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The measure on mitigating hydrogen risk during LOCA accident in nuclear power plant
Annals of Nuclear Energy 136 (2020) 107032
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
The measure on mitigating hydrogen risk during LOCA accident in nuclear power plant Xiangyuan Meng a, Xuefeng Lyu a,b,⇑, Boxue Wang a, Shuai Liu a, Yu Yu a, Zhangpeng Guo a a b
Beijing Key Laboratory of Passive Nuclear Safety Technology, School of Nuclear Science and Engineering, North China Electric Power University, Beijing 102206, China Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 21 June 2019 Received in revised form 5 September 2019 Accepted 6 September 2019
Keywords: Hydrogen risk Steam LOCA
a b s t r a c t Since there is a large empty space in PWR containment which has strong pressure bearing capability, it’s generally considered that there is no overall risk of hydrogen. However, the complex internal structure makes hydrogen easy to accumulate, burn or even explode in local compartment. Therefore, when LBLOCA with gravity injection failure happens, there may be a more serious risk of local hydrogen. GASFLOW, a three-dimensional computational fluid dynamics code, is used to study the measure on mitigating hydrogen risk. During the simulation, a steam injection device is arranged in the PZR compartment. When the accident occurs, steam is injected into the PZR compartment during the pre-set stages. By comparing the hydrogen risks in different situations, it could be found that steam injection in local space can reduce the hydrogen risk locally. The method combining steam injection device with PARs is an effective measure to mitigate the risk of hydrogen. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The nuclear safety issue has been attracted extensive attention from the academia and industry community of nuclear engineering. It covers a wide range of research topics, including both traditional physical and thermal aspects of the reactor, as well as core structure and material mechanics. Since the TMI-2 accident occurred in the United States, relevant organizations and institutions of OECD member countries have continuously carried out in-depth studies in the field of nuclear safety (Paladino et al., 2012; Reventós et al., 2008). If a LOCA accident occurs at large scale commercial PWR and the core isn’t sufficiently cooled, the zirconium cladding may react with water or steam at high temperatures. Large amounts of hydrogen will be produced. The hydrogen may be released into the containment through the break at primary loop. Regardless of the internal spray, the fluid near the break is driven by pressure difference. In other areas, fluid transport relies on the heat exchange of steam and air, steam condensation, etc. (Rosa et al., 2009). Hydrogen tends to form layers and generate accumulation in where the break located and over it. When the break occurs in 1# Steam Generator (SG) compartment, the pressurizer (PZR) compartment will also generate accumulation and form layers (Li,
⇑ Corresponding author. E-mail address: [email protected] (X. Lyu). https://doi.org/10.1016/j.anucene.2019.107032 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
2018). The layers can prevent the fluid from mixing evenly. Combustible gas can be formed by the mixture of high concentration hydrogen, air and steam in the local containment. The combustibility of the gas is related to temperature, pressure, composition and whether there is an ignition source. Once the mixed gas is ignited, there is a risk of flame acceleration (FA) or even deflagration to detonation transition (DDT) (Gamezo et al., 2007). As the last barrier to contain radioactive materials, the integrity of the containment is extremely important. The design pressure of the containment is 59 psi, lower than pressure of an explosion of combustible gas or steam. (Westinghouse, 2008) Once the hydrogen is ignited and develops into rapid turbulent combustion or explosion, it will create a peak pressure that could threaten the integrity of the containment. Further, the peak pressure may lead to a severe release of radioactive materials into the surroundings. At present, we can ignite local hydrogen to make it diffuse and combust smoothly as a hydrogen mitigation measure for local space. (Xiao and Zhou, 2006) In large scale commercial PWR, igniters and the passive autocatalytic recombiners (PARs) are used to mitigate the hydrogen risk (Chen, 2014). But it is always a complicated problem to determine the position and starting time of the igniters (Huang and Yang, 2011). In addition, steam also can affect the combustion of hydrogen. By adding steam, the maximum pressure and temperature caused by combustion will decrease. It means that steam can inhibit the combustion of hydrogen (Ma et al., 2016; Wang et al., 2016).
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X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
Considering that hydrogen is prone to generate accumulation and form layers in PZR compartment, (Li, 2018) We boldly set a steam injection device in PZR compartment, to study the feasibility of reducing hydrogen risk by injecting steam.
2. Modeling and accident sequence The accident researched is LB-LOCA of cold leg in AP1000, with gravity injection failure. The equivalent diameter of break is 350 mm in 1# Steam Generation (SG) compartment. The AP1000 computational mesh of GASFLOW model is shown as Fig. 1. In SG compartment and PZR compartment, the grid is densely distributed. The containment model is shown as Fig. 2, containment node is shown as Fig. 3.
Fig. 2. AP1000 containment model.
Fig. 3. System model diagram.
As shown in Fig. 3, the system represented by each node is as follows: node 1 and 2: the 1#, 2# SG compartment; node 3: the Core Makeup Tank (CMT) compartment; node 4: the reactor compartment; node 5: the In Containment Refueling Water Storage Tank (IRWST) compartment; node 6: the top of the containment; node 7 and node 8: the two compartments of Passive Core Cooling System(PXS); node 9: the chemical and volume control system (CVS) compartment; node 10, 11 and 12: the flow channel of convective heat transfer in the Passive Containment Cooling System (PCCS).
Fig. 1. The AP1000 computational mesh of GASFLOW model.
The assumption of accident is given in Table 1, we use the lumped parameter method to simulate the accident. The accident sequence is shown as Table 2. When the pipe, in the1# SG
X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
3
Table 1 Sequence of events hypothesis. Initial event
The cold section breaks and the gravity injection fails
ADS
Y Y N Y Y N Y
1 2/3 4
CMT ACC IRWST PCCS
Table 2 Events hypothesis. Time (second)
Events hypothesis
0 20.9 26.1 841.1 945.1 1065.1 1185.1 1818.7 1937.7 2088.9 2860.8 6400
The LOCA occurred Reactor shutdown CMT start CMT low-1 water level ADS-1 start ADS-2 start ADS-3 start CMT low-2 water level ADS-4 start (failure) The ACC empty The core starts exposing Calculate the end
compartment on the IRWST side, is broken, a large amount of coolant is released into the containment space from the break. At 20.9 s after the break, the main feed water is isolated and the CMT isolation valve is quickly opened to start safe injection. 3. Hydrogen and steam source terms Hydrogen generation rate and steam generation rate is shown as Fig. 4. The calculation shows that, the amount of steam released from 0 s to 3338 s reaches 0.9143 of the steam released total amount. After the moment of 3338 s, as the core temperature rises, the reaction of core structure metal (Zr, Fe, Cr) with steam, the erosion reaction, and water radiation decomposition can produce a large amount of hydrogen. Then, because the zircon cladding generates dense oxide films on the surface reducing the contact area between zirconium and water, hydrogen production will drop sharp. The hydrogen generation time is mainly concentrated from 4000 s to 5000 s, and the maximum generating rate can reach 0.47 kg/s. So we set 3500 s as zero computing time and stop the computation at 6500 s. The total computing time is 3000 s.
Fig. 5. The position of steam injection device.
During the hydrogen generation stage, the amount of steam released from the break is small and mainly concentrated in the period before hydrogen generation. The steam generated by the accident will flow and mix randomly. It will not accumulated in the RZR compartment. So we set a steam injection device at the bottom of the PZR compartment, as shown in Fig. 5. It starts at the 980 s after the zero computing time, and it ends at the 1040 s. Injection flow rate is 1000 g/s. Steam jet flow direction is vertical. Because this time period is in the middle of the hydrogen concentration generation. 4. Calculation and analysis GASFLOW, a three dimensional computational fluid dynamics code, is used to analyze the hydrogen behavior and the mitigation effect of steam. GASFLOW is a widely used numerical tool to analyze the hydrogen behavior in the containment. Its recent development includes the advanced turbulence model (Zhang et al., 2017; Zhang et al., 2018; Zhang et al., 2018), high-performance pressure equation solver (Zhang et al., 2017) and uncertainty quantification analysis (Zhang et al., 2018). In our previous work, we use GASLFOW to analyze the performance of different hydrogen risk mitigation measures, including the inert gas injected, igniter and recombiner (Li, 2018; Chen, 2014). In order to analyze the factors affecting hydrogen risk in containment, we assume three calculation conditions: Case 1) only injecting steam into the containment; Case 2) only starting the hydrogen PARs; Case 3) both injecting steam into the containment and starting the hydrogen PARs. 4.1. The PZR compartment
Fig. 4. Rate of hydrogen and steam release.
Because hydrogen forms layers or generates accumulation at the PZR compartment, we select point A and point B to compare data in the PZR compartment. Point A is at the bottom and point B is at the middle of the PZR compartment, as shown in Fig. 6. Both of them stick to the wall. This is chosen to compare hydrogen concentrations in PZR compartments at different heights and locations. Their mole fraction of hydrogen is shown as Figs. 7 and 8. Due to the steam injection time, the time of image analysis also started at 980s. It can be seen from the Figs. 7 and 8 that steam injection can effectively reduce the mole fraction of hydrogen in the early stage. The concentration of hydrogen at point A is higher than that at point B. There are two reasons for the above results.
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X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
Fig. 9. The position of point C and point D. Fig. 6. The position of point A and point B.
Fig. 10. The mole fraction of hydrogen at PZR point C. Fig. 7. The mole fraction of hydrogen at PZR point A.
Fig. 11. The mole fraction of hydrogen at PZR point D. Fig. 8. The mole fraction of hydrogen at PZR point B.
First, caused by large density difference between the hydrogen jet fluid and mixed gas in the upper PZR space, strong shear movement occurs in the period of large amounts hydrogen injection. Due to the effect of strong shear movement, the fluid with high concentration of hydrogen flows down. So the hydrogen easily
accumulates and stratifies in the bottom of PZR compartment. The other reason is that the density of steam is higher than that of H2. Under the action of gravity, steam diffuses into the lower space. Hydrogen diffusion will be hindered. In the PZR compartment, PARs have no obvious effect in the early stage, but they have a continuous effect in the late stage. In
X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
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Fig. 13. The position of point E.
Fig. 14. The mole fraction of hydrogen in containment dome.
Fig. 12. The two-dimensional diagram of the mole fraction of hydrogen.
Fig. 15. The mole fraction of hydrogen in containment dome (case 2and case 3).
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X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
order to focus on the comparison of the effect of steam injection, we also select point C and point D as shown in Fig. 9. Point C is at the bottom and point D is at the middle of the PZR compartment. This is chosen to contrast with point A and point B. The two points are used to compare hydrogen risk in case 2 with it in case 3, as shown in Figs. 10 and 11. From the Figs. 10 and 11, we can see Case 2 and Case 3 coincide at a later stage. This is the result of the long-term effect of PARs. PARs can reduce the hydrogen concentration, but it takes longer. In the face of local high concentrations of hydrogen, they don’t work right away. The combustion product of hydrogen is steam. After the injection of steam, the steam content in the PZR compartment increases, and the hydrogen content decreases. So the ratio of hydrogen to steam also decreases. This means that the flame acceleration factor is weakened, which reduces the possibility of flame acceleration. The PZR longitudinal section is chosen to show the twodimensional diagram which is about mole fraction of hydrogen, as shown in Fig. 12. Fig. 12(a) is case 3 and Fig. 12(b) is case 2. The moment selected is 1000 s. It can be seen the effect of injecting steam more visually. The effect of steam is further confirmed. 4.2. The upper space To explore the influence of steam injection on the overall containment mole fraction of hydrogen, we also select a point E in the containment dome, as shown in Fig. 13. It’s selected because it’s above PZR compartment. The mole fraction of hydrogen is shown as Fig. 14. From Fig. 14, steam injection is useful in containment dome. But the effect of steam injection is weaker than that in local space. The volume of the upper containment space is large, it can withstand hydrogen very well. Two conditions, Case 2 and Case 3, are shown as Fig. 15. The maximum mole fraction of hydrogen is under 0.0045 in both cases respectively. This value is much lower than the hydrogen peak in the local compartment. In contrast, the effect of PARs is more significant. Therefore, the upper space is more dependent on the PARs. 5. Conclusions Facing LB-LOCA of cold leg in 1#SG compartment and gravity injection failure, AP1000 nuclear power plant will generate severe hydrogen risk. This paper studies the mitigation effect of steam injection on hydrogen risk. The main conclusions are as follows: (1) In large scale commercial PWR, injecting steam into the PZR compartment can reduce the risk of hydrogen accumulation observably at the local space.
(2) Comparing Case 1 and Case 3, we find that PARs have a strong ability to mitigate hydrogen risk in the later stage. Comparing Case 2 and Case 3, we see steam injection can be quickly reduce hydrogen concentration. (3) The method combining steam injection and PARs is an effective measure.
Acknowledgements This research is supported by the Fundamental Research Funds for the Central Universities (2018MS041), the National Natural Science Foundation of China (No. 51306057, 11705058, U1867218). References Chen, Long, 2014. Research on Hydrogen Risk and Hydrogen Mitigation Measurement of Containment during Severe Accidents for AP1000. North China Electric Power University (in Chinese). Gamezo, V.N., Ogawa, T., Oran, E.S., 2007. Numerical simulations of flame propagation and DDT in obstructed channels filled with hydrogen–air mixture. Proceed. Combust. Instit 31 (2), 2463–2471. Huang, Xingguan, Yang, Yanhua, 2011. Analysis of characters and efficiency for ignitor in hydrogen mitigation system. Atom. Energy Sci. Technol. 45 (06), 716– 721 (in Chinese). Li, Xiaobo, 2018. Research on Hydrogen Behavior in Large Containment under Loss of Coolant Accident. North China Electric Power University (in Chinese). Ma, Fei, Tong, Lili, Cao, Xuewu, Luo, Guangnan, 2016. Investigation of steam effect on hydrogen-air combustion by numerical simulation. Atomic Energy Sci. Technol. 50 (11), 1972–1978 (in Chinese). Paladino, D., Andreani, M., Zboray, R., et al., 2012. Toward a CFD-grade database addressing LWR containment phenomena. Nucl. Eng. Des. 253 (12), 331–342. Reventós, F., Freixa, J., Batet, L., et al., 2008. An analytical comparative exercise on the OECD-SETH PKL E2.2 experiment. Nucl. Eng. Des. 238 (4), 1146–1154. Rosa, J.C.D.L., Escrivá, A., Herranz, L.E., et al., 2009. Review on condensation on the containment structures. Prog. Nucl. Energy 51 (1), 32–66. Wang, Ying, Li, Yong, Zan, Yuanfeng, Tang, Yueming, Zheng, Hu.a., Xie, Shijie, 2016. Experimental analysis of steam effect on hydrogen combustion in severe accidents. Nucl. Power Eng. 37 (S2), 125–128 (in Chinese). Westinghouse, L.L.C., 2008. Westinghouse AP1000 Design Control Document. Westinghouse Electric Company LLC. Xiao, Jianjun, Zhou, Zhiwei, 2006. Yingqing Jing. Preliminary study on mitigation measures for hydrogen explosion in severe accidents. Nucl. Power Eng. 02. 64– 67+77 (in Chinese). Zhang, Han, Li, Yabing, Xiao, Jianjun, Jordan, Thomas, 2017. Large eddy simulation of turbulent flow using the parallel computational fluid dynamics code gasflowmessage passing interface. Nucl. Eng. Technol. Zhang, Han, Li, Yabing, Xiao, Jianjun, Jordan, Thomas, 2018. Detached Eddy simulation of hydrogen turbulent dispersion in nuclear containment compartment using GASFLOW-MPI. Int. J. Hydrogen Energy 43 (29). Zhang, Han, Li, Yabing, Xiao, Jianjun, Jordan, Thomas, 2018. Large eddy simulations of the all-speed turbulent jet flow using 3-D CFD code GASFLOW-MPI. Nucl. Eng. Des. 328. Zhang, Han, Li, Yabing, Xiao, Jianjun, Jordan, Thomas, 2018. Uncertainty analysis of condensation heat transfer benchmark using CFD code GASFLOW-MPI. Nucl. Eng. Des. 340.
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
The measure on mitigating hydrogen risk during LOCA accident in nuclear power plant Xiangyuan Meng a, Xuefeng Lyu a,b,⇑, Boxue Wang a, Shuai Liu a, Yu Yu a, Zhangpeng Guo a a b
Beijing Key Laboratory of Passive Nuclear Safety Technology, School of Nuclear Science and Engineering, North China Electric Power University, Beijing 102206, China Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 21 June 2019 Received in revised form 5 September 2019 Accepted 6 September 2019
Keywords: Hydrogen risk Steam LOCA
a b s t r a c t Since there is a large empty space in PWR containment which has strong pressure bearing capability, it’s generally considered that there is no overall risk of hydrogen. However, the complex internal structure makes hydrogen easy to accumulate, burn or even explode in local compartment. Therefore, when LBLOCA with gravity injection failure happens, there may be a more serious risk of local hydrogen. GASFLOW, a three-dimensional computational fluid dynamics code, is used to study the measure on mitigating hydrogen risk. During the simulation, a steam injection device is arranged in the PZR compartment. When the accident occurs, steam is injected into the PZR compartment during the pre-set stages. By comparing the hydrogen risks in different situations, it could be found that steam injection in local space can reduce the hydrogen risk locally. The method combining steam injection device with PARs is an effective measure to mitigate the risk of hydrogen. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The nuclear safety issue has been attracted extensive attention from the academia and industry community of nuclear engineering. It covers a wide range of research topics, including both traditional physical and thermal aspects of the reactor, as well as core structure and material mechanics. Since the TMI-2 accident occurred in the United States, relevant organizations and institutions of OECD member countries have continuously carried out in-depth studies in the field of nuclear safety (Paladino et al., 2012; Reventós et al., 2008). If a LOCA accident occurs at large scale commercial PWR and the core isn’t sufficiently cooled, the zirconium cladding may react with water or steam at high temperatures. Large amounts of hydrogen will be produced. The hydrogen may be released into the containment through the break at primary loop. Regardless of the internal spray, the fluid near the break is driven by pressure difference. In other areas, fluid transport relies on the heat exchange of steam and air, steam condensation, etc. (Rosa et al., 2009). Hydrogen tends to form layers and generate accumulation in where the break located and over it. When the break occurs in 1# Steam Generator (SG) compartment, the pressurizer (PZR) compartment will also generate accumulation and form layers (Li,
⇑ Corresponding author. E-mail address: [email protected] (X. Lyu). https://doi.org/10.1016/j.anucene.2019.107032 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
2018). The layers can prevent the fluid from mixing evenly. Combustible gas can be formed by the mixture of high concentration hydrogen, air and steam in the local containment. The combustibility of the gas is related to temperature, pressure, composition and whether there is an ignition source. Once the mixed gas is ignited, there is a risk of flame acceleration (FA) or even deflagration to detonation transition (DDT) (Gamezo et al., 2007). As the last barrier to contain radioactive materials, the integrity of the containment is extremely important. The design pressure of the containment is 59 psi, lower than pressure of an explosion of combustible gas or steam. (Westinghouse, 2008) Once the hydrogen is ignited and develops into rapid turbulent combustion or explosion, it will create a peak pressure that could threaten the integrity of the containment. Further, the peak pressure may lead to a severe release of radioactive materials into the surroundings. At present, we can ignite local hydrogen to make it diffuse and combust smoothly as a hydrogen mitigation measure for local space. (Xiao and Zhou, 2006) In large scale commercial PWR, igniters and the passive autocatalytic recombiners (PARs) are used to mitigate the hydrogen risk (Chen, 2014). But it is always a complicated problem to determine the position and starting time of the igniters (Huang and Yang, 2011). In addition, steam also can affect the combustion of hydrogen. By adding steam, the maximum pressure and temperature caused by combustion will decrease. It means that steam can inhibit the combustion of hydrogen (Ma et al., 2016; Wang et al., 2016).
2
X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
Considering that hydrogen is prone to generate accumulation and form layers in PZR compartment, (Li, 2018) We boldly set a steam injection device in PZR compartment, to study the feasibility of reducing hydrogen risk by injecting steam.
2. Modeling and accident sequence The accident researched is LB-LOCA of cold leg in AP1000, with gravity injection failure. The equivalent diameter of break is 350 mm in 1# Steam Generation (SG) compartment. The AP1000 computational mesh of GASFLOW model is shown as Fig. 1. In SG compartment and PZR compartment, the grid is densely distributed. The containment model is shown as Fig. 2, containment node is shown as Fig. 3.
Fig. 2. AP1000 containment model.
Fig. 3. System model diagram.
As shown in Fig. 3, the system represented by each node is as follows: node 1 and 2: the 1#, 2# SG compartment; node 3: the Core Makeup Tank (CMT) compartment; node 4: the reactor compartment; node 5: the In Containment Refueling Water Storage Tank (IRWST) compartment; node 6: the top of the containment; node 7 and node 8: the two compartments of Passive Core Cooling System(PXS); node 9: the chemical and volume control system (CVS) compartment; node 10, 11 and 12: the flow channel of convective heat transfer in the Passive Containment Cooling System (PCCS).
Fig. 1. The AP1000 computational mesh of GASFLOW model.
The assumption of accident is given in Table 1, we use the lumped parameter method to simulate the accident. The accident sequence is shown as Table 2. When the pipe, in the1# SG
X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
3
Table 1 Sequence of events hypothesis. Initial event
The cold section breaks and the gravity injection fails
ADS
Y Y N Y Y N Y
1 2/3 4
CMT ACC IRWST PCCS
Table 2 Events hypothesis. Time (second)
Events hypothesis
0 20.9 26.1 841.1 945.1 1065.1 1185.1 1818.7 1937.7 2088.9 2860.8 6400
The LOCA occurred Reactor shutdown CMT start CMT low-1 water level ADS-1 start ADS-2 start ADS-3 start CMT low-2 water level ADS-4 start (failure) The ACC empty The core starts exposing Calculate the end
compartment on the IRWST side, is broken, a large amount of coolant is released into the containment space from the break. At 20.9 s after the break, the main feed water is isolated and the CMT isolation valve is quickly opened to start safe injection. 3. Hydrogen and steam source terms Hydrogen generation rate and steam generation rate is shown as Fig. 4. The calculation shows that, the amount of steam released from 0 s to 3338 s reaches 0.9143 of the steam released total amount. After the moment of 3338 s, as the core temperature rises, the reaction of core structure metal (Zr, Fe, Cr) with steam, the erosion reaction, and water radiation decomposition can produce a large amount of hydrogen. Then, because the zircon cladding generates dense oxide films on the surface reducing the contact area between zirconium and water, hydrogen production will drop sharp. The hydrogen generation time is mainly concentrated from 4000 s to 5000 s, and the maximum generating rate can reach 0.47 kg/s. So we set 3500 s as zero computing time and stop the computation at 6500 s. The total computing time is 3000 s.
Fig. 5. The position of steam injection device.
During the hydrogen generation stage, the amount of steam released from the break is small and mainly concentrated in the period before hydrogen generation. The steam generated by the accident will flow and mix randomly. It will not accumulated in the RZR compartment. So we set a steam injection device at the bottom of the PZR compartment, as shown in Fig. 5. It starts at the 980 s after the zero computing time, and it ends at the 1040 s. Injection flow rate is 1000 g/s. Steam jet flow direction is vertical. Because this time period is in the middle of the hydrogen concentration generation. 4. Calculation and analysis GASFLOW, a three dimensional computational fluid dynamics code, is used to analyze the hydrogen behavior and the mitigation effect of steam. GASFLOW is a widely used numerical tool to analyze the hydrogen behavior in the containment. Its recent development includes the advanced turbulence model (Zhang et al., 2017; Zhang et al., 2018; Zhang et al., 2018), high-performance pressure equation solver (Zhang et al., 2017) and uncertainty quantification analysis (Zhang et al., 2018). In our previous work, we use GASLFOW to analyze the performance of different hydrogen risk mitigation measures, including the inert gas injected, igniter and recombiner (Li, 2018; Chen, 2014). In order to analyze the factors affecting hydrogen risk in containment, we assume three calculation conditions: Case 1) only injecting steam into the containment; Case 2) only starting the hydrogen PARs; Case 3) both injecting steam into the containment and starting the hydrogen PARs. 4.1. The PZR compartment
Fig. 4. Rate of hydrogen and steam release.
Because hydrogen forms layers or generates accumulation at the PZR compartment, we select point A and point B to compare data in the PZR compartment. Point A is at the bottom and point B is at the middle of the PZR compartment, as shown in Fig. 6. Both of them stick to the wall. This is chosen to compare hydrogen concentrations in PZR compartments at different heights and locations. Their mole fraction of hydrogen is shown as Figs. 7 and 8. Due to the steam injection time, the time of image analysis also started at 980s. It can be seen from the Figs. 7 and 8 that steam injection can effectively reduce the mole fraction of hydrogen in the early stage. The concentration of hydrogen at point A is higher than that at point B. There are two reasons for the above results.
4
X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
Fig. 9. The position of point C and point D. Fig. 6. The position of point A and point B.
Fig. 10. The mole fraction of hydrogen at PZR point C. Fig. 7. The mole fraction of hydrogen at PZR point A.
Fig. 11. The mole fraction of hydrogen at PZR point D. Fig. 8. The mole fraction of hydrogen at PZR point B.
First, caused by large density difference between the hydrogen jet fluid and mixed gas in the upper PZR space, strong shear movement occurs in the period of large amounts hydrogen injection. Due to the effect of strong shear movement, the fluid with high concentration of hydrogen flows down. So the hydrogen easily
accumulates and stratifies in the bottom of PZR compartment. The other reason is that the density of steam is higher than that of H2. Under the action of gravity, steam diffuses into the lower space. Hydrogen diffusion will be hindered. In the PZR compartment, PARs have no obvious effect in the early stage, but they have a continuous effect in the late stage. In
X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
5
Fig. 13. The position of point E.
Fig. 14. The mole fraction of hydrogen in containment dome.
Fig. 12. The two-dimensional diagram of the mole fraction of hydrogen.
Fig. 15. The mole fraction of hydrogen in containment dome (case 2and case 3).
6
X. Meng et al. / Annals of Nuclear Energy 136 (2020) 107032
order to focus on the comparison of the effect of steam injection, we also select point C and point D as shown in Fig. 9. Point C is at the bottom and point D is at the middle of the PZR compartment. This is chosen to contrast with point A and point B. The two points are used to compare hydrogen risk in case 2 with it in case 3, as shown in Figs. 10 and 11. From the Figs. 10 and 11, we can see Case 2 and Case 3 coincide at a later stage. This is the result of the long-term effect of PARs. PARs can reduce the hydrogen concentration, but it takes longer. In the face of local high concentrations of hydrogen, they don’t work right away. The combustion product of hydrogen is steam. After the injection of steam, the steam content in the PZR compartment increases, and the hydrogen content decreases. So the ratio of hydrogen to steam also decreases. This means that the flame acceleration factor is weakened, which reduces the possibility of flame acceleration. The PZR longitudinal section is chosen to show the twodimensional diagram which is about mole fraction of hydrogen, as shown in Fig. 12. Fig. 12(a) is case 3 and Fig. 12(b) is case 2. The moment selected is 1000 s. It can be seen the effect of injecting steam more visually. The effect of steam is further confirmed. 4.2. The upper space To explore the influence of steam injection on the overall containment mole fraction of hydrogen, we also select a point E in the containment dome, as shown in Fig. 13. It’s selected because it’s above PZR compartment. The mole fraction of hydrogen is shown as Fig. 14. From Fig. 14, steam injection is useful in containment dome. But the effect of steam injection is weaker than that in local space. The volume of the upper containment space is large, it can withstand hydrogen very well. Two conditions, Case 2 and Case 3, are shown as Fig. 15. The maximum mole fraction of hydrogen is under 0.0045 in both cases respectively. This value is much lower than the hydrogen peak in the local compartment. In contrast, the effect of PARs is more significant. Therefore, the upper space is more dependent on the PARs. 5. Conclusions Facing LB-LOCA of cold leg in 1#SG compartment and gravity injection failure, AP1000 nuclear power plant will generate severe hydrogen risk. This paper studies the mitigation effect of steam injection on hydrogen risk. The main conclusions are as follows: (1) In large scale commercial PWR, injecting steam into the PZR compartment can reduce the risk of hydrogen accumulation observably at the local space.
(2) Comparing Case 1 and Case 3, we find that PARs have a strong ability to mitigate hydrogen risk in the later stage. Comparing Case 2 and Case 3, we see steam injection can be quickly reduce hydrogen concentration. (3) The method combining steam injection and PARs is an effective measure.
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