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Development of a mitigation system against hydrogen-air deflagrations in nuclear power plants
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Development of a mitigation system against hydrogen-air deflagrations in nuclear power plants
T
Hiroyasu Saitoha,∗, Teruhito Otsukab, Norihiko Yoshikawac, Nozomu Kannod, Seiji Takanashib, Yousuke Oozawac, Masahiro Hiratac, Masayuki Takeshitac, Kenji Sakuragic, Sayuri Kuriharaa, Yuichiro Tsunashimaa, Naohito Aokia, Kento Tanakaa a
Department of Engineering Science and Mechanics, Shibaura Institute of Technology, Toyosu 3-7-5, Koto-ku, Tokyo, 135-8548, Japan National Institute of Occupational Safety and Health (JNIOSH), Japan, 1-4-6, Umezono, Kiyose, Tokyo, 204-0024, Japan c Department of Micro-nano Systems Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8603, Japan d Department of Vehicle and Mechanical Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, Aichi, 468-8502, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Mitigation Hydrogen explosion Nuclear power plant Severe accident Deflagration Flame arrester
A novel mitigation system against hydrogen-air deflagrations in nuclear power plant buildings is proposed and developed through a series of field experiments using explosion vessels of different volume sizes. The mitigation system is installed on the outer surface of the vessels, and it comprises flame arrester and explosion air bag. The flame arrester is made by stacking 10–20 sheets of fine-mesh wire screens, and the air bag is connected for holding explosion gas. The successful mitigation mechanism is the sequence of pressure-rise reduction by the air bag expansion, flame quenching by the flame arrester, and the slow burning of the gas mixture sucked from the air bag back into the vessel due to the negative pressure caused by the rapid condensation of water vapor inside the vessel. Necessary conditions for the successful mitigation system are discussed, and the practical unit size of flame arrester sheet is recommended.
1. Introduction
systems are additionally installed in nuclear power plants, no feasible mitigation means has not been developed yet. Any preventive means cannot sufficiently reduce the inherent possibility of explosions once sensitive hydrogen-air cloud is formed by the nuclear reactor meltdown. The emergency responding measures to the severe accident triggered by major earthquake are extremely difficult under the continuous situation of earthquake aftershocks. The Fukushima accident reports have indeed recognized multiple trivial mistakes in the emergency operations. These considerations clearly lead to the necessity of developing new feasible mitigation means beyond the existing preventive means. In the present paper, a new mitigation system is proposed on the assumption of explosion occurrence, and the mitigation mechanism is demonstrated through the series of field experiments.
The disastrous accident in the Fukushima nuclear power plants in 2011 (JAEA, 2016; National Research Council, 2014) has strongly indicated that the existing accident preventive means against hydrogen explosions are insufficient. The records of the three severe accidents at Three Mile Island in 1979 (Nuclear Safety Analysis Center, 1980), Chernobyl in 1986 (World Nuclear Association, 2018), and Fukushima have shown that these accidental explosions are hydrogen-air deflagrations of relatively large volume vapor cloud inside the power plant buildings. Although the accident record at Hamaoka nuclear power plant in Japan showed the occurrence of detonations, the detonation accident occurred locally in pipes. Hydrogen-oxygen mixture is formed through the radiation decomposition of water in pipes at this accident (Du et al., 2018). It is commonly recognized through many field explosion experiments that the transition from deflagration to detonation does not occur in open space or large scale volume like nuclear power plant buildings. Therefore, the practical mitigation system should be designed against deflagration accidents. Although some preventive means including filtered ventilation
∗
2. Basic properties of hydrogen-air deflagrations Hydrogen-air mixture is one of the most sensitive explosive gases. The minimum ignition energy is 17 μJ in the stoichiometric mixture, i.e. hydrogen of 29.6 vol% in the mixture. In comparison with
Corresponding author. E-mail address: [email protected] (H. Saitoh).
https://doi.org/10.1016/j.jlp.2019.03.011 Received 9 January 2019; Received in revised form 29 March 2019; Accepted 29 March 2019 Available online 31 March 2019 0950-4230/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 4. Overpressure record of 43 vol% hydrogen-air explosion using a small size mitigation system.
Fig. 1. Conceptual diagram of the mitigation system against hydrogen explosions.
Fig. 2. Overpressure record of the stoichiometric hydrogen-air explosion using 10 cm long 100A pipe (inner diameter: 105 mm, volume: 870 cm3). Fig. 5. Experimental setup of the mitigation system for 1 m3 vessel.
Fig. 3. Small scale model experiment.
Fig. 6. Flame arrester and the holding aluminum frame.
hydrocarbon-air mixtures, e.g. 100 μJ of methane-air mixture, the energy is one order smaller. The flammability range of 4–75 vol% hydrogen volume fractions is also wide in comparison with 5–15 vol% methane fractions. The laminar burning velocity of 43 vol% hydrogenair mixture is about 3 m/s which is about 10 fold of stoichiometric methane-air mixture. The behaviors of hydrogen-air deflagrations in open space are obtained e.g. in the report of the field experiments using big latex balloons (Otsuka et al., 2007). The important experimental property for flame arrester design is the Maximum Experimental Safe Gap, which is usually abbreviated MESG. The value of the parameter is half of the flame quenching thickness. Flames quench through a gap distance below the critical value. The MESG of hydrogen-air mixture is about 0.25–0.3 mm. This MESG value corresponds to the wire screens of 50–70 mesh per inch.
We actually confirmed the quenching performance of the mesh arrester used in our mitigation system by preliminary verification (Yoshikawa et al., 2015) according to ISO standard (ISO, 2008). 3. Mechanism of the mitigation system Fig. 1 shows a schematic diagram of the mitigation system. The deflagration propagates from the ignition point induces the flow of the unburned hydrogen-air mixture into the expanding air bag through the wire screen sheets of the flame arrester. Stainless steel screens of 50–70 mesh/inch are stacked in 10–20 sheets layer. The flame arrester quenches the propagating flame, and the air bag holds the unburned mixture inside. The negative pressure to the atmosphere is induced in 10
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 7. Air bag motion (29 vol% hydrogen-air mixture, cube-center ignition, successful case).
bag finally shrinks. This back-suction effect by the negative pressure saliently enhances the mitigation system. The above mitigation mechanism is verified throughout the series of experiments, and the details are shown in the following sections. The mitigation system does not additionally require any control or power systems, and this is a strong advantage over other systems. The system prevents massive dispersion of radioactive substances into vast areas of the atmosphere. The system also prevents secondary explosions by very lean oxygen concentrations inside the vessel. The novelty of the present mitigation system is the effective concurrent function of flame arrester and explosion gas capturing (Yoshikawa and Otsuka, 2015). Although many types of flame arresters have already been developed and commercially available, these conventional systems do not prevent the dispersion of explosion gases into the atmosphere. The radioactive substance particulates are submicron size and particularly crucial in the severe accidents of nuclear power plants, and some of the explosion accidents in chemical industries also include similar dispersion of toxics into the atmosphere. The rupture disc venting systems furthermore inherently possess the possibility of the secondary explosion of exhaust gas mixture outside the first explosion location. In addition to the above defects, the conventional flame arrester systems are designed mainly for piping systems and small volume vessels, and the cost for large scale mitigation system becomes high. The system cost is important factor in developing the large scale systems. The present mitigation system uses inexpensive flame arrester units made by stacking fine-mesh wire screens. The actual large size mitigation systems are installed at several different locations on the outer surface of the nuclear power plant building for responding to the ignition source at any location. The actual mitigation system, for example of 40 m cubic building, may comprise 1000–1500 m2 flame arrester area and 100,000 m3 total volume of air bags. The installation of 4,000–6,000 flame arrester units of 50 cm square on the building surface is feasible. Furthermore, the recent aerospace technology of high altitude balloons can easily provide a single balloon of 30,000 m3 (JAXA, 2017).
Fig. 8. Overpressure record measured inside the vessel (29 vol% hydrogen-air, cube-center ignition, successful case).
Fig. 9. Back-suction of air bag mixture (29 vol% hydrogen-air mixture, cubecenter ignition, successful case).
the explosion vessel by the rapid condensation of water vapor. Similar phenomena are also confirmed by Kuznetsov et al. (2015). For example, the pressure of cooled down combustion products of stoichiometric hydrogen-air mixture is 56 vol% of the initial gas mixture at room temperature. The unburned gas mixture once held inside the air bag is sucked back into the vessel and burns slowly. This secondary burning inside the vessel does not generate noticeable pressure rise, and the air
4. Experiments and results 4.1. Laboratory scale experiments Fig. 2 shows the explosion pressure record of the stoichiometric hydrogen-air mixture in a closed vessel initially at the atmospheric pressure. The pressure records in the vessel are obtained using pressure 11
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 10. Blow off of the air bag (29 vol% hydrogen-air mixture, ignition at the bottom surface center, failure case).
Fig. 11. Overpressure record measured inside the vessel (29 vol% hydrogen-air, ignition at the bottom surface center, failure case).
transducers (Kistler, 404JA2, 4045A2, 4049A5SP22 and Kyowa, PGM10KH) in all the experiments in this paper. The peak overpressure is about 7 fold of the initial absolute pressure, and the pressure decays rapidly by heat loss to the vessel surface. The pressure further decreases below the atmospheric pressure by the rapid water vapor condensation. The absolute pressure tolerance of 8 atm for an actual nuclear power plant building, e.g. 40 m cubic reinforced concrete building, requires the wall thickness of about 15 m through a simple estimate. Thus the explosion-resistant building construction is impractical. Fig. 3 shows a preliminary experimental set up for the mitigation system. The same vessel in Fig. 2 is connected to a flame arrester and an air bag. The flame arrester is a stack of 10 sheets of mesh screen of 50 per inch, 0.23 mm wire diameter, and 0.28 mm mesh size. A pair of cross mark plates is used to prevent the bending of the sheets. An air bag of 1.8 L reinforced nylon is connected to the flame arrester. Fig. 4 shows pressure records of 43 vol% hydrogen-air (equivalence ratio, 1.8) explosions. The ignition occurs at the left side of the vessel, and the pressure records of the vessel and the air bag show a salient pressure reduction in comparison with the closed vessel explosion of
Fig. 12. 8 m3 concrete vessel and frame panel for flame arrester.
Fig. 2. Although the flame quenching mechanism by wire screens is an interesting problem, the complexity of the phenomena is formidable. The mechanism involves the complicated interactions among turbulence, chemical reactions, heat transfer, and surface catalytic effects. These controlling factors are not sufficiently elucidated, and the computational analyses contain a lot of uncertainties. The experimental investigations are also presently limited to the determination of critical 12
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 13. Flame arrester modules installed on the frame panel.
Fig. 15. Overpressure record inside the explosion vessel (26 vol% hydrogen-air mixture, ignition at the cube center, successful case).
conditions dividing the quenching and non-quenching regions, because of the knowledge lack of the complex interaction mechanism. Therefore, aiming at establishing a practical mitigation system, the present research takes a pragmatic approach by determining the critical conditions for successful quenching.
explosion. The pressure records at the four locations indicate that the explosion pressure is approximately uniform. This uniform distribution of instantaneous pressure inside the vessel is common to all the experimental results described in this paper. The pressure record in Fig. 8 shows that the first peak of the overpressure is 23 kPa at 40 ms after the ignition. The explosion pressure is saliently reduced in comparison with the constant volume explosion of 700 kPa overpressure. The hydrogen concentration inside the vessel after the explosion was 1 vol%, which indicates the slow burning of the mixture due to the back-suction effect. Figs. 10 and 11 correspond to a failure case of 29 vol% hydrogen-air mixture ignited at the center of the vessel bottom surface. These figures show the air bag blow off and the overpressure of 68 kPa at 75 ms. The second peak pressure about 100 ms corresponds to the blow off of the air bag. The close up video records have shown that the flame arrester surface moves 15–20 mm upward by a strong surface integral of explosion pressure. The flame passes through the flame arrester. The setup of 8 m3 vessel experiments are shown in Figs. 12 and 13. The vessel is 2 m cube, and the resistant pressure of the concrete is 3 atm. The flame arrester is made by stacking 5 sheets of 40 mesh/inch, 20 sheets of 70 mesh/inch, and 5 sheets of 40 mesh/inch. Two flame arrester panels are installed on the pair of opposite surfaces. The total volume of two air bags is 34.6 m3, i.e. 17.6 m3 × 2. The test gas mixtures are ignited at the center of the vessel by electrical spark. Figs. 14 and 15 show a successful case of 26 vol% hydrogen-air explosion mitigation. The air bags expand and reduce the peak explosion pressure down to about 22 kPa. Figs. 16 and 17 show a failure case of 30 vol% hydrogen-air explosion mitigation. The left air bag blows off by the flame passing through the upper part of the flame arrester, although the right mitigation system is successful as shown in Fig. 16. The right photograph is
4.2. Large scale field experiments Based on the results of the preliminary small size experiments, successful conditions for the mitigation mechanism are tested through large size field experiments using three different volume size vessels of 0.125 m3, 1 m3 and 8 m3. The results of the 0.125 m3 vessel have been already reported (Yoshikawa et al., 2015), the present paper starts the results of 1 m3 steel vessel. Fig. 5 shows a photograph of the experimental setup. The vessel is a 1 m steel cube with a mitigation system on the top surface. The flame arrester is a stack of 20 sheets of 1 m square screen of 70 mesh/inch (wire diameter 0.14 mm, mesh size 0.22 mm, and opening ratio 37.3%), and top and bottom surfaces are pressed by aluminum holding frames shown in Fig. 6. The air bag is a 1 m3 silicon-coated reinforced nylon bag. Test gases are prepared by flow mixing of bomb hydrogen gas and air supply from a compressor, and the expected hydrogen concentrations are obtained using a portable gas meter (Riken, GX-8000). A high-speed camera (Photron, FASTCAM SA1.1) and a compact digital camera record the motion of air bags, and the inner pressure record are obtained at four different locations on the inner surface of the vessel. Figs. 7–9 show the records of 29 vol% hydrogen-air mixture explosion ignited at the center of the vessel. The air bag motion in Fig. 7 shows the expansion and the later shrink at 660 ms due to the backsuction effect. Fig. 9 shows that the back-suction effect induces the strong stick of the air bag to the flame arrester surface after the
Fig. 14. Motion records of air bags (26 vol% hydrogen-air mixture, ignition at the cube center, successful case). 13
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 16. Motion records of air bags. (30 vol% hydrogen-air mixture, ignition at the cube center, failure case). a) Time-series images, b) Close-up view of the flame jet around t = 87 ms.
Fig. 18. Improved flame arrester design.
flame arrester. The complicated designs of flame arresters with many bolts and nuts increase the failure risk. The following field experiments concentrate on the determination of flame arrester designs, especially on manufacturing process, size, and quantity of screen sheets. The improved flame arrester is shown in Fig. 18. The unit size of flame arrester is 50 cm square of stacked 20 sheets of 60 mesh/inch screens (wire diameter, 0.14 mm, mesh size, 0.28 mm, opening ratio, 44.8%). The four sides are tightly sealed with gum tapes, and a pair of holding
Fig. 17. Overpressure record inside the explosion vessel (30 vol% hydrogen-air mixture, ignition at the cube center, failure case).
a close-up view. The pressure record also shows the failure of the mitigation system. The results of the 1 m3 and 8 m3 vessel experiments have particularly suggested the necessity of more reliable manufacturing of the 14
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 19. Motion of air bags (28 vol% hydrogen-air mixture, 2 air bags, successful case). a) Side view, b) View from the direction of the arrow indicated in the a).
28 vol% hydrogen-air mixture. The peak overpressure is reduced 28 kPa. Fig. 21 shows the experimental results for determining the critical quantity of flame arrester sheets. The left side four flame arrester units are the same as Fig. 19. On the right side vessel surface, two sets of 10 sheets are on the upper left and the lower right parts, and two sets of 9 sheets are on the upper right and the lower left parts. The photograph taken from the front show the critical quantity of sheets is 10 less than which flame quenching fails. 5. Conclusion The effective mechanism of the novel hydrogen explosion mitigation system is described and demonstrated through a series of field experiments. The system prevents the massive dispersion of radioactive substances into the atmosphere. The actual installations of the systems on different locations of the building surfaces require that the total volume of the several air bags is about 1.5–2 fold of the building inner volume. The recommended unit size of the flame arrester is 50 cm square made by stacking 15–20 sheets of 60–70 mesh/inch wirescreens. In addition to the nuclear power plant systems, the concurrent effective mitigation function of flame quenching and explosion gas capture can also be applied to other safety systems including chemical process industries, grain elevators, and fuel pipe lines.
Fig. 20. Overpressure record (28 vol% hydrogen-air mixture, two air bags, successful case).
frames is set on the both surfaces. The seal packing sheets are also inserted between the vessel body and the flame arresters. The quantity of bolts and nuts is largely reduced in comparison with the former flame arresters. The installation of flame arresters becomes easier and less time-consuming. The explosion vessel is 1 m cubic steel frame, and the four side surfaces are used for setting flame arresters or blank steel plates. Two transparent vinyl air bags of 1.5 m3 each are installed on the pair opposite sides of the vessel. The gas mixtures are ignited at the center of the cubic vessel. Figs. 19 and 20 show a successful mitigation case of two air bags for
Acknowledgement The present experimental study was conducted as one of the light water reactor hydrogen safety programs of the Japan Atomic Energy Agency, under the financial support of the Ministry of Economy, Trade 15
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 21. Critical sheet quantity for mitigation (31 vol% hydrogen-air mixture). a) side view (frame interval, 33 ms), b) front view (frame interval, 1 ms).
and Industry of Japan (METI), during the period of 2012–2016.
Nuclear Safety Analysis Center, 1980. Analysis of Three Mile Island-Unit 2 Accident, EPRI-NSAC–80-1. Electric Power Research Institute, California. National Research Council, 2014. Lessons Learned from the Fukushima Nuclear Accident for Improving Safety of U.S. Nuclear Plants. The National Academies Press, Washington, DC. Otsuka, T., Saitoh, H., Mizutani, T., Morimoto, K., Yoshikawa, N., 2007. Hazard evaluation of hydrogen-air deflagration with flame propagation velocity measurement by image velocimetry using brightness subtraction. J. Loss Prev. Process. Ind. 20, 427–432. https://doi.org/10.1016/j.jlp.2007.04.031. World Nuclear Association, 2018. Chernobyl Accident 1986. http://www.world-nuclear. org/information-library/safety-and-security/safety-of-plants/chernobyl-accident. aspx/, Accessed date: 13 February 2019. Yoshikawa, N., Otsuka, T., Saitoh, H., Nozomu, K., Oozawa, Y., Kurihara, S., Hirata, M., Sakuragi, K., Takanashi, S., 2015. Basic design and fundamental experiments of a damage reduction system against hydrogen explosions. J. Jpn. Soc. Saf. Eng. 35 (2), 122–130. (in Japanese). https://doi.org/10.18943/safety.54.2_122. Yoshikawa, N. and Otsuka, T., 2015. Japan patent No.5747019B2.
References Du, Y., Zhou, F., Ma, L., Zheng, J., Xu, C., Chen, G., 2018. Dynamic fracture response of pre-flawed elbow pipe subjected to internal hydrogen-oxygen detonation. Int. J. Hydrogen Energy 43, 19625–19635. https://doi.org/10.1016/j.ijhydene.2018.08. 211. ISO, 2008. Flame Arresters – Performance Requirements, Test Methods and Limits for Use. ISO 16852:2008(E). JAEA, 2016. Fukushima Nuclear Accident Archive. https://f-archive.jaea.go.jp/index. php?locale=eng/, Accessed date: 24 December 2018. JAXA, 2017. B17-04 Scientific Balloon Testing. http://global.jaxa.jp/projects/sas/ balloon/topics.html/, Accessed date: 13 February 2019. Kuznetsov, M., Friedrich, A., Stern, G., Kotchourko, N., Jallais, S., L'Hostis, B., 2015. Medium-scale experiments on vented hydrogen deflagration. J. Loss Prev. Process. Ind. 36, 416–428. https://doi.org/10.1016/j.jlp.2015.04.013.
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Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Development of a mitigation system against hydrogen-air deflagrations in nuclear power plants
T
Hiroyasu Saitoha,∗, Teruhito Otsukab, Norihiko Yoshikawac, Nozomu Kannod, Seiji Takanashib, Yousuke Oozawac, Masahiro Hiratac, Masayuki Takeshitac, Kenji Sakuragic, Sayuri Kuriharaa, Yuichiro Tsunashimaa, Naohito Aokia, Kento Tanakaa a
Department of Engineering Science and Mechanics, Shibaura Institute of Technology, Toyosu 3-7-5, Koto-ku, Tokyo, 135-8548, Japan National Institute of Occupational Safety and Health (JNIOSH), Japan, 1-4-6, Umezono, Kiyose, Tokyo, 204-0024, Japan c Department of Micro-nano Systems Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8603, Japan d Department of Vehicle and Mechanical Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, Aichi, 468-8502, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Mitigation Hydrogen explosion Nuclear power plant Severe accident Deflagration Flame arrester
A novel mitigation system against hydrogen-air deflagrations in nuclear power plant buildings is proposed and developed through a series of field experiments using explosion vessels of different volume sizes. The mitigation system is installed on the outer surface of the vessels, and it comprises flame arrester and explosion air bag. The flame arrester is made by stacking 10–20 sheets of fine-mesh wire screens, and the air bag is connected for holding explosion gas. The successful mitigation mechanism is the sequence of pressure-rise reduction by the air bag expansion, flame quenching by the flame arrester, and the slow burning of the gas mixture sucked from the air bag back into the vessel due to the negative pressure caused by the rapid condensation of water vapor inside the vessel. Necessary conditions for the successful mitigation system are discussed, and the practical unit size of flame arrester sheet is recommended.
1. Introduction
systems are additionally installed in nuclear power plants, no feasible mitigation means has not been developed yet. Any preventive means cannot sufficiently reduce the inherent possibility of explosions once sensitive hydrogen-air cloud is formed by the nuclear reactor meltdown. The emergency responding measures to the severe accident triggered by major earthquake are extremely difficult under the continuous situation of earthquake aftershocks. The Fukushima accident reports have indeed recognized multiple trivial mistakes in the emergency operations. These considerations clearly lead to the necessity of developing new feasible mitigation means beyond the existing preventive means. In the present paper, a new mitigation system is proposed on the assumption of explosion occurrence, and the mitigation mechanism is demonstrated through the series of field experiments.
The disastrous accident in the Fukushima nuclear power plants in 2011 (JAEA, 2016; National Research Council, 2014) has strongly indicated that the existing accident preventive means against hydrogen explosions are insufficient. The records of the three severe accidents at Three Mile Island in 1979 (Nuclear Safety Analysis Center, 1980), Chernobyl in 1986 (World Nuclear Association, 2018), and Fukushima have shown that these accidental explosions are hydrogen-air deflagrations of relatively large volume vapor cloud inside the power plant buildings. Although the accident record at Hamaoka nuclear power plant in Japan showed the occurrence of detonations, the detonation accident occurred locally in pipes. Hydrogen-oxygen mixture is formed through the radiation decomposition of water in pipes at this accident (Du et al., 2018). It is commonly recognized through many field explosion experiments that the transition from deflagration to detonation does not occur in open space or large scale volume like nuclear power plant buildings. Therefore, the practical mitigation system should be designed against deflagration accidents. Although some preventive means including filtered ventilation
∗
2. Basic properties of hydrogen-air deflagrations Hydrogen-air mixture is one of the most sensitive explosive gases. The minimum ignition energy is 17 μJ in the stoichiometric mixture, i.e. hydrogen of 29.6 vol% in the mixture. In comparison with
Corresponding author. E-mail address: [email protected] (H. Saitoh).
https://doi.org/10.1016/j.jlp.2019.03.011 Received 9 January 2019; Received in revised form 29 March 2019; Accepted 29 March 2019 Available online 31 March 2019 0950-4230/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 4. Overpressure record of 43 vol% hydrogen-air explosion using a small size mitigation system.
Fig. 1. Conceptual diagram of the mitigation system against hydrogen explosions.
Fig. 2. Overpressure record of the stoichiometric hydrogen-air explosion using 10 cm long 100A pipe (inner diameter: 105 mm, volume: 870 cm3). Fig. 5. Experimental setup of the mitigation system for 1 m3 vessel.
Fig. 3. Small scale model experiment.
Fig. 6. Flame arrester and the holding aluminum frame.
hydrocarbon-air mixtures, e.g. 100 μJ of methane-air mixture, the energy is one order smaller. The flammability range of 4–75 vol% hydrogen volume fractions is also wide in comparison with 5–15 vol% methane fractions. The laminar burning velocity of 43 vol% hydrogenair mixture is about 3 m/s which is about 10 fold of stoichiometric methane-air mixture. The behaviors of hydrogen-air deflagrations in open space are obtained e.g. in the report of the field experiments using big latex balloons (Otsuka et al., 2007). The important experimental property for flame arrester design is the Maximum Experimental Safe Gap, which is usually abbreviated MESG. The value of the parameter is half of the flame quenching thickness. Flames quench through a gap distance below the critical value. The MESG of hydrogen-air mixture is about 0.25–0.3 mm. This MESG value corresponds to the wire screens of 50–70 mesh per inch.
We actually confirmed the quenching performance of the mesh arrester used in our mitigation system by preliminary verification (Yoshikawa et al., 2015) according to ISO standard (ISO, 2008). 3. Mechanism of the mitigation system Fig. 1 shows a schematic diagram of the mitigation system. The deflagration propagates from the ignition point induces the flow of the unburned hydrogen-air mixture into the expanding air bag through the wire screen sheets of the flame arrester. Stainless steel screens of 50–70 mesh/inch are stacked in 10–20 sheets layer. The flame arrester quenches the propagating flame, and the air bag holds the unburned mixture inside. The negative pressure to the atmosphere is induced in 10
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 7. Air bag motion (29 vol% hydrogen-air mixture, cube-center ignition, successful case).
bag finally shrinks. This back-suction effect by the negative pressure saliently enhances the mitigation system. The above mitigation mechanism is verified throughout the series of experiments, and the details are shown in the following sections. The mitigation system does not additionally require any control or power systems, and this is a strong advantage over other systems. The system prevents massive dispersion of radioactive substances into vast areas of the atmosphere. The system also prevents secondary explosions by very lean oxygen concentrations inside the vessel. The novelty of the present mitigation system is the effective concurrent function of flame arrester and explosion gas capturing (Yoshikawa and Otsuka, 2015). Although many types of flame arresters have already been developed and commercially available, these conventional systems do not prevent the dispersion of explosion gases into the atmosphere. The radioactive substance particulates are submicron size and particularly crucial in the severe accidents of nuclear power plants, and some of the explosion accidents in chemical industries also include similar dispersion of toxics into the atmosphere. The rupture disc venting systems furthermore inherently possess the possibility of the secondary explosion of exhaust gas mixture outside the first explosion location. In addition to the above defects, the conventional flame arrester systems are designed mainly for piping systems and small volume vessels, and the cost for large scale mitigation system becomes high. The system cost is important factor in developing the large scale systems. The present mitigation system uses inexpensive flame arrester units made by stacking fine-mesh wire screens. The actual large size mitigation systems are installed at several different locations on the outer surface of the nuclear power plant building for responding to the ignition source at any location. The actual mitigation system, for example of 40 m cubic building, may comprise 1000–1500 m2 flame arrester area and 100,000 m3 total volume of air bags. The installation of 4,000–6,000 flame arrester units of 50 cm square on the building surface is feasible. Furthermore, the recent aerospace technology of high altitude balloons can easily provide a single balloon of 30,000 m3 (JAXA, 2017).
Fig. 8. Overpressure record measured inside the vessel (29 vol% hydrogen-air, cube-center ignition, successful case).
Fig. 9. Back-suction of air bag mixture (29 vol% hydrogen-air mixture, cubecenter ignition, successful case).
the explosion vessel by the rapid condensation of water vapor. Similar phenomena are also confirmed by Kuznetsov et al. (2015). For example, the pressure of cooled down combustion products of stoichiometric hydrogen-air mixture is 56 vol% of the initial gas mixture at room temperature. The unburned gas mixture once held inside the air bag is sucked back into the vessel and burns slowly. This secondary burning inside the vessel does not generate noticeable pressure rise, and the air
4. Experiments and results 4.1. Laboratory scale experiments Fig. 2 shows the explosion pressure record of the stoichiometric hydrogen-air mixture in a closed vessel initially at the atmospheric pressure. The pressure records in the vessel are obtained using pressure 11
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 10. Blow off of the air bag (29 vol% hydrogen-air mixture, ignition at the bottom surface center, failure case).
Fig. 11. Overpressure record measured inside the vessel (29 vol% hydrogen-air, ignition at the bottom surface center, failure case).
transducers (Kistler, 404JA2, 4045A2, 4049A5SP22 and Kyowa, PGM10KH) in all the experiments in this paper. The peak overpressure is about 7 fold of the initial absolute pressure, and the pressure decays rapidly by heat loss to the vessel surface. The pressure further decreases below the atmospheric pressure by the rapid water vapor condensation. The absolute pressure tolerance of 8 atm for an actual nuclear power plant building, e.g. 40 m cubic reinforced concrete building, requires the wall thickness of about 15 m through a simple estimate. Thus the explosion-resistant building construction is impractical. Fig. 3 shows a preliminary experimental set up for the mitigation system. The same vessel in Fig. 2 is connected to a flame arrester and an air bag. The flame arrester is a stack of 10 sheets of mesh screen of 50 per inch, 0.23 mm wire diameter, and 0.28 mm mesh size. A pair of cross mark plates is used to prevent the bending of the sheets. An air bag of 1.8 L reinforced nylon is connected to the flame arrester. Fig. 4 shows pressure records of 43 vol% hydrogen-air (equivalence ratio, 1.8) explosions. The ignition occurs at the left side of the vessel, and the pressure records of the vessel and the air bag show a salient pressure reduction in comparison with the closed vessel explosion of
Fig. 12. 8 m3 concrete vessel and frame panel for flame arrester.
Fig. 2. Although the flame quenching mechanism by wire screens is an interesting problem, the complexity of the phenomena is formidable. The mechanism involves the complicated interactions among turbulence, chemical reactions, heat transfer, and surface catalytic effects. These controlling factors are not sufficiently elucidated, and the computational analyses contain a lot of uncertainties. The experimental investigations are also presently limited to the determination of critical 12
Journal of Loss Prevention in the Process Industries 60 (2019) 9–16
H. Saitoh, et al.
Fig. 13. Flame arrester modules installed on the frame panel.
Fig. 15. Overpressure record inside the explosion vessel (26 vol% hydrogen-air mixture, ignition at the cube center, successful case).
conditions dividing the quenching and non-quenching regions, because of the knowledge lack of the complex interaction mechanism. Therefore, aiming at establishing a practical mitigation system, the present research takes a pragmatic approach by determining the critical conditions for successful quenching.
explosion. The pressure records at the four locations indicate that the explosion pressure is approximately uniform. This uniform distribution of instantaneous pressure inside the vessel is common to all the experimental results described in this paper. The pressure record in Fig. 8 shows that the first peak of the overpressure is 23 kPa at 40 ms after the ignition. The explosion pressure is saliently reduced in comparison with the constant volume explosion of 700 kPa overpressure. The hydrogen concentration inside the vessel after the explosion was 1 vol%, which indicates the slow burning of the mixture due to the back-suction effect. Figs. 10 and 11 correspond to a failure case of 29 vol% hydrogen-air mixture ignited at the center of the vessel bottom surface. These figures show the air bag blow off and the overpressure of 68 kPa at 75 ms. The second peak pressure about 100 ms corresponds to the blow off of the air bag. The close up video records have shown that the flame arrester surface moves 15–20 mm upward by a strong surface integral of explosion pressure. The flame passes through the flame arrester. The setup of 8 m3 vessel experiments are shown in Figs. 12 and 13. The vessel is 2 m cube, and the resistant pressure of the concrete is 3 atm. The flame arrester is made by stacking 5 sheets of 40 mesh/inch, 20 sheets of 70 mesh/inch, and 5 sheets of 40 mesh/inch. Two flame arrester panels are installed on the pair of opposite surfaces. The total volume of two air bags is 34.6 m3, i.e. 17.6 m3 × 2. The test gas mixtures are ignited at the center of the vessel by electrical spark. Figs. 14 and 15 show a successful case of 26 vol% hydrogen-air explosion mitigation. The air bags expand and reduce the peak explosion pressure down to about 22 kPa. Figs. 16 and 17 show a failure case of 30 vol% hydrogen-air explosion mitigation. The left air bag blows off by the flame passing through the upper part of the flame arrester, although the right mitigation system is successful as shown in Fig. 16. The right photograph is
4.2. Large scale field experiments Based on the results of the preliminary small size experiments, successful conditions for the mitigation mechanism are tested through large size field experiments using three different volume size vessels of 0.125 m3, 1 m3 and 8 m3. The results of the 0.125 m3 vessel have been already reported (Yoshikawa et al., 2015), the present paper starts the results of 1 m3 steel vessel. Fig. 5 shows a photograph of the experimental setup. The vessel is a 1 m steel cube with a mitigation system on the top surface. The flame arrester is a stack of 20 sheets of 1 m square screen of 70 mesh/inch (wire diameter 0.14 mm, mesh size 0.22 mm, and opening ratio 37.3%), and top and bottom surfaces are pressed by aluminum holding frames shown in Fig. 6. The air bag is a 1 m3 silicon-coated reinforced nylon bag. Test gases are prepared by flow mixing of bomb hydrogen gas and air supply from a compressor, and the expected hydrogen concentrations are obtained using a portable gas meter (Riken, GX-8000). A high-speed camera (Photron, FASTCAM SA1.1) and a compact digital camera record the motion of air bags, and the inner pressure record are obtained at four different locations on the inner surface of the vessel. Figs. 7–9 show the records of 29 vol% hydrogen-air mixture explosion ignited at the center of the vessel. The air bag motion in Fig. 7 shows the expansion and the later shrink at 660 ms due to the backsuction effect. Fig. 9 shows that the back-suction effect induces the strong stick of the air bag to the flame arrester surface after the
Fig. 14. Motion records of air bags (26 vol% hydrogen-air mixture, ignition at the cube center, successful case). 13
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Fig. 16. Motion records of air bags. (30 vol% hydrogen-air mixture, ignition at the cube center, failure case). a) Time-series images, b) Close-up view of the flame jet around t = 87 ms.
Fig. 18. Improved flame arrester design.
flame arrester. The complicated designs of flame arresters with many bolts and nuts increase the failure risk. The following field experiments concentrate on the determination of flame arrester designs, especially on manufacturing process, size, and quantity of screen sheets. The improved flame arrester is shown in Fig. 18. The unit size of flame arrester is 50 cm square of stacked 20 sheets of 60 mesh/inch screens (wire diameter, 0.14 mm, mesh size, 0.28 mm, opening ratio, 44.8%). The four sides are tightly sealed with gum tapes, and a pair of holding
Fig. 17. Overpressure record inside the explosion vessel (30 vol% hydrogen-air mixture, ignition at the cube center, failure case).
a close-up view. The pressure record also shows the failure of the mitigation system. The results of the 1 m3 and 8 m3 vessel experiments have particularly suggested the necessity of more reliable manufacturing of the 14
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Fig. 19. Motion of air bags (28 vol% hydrogen-air mixture, 2 air bags, successful case). a) Side view, b) View from the direction of the arrow indicated in the a).
28 vol% hydrogen-air mixture. The peak overpressure is reduced 28 kPa. Fig. 21 shows the experimental results for determining the critical quantity of flame arrester sheets. The left side four flame arrester units are the same as Fig. 19. On the right side vessel surface, two sets of 10 sheets are on the upper left and the lower right parts, and two sets of 9 sheets are on the upper right and the lower left parts. The photograph taken from the front show the critical quantity of sheets is 10 less than which flame quenching fails. 5. Conclusion The effective mechanism of the novel hydrogen explosion mitigation system is described and demonstrated through a series of field experiments. The system prevents the massive dispersion of radioactive substances into the atmosphere. The actual installations of the systems on different locations of the building surfaces require that the total volume of the several air bags is about 1.5–2 fold of the building inner volume. The recommended unit size of the flame arrester is 50 cm square made by stacking 15–20 sheets of 60–70 mesh/inch wirescreens. In addition to the nuclear power plant systems, the concurrent effective mitigation function of flame quenching and explosion gas capture can also be applied to other safety systems including chemical process industries, grain elevators, and fuel pipe lines.
Fig. 20. Overpressure record (28 vol% hydrogen-air mixture, two air bags, successful case).
frames is set on the both surfaces. The seal packing sheets are also inserted between the vessel body and the flame arresters. The quantity of bolts and nuts is largely reduced in comparison with the former flame arresters. The installation of flame arresters becomes easier and less time-consuming. The explosion vessel is 1 m cubic steel frame, and the four side surfaces are used for setting flame arresters or blank steel plates. Two transparent vinyl air bags of 1.5 m3 each are installed on the pair opposite sides of the vessel. The gas mixtures are ignited at the center of the cubic vessel. Figs. 19 and 20 show a successful mitigation case of two air bags for
Acknowledgement The present experimental study was conducted as one of the light water reactor hydrogen safety programs of the Japan Atomic Energy Agency, under the financial support of the Ministry of Economy, Trade 15
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Fig. 21. Critical sheet quantity for mitigation (31 vol% hydrogen-air mixture). a) side view (frame interval, 33 ms), b) front view (frame interval, 1 ms).
and Industry of Japan (METI), during the period of 2012–2016.
Nuclear Safety Analysis Center, 1980. Analysis of Three Mile Island-Unit 2 Accident, EPRI-NSAC–80-1. Electric Power Research Institute, California. National Research Council, 2014. Lessons Learned from the Fukushima Nuclear Accident for Improving Safety of U.S. Nuclear Plants. The National Academies Press, Washington, DC. Otsuka, T., Saitoh, H., Mizutani, T., Morimoto, K., Yoshikawa, N., 2007. Hazard evaluation of hydrogen-air deflagration with flame propagation velocity measurement by image velocimetry using brightness subtraction. J. Loss Prev. Process. Ind. 20, 427–432. https://doi.org/10.1016/j.jlp.2007.04.031. World Nuclear Association, 2018. Chernobyl Accident 1986. http://www.world-nuclear. org/information-library/safety-and-security/safety-of-plants/chernobyl-accident. aspx/, Accessed date: 13 February 2019. Yoshikawa, N., Otsuka, T., Saitoh, H., Nozomu, K., Oozawa, Y., Kurihara, S., Hirata, M., Sakuragi, K., Takanashi, S., 2015. Basic design and fundamental experiments of a damage reduction system against hydrogen explosions. J. Jpn. Soc. Saf. Eng. 35 (2), 122–130. (in Japanese). https://doi.org/10.18943/safety.54.2_122. Yoshikawa, N. and Otsuka, T., 2015. Japan patent No.5747019B2.
References Du, Y., Zhou, F., Ma, L., Zheng, J., Xu, C., Chen, G., 2018. Dynamic fracture response of pre-flawed elbow pipe subjected to internal hydrogen-oxygen detonation. Int. J. Hydrogen Energy 43, 19625–19635. https://doi.org/10.1016/j.ijhydene.2018.08. 211. ISO, 2008. Flame Arresters – Performance Requirements, Test Methods and Limits for Use. ISO 16852:2008(E). JAEA, 2016. Fukushima Nuclear Accident Archive. https://f-archive.jaea.go.jp/index. php?locale=eng/, Accessed date: 24 December 2018. JAXA, 2017. B17-04 Scientific Balloon Testing. http://global.jaxa.jp/projects/sas/ balloon/topics.html/, Accessed date: 13 February 2019. Kuznetsov, M., Friedrich, A., Stern, G., Kotchourko, N., Jallais, S., L'Hostis, B., 2015. Medium-scale experiments on vented hydrogen deflagration. J. Loss Prev. Process. Ind. 36, 416–428. https://doi.org/10.1016/j.jlp.2015.04.013.
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