1400 1600 1800 2000
Time(second) Fig. 4. Average pressure in containment.
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
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muti-dimensional computational fluid dynamics code GASFLOW, Flame Acceleration Factor is used to assess the possibility of flame acceleration.
dindex
d /H2 ; /H2 o ; /o2 ; T ¼ dcritical /H2 ; /o2 ; T
where the /H2 , /H2 o , /o2 are average volume percentage concentrations of hydrogen, steam and oxygen, respectively. is expansion factor of gas mixture. d /H2 ; /H2 o ; /o2 ; T dcritical /H2 ; /o2 ; T is critical expansion factor of gas mixture. If dindex < 1, hydrogen flame acceleration will not happen. If dindex 1, flame acceleration may occur for gas mixture. Figs. 5 and 6 are flame acceleration factors in 1# SG compartment and the upper space of containment, respectively. As shown in Figs. 5 and 6, it can be seen that flame acceleration factor is greater than 1 in 1 # SG compartment and the upper space of containment. Especially in case 1, the time period that the flame acceleration factor greater than 1 is the longest and flame acceleration risk is greater. With the prolongation of CO2 injection time, a large amount of CO2 mixes with hydrogen in the upper space of containment, and then goes into the 1 # SG compartment under gravity, which reduces the flame acceleration factor in 1 # SG compartment and the upper space of containment. The larger the total amount of inert gas is, the earlier the flame acceleration factor begins to decrease. Deflagration to detonation transition (DDT) is the change of flame from combustion to explosion. DDT factor, Rindex is used to evaluate the possibility of deflagration to detonation transition in GASFLOW code. The formula is as follows:
Rindex ¼ D=7k 1
D ¼ V3
1.4
case1 case2 case3
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
200
400
600
800 1000 1200 1400 1600 1800 2000
Time(second) Fig. 6. Flame acceleration factor in upper space of containment.
volume of gas clouds beyond the lower limit of hydrogen flammable concentration in the space. If Rindex < 1, DDT will not happen. If Rindex 1, DDT may occur (Travis et al., 2011). Figs. 7 and 8 show the DDT factor of the 1 # SG compartment and the upper space of containment in three cases, respectively. From Figs. 7 and 8, it can be seen that DDT in 1 # SG compartment is more likely to occur in a longer period of time in case 1, and DDT in 1 # SG compartment will not occur in case 3. DDT factor in the upper space of containment does not exceed 1 in all three cases, which indicates that there is no DDT risk in the upper space of containment. Therefore, we must inject sufficient CO2 into the containment to reduce hydrogen risk when post-inerting measure is taken.
4.0
1.6
DDT factor in 1#SG compartment
Flame acceleration in 1#SG compartment
where D is characteristic dimensions of combustible mixture gases. k is the explosion unit average length of mixture gases. V is the
1.6
Flame acceleration factor in the upper space of containment
4
1.4
case1 case2 case3
1.2 1.0 0.8 0.6 0.4 0.2
3.6
case1 case2 case3
3.2 2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0
0.0 0
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800 1000 1200 1400 1600 1800 2000
Time(second) Fig. 5. Flame acceleration factor in 1# SG compartment.
0
200
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800 1000 1200 1400 1600 1800 2000
Time(second) Fig. 7. DDT factor in 1#SG compartment.
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
DDT factor in the upper space of containment
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Appendix A. Supplementary data 0.45
Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.107125.
0.40
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References
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Time(second) Fig. 8. DDT factor in the upper space of containment.
4. Conclusion In this paper, we study the effect of total amount of inert gas on hydrogen risk in AP1000 nuclear power plant at severe accident of large break of cold leg in 1# steam generation compartment with gravity injection failure and hydrogen igniters without power. The main conclusions are as follows: (1) Post-inerting can reduce hydrogen risk in containment at severe accident. (2) The more CO2 is injected into containment, the lower the possibility of flame acceleration and deflagration to detonation transition in 1#SG compartment and the upper space of containment is, and the better the effect of mitigating hydrogen risk is. (3) During the process of post-inerting, the total amount of inert gas directly affects the final pressure in containment. In order to prevent the containment overpressure, it is necessary to ensure the normal function of containment filtration and discharge system at severe accident condition.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 51306057, 11635005), the Fundamental Research Funds for the Central Universities (No. 2018MS041, 2018ZD10).
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Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP Xuefeng Lyu ⇑, Boxue Wang, Shuai Liu, Xiong Huang, Xiangyuan Meng Beijing Key Laboratory of Passive Nuclear Safety Technology, School of Nuclear Science and Engineering, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 5 August 2019 Received in revised form 10 October 2019 Accepted 14 October 2019 Available online xxxx Keywords: Hydrogen risk Post-inerting Total amount of inert gas
a b s t r a c t To study the hydrogen risk at severe accident of LB-LOCA of cold leg in 1# SG compartment with gravity injection failure and hydrogen igniters without power in AP1000 NPP, we investigate the effect of total amount of inert gas on reducing hydrogen risk during AP1000 post-inerting. The total amount of inert gas is 42 tons, 56 tons and 70 tons respectively. We analyze the average volume fraction of hydrogen, flame acceleration factor and deflagration to detonation transition factor in 1# SG compartment and in the upper space of containment, and the pressure in containment. The results show that, post-inerting can reduce hydrogen risk in containment at severe accident, the more the total amount of inert gas is injected, the lower the possibility of FA and DDT is. Moreover, we must ensure that containment filtration and discharge system is functioning normally at accident conditions to prevent containment from overpressure. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The reactors in China are mainly pressurized water reactor (PWR). Under the condition of Loss of Coolant Accident (LOCA), if the core is not cooled enough, the fuel cladding will react with water or steam to produce a large amount of hydrogen (Sehgal, 2001). Local accumulation of hydrogen in containment may lead to rapid turbulent combustion or explosion. The combustion or explosion of hydrogen not only affects the effective implementation of safety functions of the safety systems in containment, but also may destroy containment and lead to leakage of radioactive materials. Therefore, study of hydrogen risk mitigation measures for PWR nuclear power plants is greatly significant to ensure the safety of nuclear power plants. At present, many scholars have studied hydrogen behavior and hydrogen risk at severe accidents in nuclear power plants. R. Gharari summarized hydrogen generation, explosion and hydrogen risk mitigation measures at light water reactor severe accidents (Gharari et al., 2018). Y. Halouane validated the CFD model of passive hydrogen composite dehydrogenation by using the data measured on the scaling experimental bench (Halouane and Dehbi, 2018). L. L. Tong studied hydrogen generation and hydrogen risk at the accident of passive advanced pressurized water reactor nuclear power plant (Tong, 2016). In the previous work of the ⇑ Corresponding author.
authors, the effectiveness of hydrogen control system in AP1000 containment is analyzed by using three-dimensional fluid dynamics code (Lyu et al., 2018). OECD/NEA has organized a series of International Standard Problem (ISP) tests to study the distribution of hydrogen in containment at severe accidents (Karwat,1993; Malet et al., 2010). Lin Qian and Li Jingxi respectively studied the hydrogen elimination measures of AP1000 nuclear power plant (Qian and Quan-fu, 2012; Jingxi et al., 2012). Huang Xingguan numerically simulated and analyzed hydrogen risk and effect of hydrogen mitigation system in containment of Ling’ao Nuclear Power Plant (Xingguan et al., 2011). Wang Hui analyzed the hydrogen behavior and risk at severe accident of advanced pressurized water reactor nuclear power plant (Hui et al., 2015). Zhou Kefeng simulated the state of reactor core in the process of power failure accident of CPR1000 nuclear power plant, and evaluated the generation, distribution and behavior of hydrogen (Ke-feng et al., 2018). Meilan Chen studied on the mitigation of strategies for combustible gases in NPP, especially in AP1000 and EPR (Chen and Chen, 2017). Watada M and Sumer Sahin respectively studied the effectiveness of igniters and hydrogen combiners under different severe accidents in different nuclear power plants (Watada and Furuta, 1996; Sahin and Sarwar, 2013). Long Chen found that only open the top three ADS couldn’t prevent the generation of hydrogen on AP1000 during LB-LOCA (Chen et al., 2013). Xiaolong Zhang put forward and validated a new method for hydrogen deflagration to detonation transition prediction under severe accidents (Zhang et al., 2019).
E-mail address: [email protected] (X. Lyu). https://doi.org/10.1016/j.anucene.2019.107125 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
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Briefly, the research on hydrogen risk in PWR nuclear power plant is mainly focused on numerical simulate hydrogen behavior and the effect of igniters on mitigating hydrogen risk. In the previous work of the authors, the effects of initial injection time, injection rate, injection point of inert gas on hydrogen risk are studied (Lyu and Chen, 2016; Lyu et al., 2017; Lyu et al., 2019). In this paper, we will study the effect of total amount of inert gas on mitigating hydrogen risk during post-inerting process of nuclear power plant. 2. Containment model and inert gas selection 2.1. AP1000 containment model We simulate a LB-LOCA of cold leg in 1# steam generation (SG) compartment with gravity injection failure in AP1000 nuclear power plant, which is the same as the reference (Lyu et al., 2019). In this hypothetical accident, the equivalent diameter of break is 300 mm, the hydrogen igniters are all out of work, and the passive hydrogen recombiners operate normally, the RPV bottom head remains intact. According to the simulation results in reference (Lyu et al., 2019), the hydrogen generation time is mainly concentrated in 5700 s to 6700 s. Based on the hydrogen and steam source terms, we will simulate the hydrogen behavior in the containment of AP1000 nuclear power plant at severe accident by using the multi-dimensional computational fluid dynamics code Gasflow (Travis et al., 2007). GASFLOW is developed by Fzk in Germany as a best-estimate tool to characterize local phenomena within a flow field, it has been widely applied in the prediction of hydrogen turbulent dispersion and heat transfer inside the containment (Li et al., 2018; Zhang et al., 2017; Zhang et al., 2018). Pal Kostka proved that the modeling of hydrogen distribution in the VVER-440/213 containment has reasonable results and remarkable physical insights (Kostka, 2002). Xiao Jian-jun indicated that the combination of recombiners and igniters is a safe and effective way to reduce the risk of hydrogen combustion (Jian-jun and Zhi-wei, 2006). Zhiqiang Zou analyzed and compared three kinds of different hydrogen mitigation measures in subcompartment (Zou et al., 2013). Yabing Li numerically validated a dynamic film model of GASFLOW-MPI, which can provide reasonable predictions for both heat and mass transfer between film and gas (Li et al., 2019). Zhang developed the advanced LES and DES turbulence models for GASFLOW-MPI (Zhang et al., 2017; Zhang et al., 2018; Zhang et al., 2018) and applied to the well-known benchmark based on THAI facility (Zhang et al., 2019). The results show that new turbulence models could capture the local unsteady turbulence behavior accurately. Zhang also quantified the uncertainties propagated from the boundary conditions, physical models and mesh size in the condensation heat transfer benchmark (Zhang et al., 2018). The diameter of AP1000 steel containment is about 40 m and the height is about 66 m. Based on the containment geometric model of AP1000 nuclear power plant, we establish a containment model in cylindrical coordinate system. As shown in Fig. 1, the model mainly includes two SG compartments, one pressure vessel compartment, one pressurizer compartment, one in-containment refueling water storage tank (IRWST), upper space and so on. 2.2. Simplification and assumption Since the release time of hydrogen is mainly between 5700 s and 6700 s, we begin with the analysis of hydrogen behavior from 5700 s, which is set as zero time of calculation, and the total calculation time is 2000 s. According to the results by the severe accident analysis program, the average temperature in containment
Fig. 1. AP1000 containment model.
is 382 K and the pressure is 0.253 MPa at 5700 s. The volume fractions of steam and air are 0.374 and 0.626 respectively. If hydrogen is ignited at a lower concentration, it is eliminated by a slower diffusion combustion mode, which actually reduces the risk of flame acceleration and deflagration to detonation transition. In order to eliminate this uncertainty and maximize the hydrogen risk, it is conservatively assumed that hydrogen does not diffuse combustion at low concentration in containment. 2.3. Inert gas selection Helium and Neon have good chemical stability, but they are difficult to liquefy during storage, so they are not used in nuclear power plant. Some commonly used fire extinguishing gases, such as Halon 1301, can produce refractory corrosive and radioactive materials, which are also not suitable to be used as inert gases. CO2 and N2 are usually used as inert gases. Table 1 shows the physical properties of CO2 and N2. Liquid injection and gas injection are commonly used inert gas injection modes. The existing gas injection system can completely accomplish the injection of inert gas into containment, and doesn’t affect the normal operation of nuclear power plant. In this paper, CO2 is selected as inert gas, and injected at 300 K and 3 MPa. The injection position is in the upper space of containment.
Table 1 Physical properties of CO2 and N2. Physical properties
unit
CO2
N2
Density (0 °C, 1 bar) Melting Temperature Enthalpy of fusion Saturation temperature (103 bar) Critical temperature Critical pressure Specific heat capacity (25 °C, 1 bar) Thermal conductivity (25 °C, 1 bar) Kinetic viscosity
Kg/m3 °C kJ/kg °C °C bar kJ/(kgK) w/(mK) 105Ns/m2
1.977 56.6 184 78.2 31.1 73.92 0.846 0.016 1.48
1.25 209.8 25.75 195.7 147.16 33.93 1.308 0.026 1.78
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
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In order to study the effect of total amount of inert gas on mitigating hydrogen risk, we assume that inert gas is injected into the containment from 600 s. The total amount of inert gas injected is 42 tons (case1), 56 tons (case2) and 70 tons (case3), respectively. Since hydrogen is released in 1 # SG compartment in this hypothetical accident and more likely to flow upward because of its lower density, the hydrogen risk in 1 # SG compartment and the upper space of containment is relatively high, so we mainly analyze hydrogen distribution and hydrogen risk in 1 # SG compartment and the upper space of containment. 3. Results and discussion 3.1. The volume fraction of hydrogen
Average fraction of hydrogen in 1#SG compartment
Figs. 2 and 3 show the variation of hydrogen average volume fraction with time in 1 # SG compartment and the upper containment space, respectively. As can be seen from Fig. 2, the average
volume fraction of hydrogen in case 1 is significantly higher than that in case 2 and case 3 in 1 # SG compartment. The injection position of CO2 is in the upper space of containment. A large amount of CO2 migrates rapidly to the lower space of containment under the action of gravity and diffusion force, which results in that H2 does not diffuse to the upper space of containment in time, more hydrogen still accumulates in 1 # SG compartment. So, hydrogen volume fraction increases in 1 # SG compartment until it reaches the highest value. Then, with the increase of inert gas in 1 # SG compartment, the average volume fraction of hydrogen decreases gradually. The injection amount in case 1 is less than that of the other two cases, so the average volume fraction of hydrogen in case 1 is higher than that in case 2 and case 3. In Fig. 3, the hydrogen volume fraction in the upper space of containment at different cases is almost the same trend. The average volume fraction of hydrogen in case 1 is higher than that in case 2 and case 3, the reason is that the inert gas injected in case 1 is the least. 3.2. The average pressure in containment The influence of total amount of inert gas on the average pressure in containment during post-inerting is shown in Fig. 4. When the inert gas is injected into the containment at 600 s, the pressure basically shows a positive upward trend. The greater the total amount of CO2, the greater the pressure rise. At the end of the calculation, the average pressure in containment in case 1, case 2 and case 3 is 5.1297 bar, 6.7757 bar and 7.6306 bar respectively, which exceeds the containment design pressure by 4.07 bar, and seriously threatens the integrity of containment. Therefore, in order to mitigate the hydrogen risk in containment, it is necessary to ensure the normal operation of containment filtration and discharge system at severe accident condition.
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Flame acceleration is the process that the mixed gas produced by flame combustion expands, and accompanied by strong turbulence generation. It is often used to judge a process from the laminar state to the rapid transformation of turbulent combustion. In
Time(second) Fig. 2. Average volume fraction of hydrogen in 1#Scompartment.
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Average pressure(bar)
Average fraction of hydorogen in the upper space of containment
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Time(second) Fig. 3. Average volume fraction of hydrogen in the upper space of containment.
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Time(second) Fig. 4. Average pressure in containment.
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
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muti-dimensional computational fluid dynamics code GASFLOW, Flame Acceleration Factor is used to assess the possibility of flame acceleration.
dindex
d /H2 ; /H2 o ; /o2 ; T ¼ dcritical /H2 ; /o2 ; T
where the /H2 , /H2 o , /o2 are average volume percentage concentrations of hydrogen, steam and oxygen, respectively. is expansion factor of gas mixture. d /H2 ; /H2 o ; /o2 ; T dcritical /H2 ; /o2 ; T is critical expansion factor of gas mixture. If dindex < 1, hydrogen flame acceleration will not happen. If dindex 1, flame acceleration may occur for gas mixture. Figs. 5 and 6 are flame acceleration factors in 1# SG compartment and the upper space of containment, respectively. As shown in Figs. 5 and 6, it can be seen that flame acceleration factor is greater than 1 in 1 # SG compartment and the upper space of containment. Especially in case 1, the time period that the flame acceleration factor greater than 1 is the longest and flame acceleration risk is greater. With the prolongation of CO2 injection time, a large amount of CO2 mixes with hydrogen in the upper space of containment, and then goes into the 1 # SG compartment under gravity, which reduces the flame acceleration factor in 1 # SG compartment and the upper space of containment. The larger the total amount of inert gas is, the earlier the flame acceleration factor begins to decrease. Deflagration to detonation transition (DDT) is the change of flame from combustion to explosion. DDT factor, Rindex is used to evaluate the possibility of deflagration to detonation transition in GASFLOW code. The formula is as follows:
Rindex ¼ D=7k 1
D ¼ V3
1.4
case1 case2 case3
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
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400
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800 1000 1200 1400 1600 1800 2000
Time(second) Fig. 6. Flame acceleration factor in upper space of containment.
volume of gas clouds beyond the lower limit of hydrogen flammable concentration in the space. If Rindex < 1, DDT will not happen. If Rindex 1, DDT may occur (Travis et al., 2011). Figs. 7 and 8 show the DDT factor of the 1 # SG compartment and the upper space of containment in three cases, respectively. From Figs. 7 and 8, it can be seen that DDT in 1 # SG compartment is more likely to occur in a longer period of time in case 1, and DDT in 1 # SG compartment will not occur in case 3. DDT factor in the upper space of containment does not exceed 1 in all three cases, which indicates that there is no DDT risk in the upper space of containment. Therefore, we must inject sufficient CO2 into the containment to reduce hydrogen risk when post-inerting measure is taken.
4.0
1.6
DDT factor in 1#SG compartment
Flame acceleration in 1#SG compartment
where D is characteristic dimensions of combustible mixture gases. k is the explosion unit average length of mixture gases. V is the
1.6
Flame acceleration factor in the upper space of containment
4
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case1 case2 case3
1.2 1.0 0.8 0.6 0.4 0.2
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Time(second) Fig. 5. Flame acceleration factor in 1# SG compartment.
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Time(second) Fig. 7. DDT factor in 1#SG compartment.
Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125
DDT factor in the upper space of containment
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Appendix A. Supplementary data 0.45
Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.107125.
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References
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Time(second) Fig. 8. DDT factor in the upper space of containment.
4. Conclusion In this paper, we study the effect of total amount of inert gas on hydrogen risk in AP1000 nuclear power plant at severe accident of large break of cold leg in 1# steam generation compartment with gravity injection failure and hydrogen igniters without power. The main conclusions are as follows: (1) Post-inerting can reduce hydrogen risk in containment at severe accident. (2) The more CO2 is injected into containment, the lower the possibility of flame acceleration and deflagration to detonation transition in 1#SG compartment and the upper space of containment is, and the better the effect of mitigating hydrogen risk is. (3) During the process of post-inerting, the total amount of inert gas directly affects the final pressure in containment. In order to prevent the containment overpressure, it is necessary to ensure the normal function of containment filtration and discharge system at severe accident condition.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 51306057, 11635005), the Fundamental Research Funds for the Central Universities (No. 2018MS041, 2018ZD10).
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Please cite this article as: X. Lyu, B. Wang, S. Liu et al., Effect on hydrogen risk of total amount of inert gas during post-inerting in AP1000 NPP, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107125