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Performance Test of the Quenching Meshes for Hydrogen Control a

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Seong-Wan HONG , Yong-Seung SHIN , Jin-Ho SONG & Soon-Heung CHANG

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Korea Advanced Institute of Science and Technology , 373-1 Kusong-song, Yusung , Daejon , 307-701 , Korea b

Korea Atomic Energy Research Institute , P.O.Box 105, Yusung , Daejeon , 305-600 , Korea Published online: 07 Feb 2012.

To cite this article: Seong-Wan HONG , Yong-Seung SHIN , Jin-Ho SONG & Soon-Heung CHANG (2003) Performance Test of the Quenching Meshes for Hydrogen Control, Journal of Nuclear Science and Technology, 40:10, 814-819 To link to this article: http://dx.doi.org/10.1080/18811248.2003.9715423

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Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 40, No. 10, p. 814–819 (October 2003)

ORIGINAL PAPER

Performance Test of the Quenching Meshes for Hydrogen Control Seong-Wan HONG1,* , Yong-Seung SHIN2 , Jin-Ho SONG2 and Soon-Heung CHANG1 1

Korea Advanced Institute of Science and Technology, 373-1 Kusong-song, Yusung, Daejon, 307-701, Korea 2 Korea Atomic Energy Research Institute, P.O.Box 105, Yusung, Daejeon, 305-600, Korea (Received March 7, 2003 and accepted in revised form June 26, 2003)

Downloaded by [117.164.155.135] at 17:36 21 March 2014

The quenching distance of hydrogen gas was experimentally investigated by considering the effects of the initial pressure and steam addition. The quenching distance decreases with the initial pressure and there is a little increase with the addition of steam. Performance tests have been carried out to check the applicability of quenching mesh for the purpose of arresting hydrogen flame propagation during a severe accident in nuclear power plants. The experimental facility for the performance test of the quenching mesh consisted of a model compartment, a visualization system and an ignition system. Dimensions of the single model compartment were 300×300×300 mm. Three-compartments are connected in parallel. The quenching mesh is located between the first and second compartments. It was observed that the flame from the first compartment where the ignition starts does not propagate to the second compartment. The quenching mesh played a role of preventing flame propagation. KEYWORDS: hydrogen combustion, flame propagation, quenching distance, quenching mesh, flame visualization, DDT, equipment survivability

I. Introduction Hydrogen combustion has been known as one of the major factors that influences the safety of nuclear power plant (NPP) reactors because the large amount of hydrogen generated under severe accidents could result in an explosive reaction, which creates direct loads on the containment building, internal structures and equipment. Since the TMI-2 accident,1) lots of effort has been directed to the control of the hydrogen combustion phenomena in the containment under severe accidents.2) As a result of this effort, the installation of igniters has been proposed as a method of preventing damaging burns in a nuclear power plant by ensuring ignition near the limits of flammability.3) Ulchin 3, 4 are the first Korean nuclear power plants followed the 10CFR50.34(f) requirement for hydrogen control. Igniters are to be installed in the YGN 5, 6 to control a hydrogen amount equivalent to 100% oxidation of Zr. The igniters and PARs (Passive Autocatalytic Recombiners) will be installed in APR–1400. A major issue for hydrogen control is how to prevent the DDT (Deflagration to Detonation Transition) because DDT can lead to structural damage from the severe accident sequences involving hydrogen combustion. The ultimate goal in hydrogen mitigation is to design countermeasures that would prevent FA (Flame acceleration) and DDT. New containment designs could, in principle, be constructed to carry higher dynamic loads, however, at the expense of additional costs. Igniter systems have been proved to be an important part of the total plant capabilities for protecting containment integrity under severe accident conditions.4, 5) But, the availability of the igniters depends on electric power. Igniters are at predetermined, fixed locations, and care must be taken in selecting adequate locations, in a manner that is independent of the accident scenario. Special consideration must be given to the situations of hydrogen accumulation as a result of temporary ∗

steam-inerted conditions. Passive antocatalytic recombiners are also other devices that recombine the hydrogen present in the containment without a need for external power or operation. The installation of PARs is highly influenced by geometric and operational constraints. Access for maintenance should remain free and PARs must be accessible for periodic surveillance. Combustion behavior in sub-compartments is uncertain and depends on the geometry, hydrogen and steam concentration and the availability of the ignition sources. Depending on the geometry and gas concentrations, turbulent deflagration, accelerated flames and transition to detonation cannot be excluded in all the cases. Deflagration to detonation transition still cannot be totally excluded if the flame initiated from the igniters is accelerated. Therefore, existing means for hydrogen control are not effective enough in properly removing the hydrogen threat in nuclear power plants. However, if the quenching mesh is installed between the compartments, flame acceleration can be prevented, resulting in the removal of the DDT. Installation of the quenching meshes between the compartments and near the equipment has been suggested to prevent flame propagation among the compartments and to maintain the equipments survivability.6) The principle of quenching mesh is based on both the heat loss from energy aspect and the radical recombination from the chemical aspect at the mesh. The heat of the burnt gas could be rapidly lost at the mesh, resulting in a reduction of the flames intensity. From the chemical aspect, the mesh changes active radicals that promote a chain molecular reaction into stable chemical elements, resulting in a reduction of the flame intensity due to the slow reaction rate. In other words, heat loss to the wall and wall termination (radical termination) are dominant chemical processes in wall-induced quenching, while chain-branching reactions and dissociations of reactants plays a significant role in flame propagation. So, the flame can be extinguished at the quenching mesh. The application examples of the quench-

Corresponding author, Tel. +82-42-868-8997, Fax. +82-42-8612574, E-mail: [email protected]

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ing mesh are the flame arresters. Flame arresters protect storage, distribution and chemical processing facilities containing flammable gases from fire and explosion. When correctly applied, flame arresters are effective devices that isolate the sources. The quenching distance is a key quantity that must be characterized in order to design the quenching meshes because it depends on the gas species. The quenching distance for hydrogen gas was measured in dry and wet atmospheric conditions because the conditions of the containment in a severe accident would be high pressure and high temperature with steam. Experiments in a closed small-scale model compartment were conducted to investigate which parameters have major effects on the performance of the quenching meshes for flame propagation prevention to the other compartment under the conditions of an accompanying chamber pressure increase during the combustion process.7) From the small-scale test, it was found that when the initial pressure is lower than 0.25 bar, the re-ignition in the second compartment does not occur although hot gas is spouted. Meanwhile, the mixture in the second compartment re-ignites with the hot gas jet at a relatively high initial pressure, 0.5 bar. This result could not be applied to the nuclear power plants because the pressure in the containment of the nuclear power plants is greater than the atmosphere. To check the applicability of the quenching meshes in nuclear power plants, the performance of the quenching mesh has to be verified under the atmospheric pressure in nuclear power plants. The performance test is carried out on a largescale at atmospheric pressure.

II. Measurement of Quenching Distance The quenching distance when an electric spark was applied could be expressed from the relation between the gap size and the applied ignition energy.8) The minimum ignition energy, minimum value of the applied energy to the ignition source, lessens with an increasing gap size but is not lessened any more if the gap size reaches a critical size. This critical size depends on the fuel species, equivalence ratio, initial temperature and pressure, and the other parameters such as steam. The minimum distance, where the minimum ignition energy is constant at a certain condition, is defined as the quenching distance. The electric spark attached to the flange is used to easily judge the gap distance. Figure 1 shows the experimental apparatus for the measurement of the quenching distance which consists of a combustion chamber, an electric spark circuit for ignition, a mixing chamber, and an electric oven. The combustion chamber was a cylindrical closed vessel, as shown in Fig. 2, with a 50 mm inside diameter and 10 mm in depth. The combustion chamber has inlet and outlet ports, which are indicated as the pre-mixture from mixing chamber and exhaust of burnt gas using vacuum pump in Fig. 2, to fill and purge the gas and a pressure regulator to adjust the initial pressure of the mixture. The spark electrodes mounted at the center of the combustion chamber were flanged with glass plates. The glass plates have the effect of suppressing the ignition when the electrodes are separated by a critical distance8) which is called the quenching distance in the present study. The gap length between the

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Schlieren m irror CDI Covex lens

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Fig. 1 Schematic of experimental setup

Fig. 2 Combustion chamber for quenching distance measurement

electrodes can be closely adjusted using a built-in micrometer. Ignition line is provided to derive the ignition of the mixture. There is a mixing chamber where the equivalence ratio of the mixture was determined based on the partial pressures, and supplied to a combustion chamber. Since the steam addition method was based on saturated vapor pressure corresponding to the mixture temperature, it was very important to keep the temperature of the mixtures constant in the processes of mixing, supplying, and adding steam prior to ignition. All experimental apparatus, including mixture supply lines, were put together in an electric oven operated at a constant temperature, so the experiment could be done at a constant mixture temperature. More details of the test facility are described in.7) When the combustion chamber was filled with a test mixture in predetermined conditions, the ignition system was activated. After ignition, the pressure of the combustion chamber was monitored using a pressure regulator connected to the combustion chamber. Any pressure increase in the combustion chamber would be an indication of flame quenching or propagation. Since the containment conditions during most severe accident sequences are characterized by pressures up to ∼2 bar and temperatures up to 400 K, experimental data for the quenching distance in this study were developed within these ranges of conditions. Figure 3 shows the representative shadow images for flame propagation and quenching. If the distance between the gaps

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Fig. 3 Shadow image on flame propagation/quenching


Fig. 5 Quenching distance at various hydrogen–air mixtures

Fig. 4 Quenching distance measured for hydrogen–air

of the glass plate is larger than a critical distance for a specified initial condition, the flame from the ignition source propagates to the outside, as shown in the top of Fig. 3. Meanwhile, if the distance between the gaps of the glass plate is smaller than a critical distance for a specified initial condition, the flame from the ignition source quenches, as shown in the bottom of Fig. 3. Quenching distance measurements for hydrogen-air mixtures with/without steam addition were performed at 300 K. Quenching distances measured for hydrogen-air mixtures with hydrogen concentrations at 300 K and atmospheric pressure are in good agreement with the previous results measured at 300 K and atmospheric pressure,8) as plotted in Fig. 4. Quenching distances at various hydrogen-air mixtures measured over a range of initial pressures are shown in Fig. 5. From this figure, the minimum quenching distance appears near a stoichiometric hydrogen concentration at various initial pressure conditions and decreases with an initial pressure increase. The effect of steam addition on quenching distances is shown in Fig. 6 for a stoichiometric hydrogen-air mixture. Steam is added under a saturated condition such that the initial temperature increases with steam addition. The effect of temperature increase is known to decrease the quenching distance.8, 9) Considering this effect, the result shows that the addition of steam increases the quenching distance. The steam added in the hydrogen-air mixture plays the role of heat sink

Fig. 6 The effect of steam addition on quenching distance

due to its large heat capacity. So, the quenching distance increases through the addition of steam to the hydrogen-air mixture. And it is confirmed that quenching distances decrease with an initial pressure increase, even though steam is added. Combustion process in a closed compartment results in an increase in chamber pressure. Considering this pressure rise, the size of the meshes for the control of the flame propagation should be smaller than the quenching distance measured in the experimental facility. According to Phillips,10) MESG (maximum experimental safe gap), which is the gap size of the quenching mesh, is approximately a half of the minimum quenching distance. From the test results, it was confirmed that quenching distance maintains a minimum in a stoichiometric mixture and can be approximated to be inversely proportional to the initial pressure and it is not varied much even though steam is added. Atmospheric conditions in severe accidents are not any more sensitive than a stoichiometric mixture. Considering safety, the quenching distance of the stoichiometric mixture at an atmospheric pressure was selected for designing the quenching mesh for the control of hydrogen combustion. So, the size of metallic meshes was selected as 0.3 mm that is a half of the minimum quenching distance, 0.6 mm, at the atmospheric pressure.

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The experimental apparatus consists of the combustion compartment, the visualization system where the laser is used as a light source and the ignition system, as schematically shown in Fig. 7. Figure 8 shows the combustion chamber. Dimensions of one compartment are 300×300×300 mm. Three-compartments of this size are connected in parallel. Both side planes of the compartments are made of quartz for optical transparency which size is 200×200 mm. For the visualization and image recording of the hydrogen flame propagation, Schlieren/shadow system and a high-speed camera are used, respectively. Two 8-inch mirrors are used to get the shadow image which provides a two dimensional image. In this test, the edge is applied between the mirror and the screen to get the Schlieren image which provides a three dimensional image. The image is generated by the density difference of the gas in the combustion chamber and is taken

Fig. 8 Combustion chamber

Fig. 9 Quenching mesh

by a high-speed camera with a 1,000 frame rate per second. Figure 9 shows the quenching mesh with 0.3 mm quenching distance. Several measurement ports such as static and dynamic pressure are provided at the top of the chamber. Electric spark igniter is installed on the right-end center of the first compartment, as shown Fig. 8. Therefore, ignition is started from the end-center of the first compartment. The ignition energy level can be changed by controlling the input voltage to the capacitor which provides the high voltage, 16.5 kV maximum for ignition. The gap of the ignition node is about 2 mm. The static pressure was measured at the Ch-3-2 location of the third compartment. The dynamic pressure sensors (PCB Piezotronics Inc. W112A02) at the Ch-1-3 and Ch-2-1 are provided to estimate the flame speed between the before and after quenching mesh. The quenching mesh is installed between the first and second compartments. The hydrogen concentration is controlled using a digital pressure indicator. The code for a severe accident analysis generally states that hydrogen burn starts at a 7–8% hydrogen concentration. So, the test was carried out at these hydrogen concentrations.

Fig. 7 Schematic of experimental apparatus



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Fig. 10 Flame images (time interval is 20 ms) at the second

Figure 10 shows the shadow flame images in the second compartment without a quenching mesh at a 7% hydrogen concentration. The flame started from the igniter of the first compartment and was easily propagated to the second compartment with time, as shown in Fig. 10. In this case, the average flame velocity in the second compartment is about 1 m/s. Figure 11 shows the Schlieren flame images in the second compartment with quenching meshes at a 7% hydrogen concentration. With quenching meshes, it was observed that the flame front from the first compartment seems not to have propagated to the second compartment at the same hydrogen concentration. Only the hot gas, not the flame, flows to the second compartment. The flame, at the starting a point ignition, is generally round shaped at the flames front surface. However, the gas of Fig. 11 does not have this shape and also its moving speed is very slow. So, it is thought that the flame does not propagate through the mesh boundary. Figure 12 shows the mesh after the test. The upper part of the mesh is imprinted with a red-yellow color because the hot burnt gas passes through the upper part of the chamber. This imprint at the upper part of the quenching mesh is caused from the asymmetry flame because the burnt gas is lighter than the unburned gas and the flame propagation from the point of the ignition source easily proceeds in an the upper direction. Figures 13 and 14 show the static and dynamic pressure for the complete and incomplete combustion of the initial mixture

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Fig. 11 Flame images (time interval is 20 ms) at the second

Fig. 12 Mesh picture after test

for the 8% hydrogen concentration with dry air, respectively. From these pressure histories, the tests were classified as to whether burn occurred completely or not. For complete burn, the pressure increases to 2.5 bar because the flame from the first compartment propagates to the second and third compartments, as shown in Fig. 10, and decreases to the atmospheric pressure in 10 s. However, the pressure increases to 0.5 bar for the incomplete burn because the flame from the JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

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Fig. 13 Pressure history for complete burn

plant is experimentally investigated. Quenching distances of hydrogen-air mixtures without steam are measured with pressure increases from the atmospheric pressure. The minimum quenching distance appears near the stoichiometric hydrogen concentration at various initial pressure conditions and decreases with the initial pressure increase. Those of stoichiometric hydrogen-air mixtures with steam with respect to the initial pressure are also measured. The quenching distances are increased through the addition of steam to the hydrogenair mixture. To check the propriety of the quenching meshes in nuclear power plants, the performance of the quenching mesh was verified under a near atmospheric pressure as in the postulated severe accident of a nuclear power plant. The flame without a quenching mesh is easily propagated to the second compartment. When a quenching mesh with 0.3 mm distance is installed in the flames path, the quenching mesh could play a role of quenching/at least weakening the flame. The experiment will be continued further to check the applicability of the quenching meshes in nuclear power plants because the hydrogen burn analysis code predicts a faster flame speed.

Acknowledgment This work has been carried out under the Nuclear R&D Program by MOST, Korea. References

Fig. 14 Pressure history for incomplete burn

first compartment does not propagate to the second and third compartments, as shown in Fig. 11. The burn only occurs in the first compartment due to the quenching mesh between the first and second compartments. The dynamic pressure sensor detects the flame propagation between the first and second compartments. It is observed that a detecting time delay exists between Ch-1-3 and Ch 2-1 for the complete burn. A test was also conducted using the operation of a small fan during the test to distribute uniformly the mixture. The flame propagation/suspension shape towards the second compartment has no difference when comparing it to the experiment without a fan. However, the burnt mixture is mixed faster.

IV. Conclusions A study of quenching meshes for the control of hydrogen combustion in severe accidents in a nuclear power


1) NASC, Three Mile Island–Unit 2 Accident, Nuclear Safety Analysis Center Report, (1980). 2) A. L. Camp, et al., Light Water Reactor Hydrogen Manual, SAND82-1137, NUREG/CR-2726, Sandia National Laboratories, (1983). 3) J. C. Cummings, et al., Review of the Grand Gulf Hydrogen Igniter System, SAND82-0218, NUREG/CR-2530, (1983). 4) R. K. Kumar, et al., Intermediate Scale Combustion Studies of Hydrogen-Air-Steam Mixtures, EPRI Report, NP-2955, (1984). 5) L. B. Thompson, et al., “Large scale hydrogen combustion experiments,” ANS Int. Conf. on Containment Design, Toronto, Canada, p. 120–125 (1984). 6) H. J. Kim, et al., “Laser Rayleigh measurement of mixing processes and control of hydrogen combustion using quenching meshes,” Nucl. Eng. Des., 187, 291–302 (1999). 7) H. J. Kim, S. W. Hong, et al., “A Study on quenching meshes as a possible controlling tool of hydrogen explosion in nuclear power plants,” ICONE10 Conf., Arlington, USA, (2002). 8) B. Lewis, Combustion, Flames and Explosion of Gases, 3rd Ed., Academic Press, Orlando, (1987). 9) I. Glassman, Combustion, 2nd Ed., Academic Press, Orlando, Florida, (1987). 10) H. Phillips, “On the transmission of explosion through a gap smaller than the quenching distance,” Combustion Flame, 7, 129–135 (1963).