Safety Implementation of Hydrogen Igniters and Recombiners for Nuclear Power Plant Severe Accident Management

TSINGHUA SCIENCE AND TECHNOLOGY I S S N 1 0 0 7 - 0 2 1 4 0 9 / 1 8 p p 5 4 9 -558 Volume 11, Number 5, October 2006

Safety Implementation of Hydrogen Igniters and Recombiners for Nuclear Power Plant Severe Accident Management XIAO Jianjun (肖建军), ZHOU Zhiwei (周志伟)**, JING Xingqing (经荥清) Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China Abstract: Hydrogen combustion in a nuclear power plant containment building may threaten the integrity of the containment. Hydrogen recombiners and igniters are two methods to reduce hydrogen levels in containment buildings during severe accidents. The purpose of this paper is to evaluate the safety implementation of hydrogen igniters and recombiners. This paper analyzes the risk of deliberate hydrogen ignition and investigates three mitigation measures using igniters only, hydrogen recombiners only or a combination of recombiners and igniters. The results indicate that steam can effectively control the hydrogen flame acceleration and the deflagration-to-detonation transition. Key words: nuclear power plant; severe accident management; hydrogen mitigation; containment; flame acceleration; deflagration-to-detonation transition

Introduction During a severe accident in a light water reactor (LWR), oxidation of the metallic components in the reactor core and the interaction of core and concrete components will produce hydrogen. Hydrogen transport in the containment will result in its mixing with air and water-steam, so that a flammable gas mixture may form somewhere in the containment. Hydrogen combustion in the containment building could rapidly increase the pressure and temperature which may threaten the containment integrity. A promising hydrogen mitigation method for severe accident conditions is deliberate ignition of the flammable gas mixture, to ignite the hydrogen at an early stage when the low hydrogen concentration will ensure that only slow combustion will occur. In 2002, the National Nuclear Safety Administration Received: 2005-03-25; revised: 2005-05-26

﹡ ﹡To whom correspondence should be addressed. E-mail: [email protected] Tel: 86-10-62784826

of China (NNSA) issued a policy statement—Technology policy about a few important safety problems in the design of a new nuclear power plant[1]—which stipulated a number of enhanced safety objectives. With respect to hydrogen behavior in the containment during severe accidents, the statement stipulated that large scale hydrogen combustion or detonation, which will threaten the containment integrity, must be eliminated. Therefore, hydrogen behavior and mitigation measures in the containment during severe accidents must be investigated for any new nuclear power plant project in China[2,3]. This paper analyzes the deliberate ignition in a single closed room with hydrogen injection using the 3-D CFD code GASFLOW which was jointly developed by FzK and LANL[4,5]. This paper presents a mechanistic approach based on flame acceleration (FA) and deflagration-to-detonation transition (DDT) criteria to evaluate the ignition risk to the containment and gives guidelines on igniter location and first ignition time. This paper also investigates three hydrogen mitigation measures.

Tsinghua Science and Technology, October 2006, 11(5): 549-558

550

1

Criteria and Numerical Models

1.1

σ criterion

The flame starts as a slow quasi-laminar deflagration of premixed H2-air-steam after deliberate ignition. The flame will propagate along the hydrogen concentration gradient towards higher H2-concentration mixture regions with intense turbulence. This effect and also the self-induced turbulence from expansion of the burned mixture behind the flame can induce transition from slow laminar to fast turbulent deflagration, i.e., FA. The dominant influencing parameters for FA are the mixture composition, turbulence generation, confinement, and length. The expansion ratio, σ, is defined as the ratio of the densities of the reactants and the products. A σ contour plot for the H2-air-steam mixture at 373 K, calculated using the STANJAN code[6], is given in Fig. 1. The physically relevant σ range is defined by the flammability limit. At 373 K, the critical σ value is 2.9 ± 0.1 for lean mixtures ( φ < 1 ) and 3.75 ± 0.25 for high O2

ϕO concentration mixtures ( φ ≥ 1 ) . Here, φ = , 2ϕ H O and ϕ represents the volume fraction. [7]

2

2

The numerator is the expansion ratio of the average mixture in the compartment. The denominator is the critical expansion ratio of the average mixture. When σ index < 1, flame acceleration is excluded, whereas for

σ index ≥ 1, there is a potential for flame acceleration. The σ criterion can be used to conservatively estimate the FA potential. If the criterion is not fulfilled, a slow quasi-laminar deflagration is expected, which can be modeled with an appropriate numerical tool. If the σ criterion predicts FA, the mixture may also undergo transition to detonation. 1.2 σ criterion model

The calculation of σ index includes the following steps in the GASFLOW code: (1) Define the control volume, which should agree with the physical room boundaries as much as possible; (2) Calculate the average mixture composition in the room; (3) Calculate the expansion ratio σ or the average mixture using a σ index = σ ( ρ H , ρ H O , ρO ,T ) table for 2

2

2

H2-air-steam-N2 mixtures; (4) Calculate the critical expansion ratio σ critical ; (5) Evaluate σ index = σ / σ critical . 1.3 λ criterion

Analyses show that the occurrence of DDT can be correlated with the geometrical size of the reacting gas mixture[8]. Tests in scaled down facilities have also shown that the detonation cell width of the mixture correlated with DDT in various mixtures. The results of these tests are that a minimum scale for DDT onset is given by R = D /(7λ ) (2) Fig. 1 Expansion ratio and flame acceleration limits of H2-air-steam mixtures at 373 K

where ρH , ρH O , and ρO are the average hydrogen,

where D is the characteristic size of the reactive gas cloud, λ is the average detonation cell width of the (generally nonuniform) gas mixture, and R is a nondimensional onset number. DDT is possible if Condition (2) is met. The geometric parameters are related to the size of gas clouds, rooms, rectangular channels, and round tubes. For unconfined or partly confined clouds of combustible mixtures, the volumetric average is recommended as the characteristic size D,

steam, and oxygen concentrations in the compartment.

(3)

Flame acceleration also occurs for larger σ values (dark gray region). σindex is defined in the GASFLOW code to evaluate the acceleration potential

σ index =

σ ( ρH , ρH O , ρO , T ) 2

2

σ critical ( ρ H , ρO , T ) 2

2

2

2

(1)

2

2

D = V 1/ 3

where V is the cloud volume exceeding the lower

XIAO Jianjun (肖建军) et al：Safety Implementation of Hydrogen Igniters and …

flammability limit. The accuracy of the detonation cell width λ of the reacting mixture calculated with available theoretical models is insufficient for nuclear power plant (NPP) analysis purposes. Experimental detonation cell width data has been collected to predict the value of λ [9]. The experimentally available detonation cell data was extrapolated up to 5 m because this covers all NPP situations of interest. Mixtures with l m to 2 m and 5 m cell sizes would require critical volumes of 2744 m3 and 42 875 m3 for detonation. These estimated volumes are typical compartment and containment dimensions. Mixtures with λ > 5 m cannot undergo a DDT in nuclear containments according to Eq. (2). 1.4

The described λ criterion was implemented into the 3-D field code GASFLOW as follows. (1) Characteristic cloud dimension D The characteristic dimension Dn(t) of the H2-airsteam cloud in room number n, which evolves from the source, is calculated by (4) Dn (t ) = Vn1 / 3 (t )

∑∆

i,n

(5)

(t )

i

where ∆ i , n (t ) indicates the computational cells in containment room n which contain a burnable mixture at time t. In the case of a dry H2-air mixture, these are the grid cells containing between 4% and 75% H2, the lower and upper flammability limits. The lower H2 limit of 4% can also be raised to 12% to identify highly transient hydrogen release phases that can lead to the cloud cells with high H2 concentrations that are dispersed in a large low hydrogen concentration cloud. (2) Average detonation cell width λ The average compositions of the H2-air-steam cloud in room n at time t are

⎛ ⎞ [ϕH2 (t )]n = ⎜ ∑ × ϕH2 ,i ⋅ ∆Vi ⎟ ⎝ i ⎠n

(

⎛

)

⎞ × ϕ H 2 O ,i ⋅ ∆Vi ⎟ ∑ ⎝ i ⎠n

[ϕ H2 O (t )]n = ⎜

This average composition is used to calculate the average equivalence ratio of the cloud, which in H2air-steam mixtures is φn = 2.3866ϕ H2 , n /(1 − ϕ H 2 , n − ϕ H 2O, n )

(8)

The average detonation cell width λn of the cloud mixture in room n can now be evaluated from measured or calculated data for λn (φ , ϕ H 2 O ). (3) DDT index R The DDT index R is evaluated for room n according to Rn (t ) = Dn (t ) /[7λn (t )] (9) If Rn < 1, detonation transition is excluded or unlikely to occur. If Rn ≥ 1, DDT cannot be excluded according to the criterion.

λ criterion model

Vn (t ) =

551

(

)

Vn

Vn

(6)

(7)

where ϕH2 ,i (t ) is the hydrogen volume fraction of the H2-air-steam cloud in cell i of room n, and ϕ H 2 O ,i (t ) is the corresponding steam volume fraction.

2

Numerical Models of Hydrogen Igniter and Recombiner

2.1

Igniter model

To form a flammable mixture, the hydrogen volume fraction must exceed 4% for steam volume fractions up to 30%. For steam volume fractions of 30%-65%, the minimum hydrogen volume fraction increases from 4% to 12%. Above 65% steam volume fraction, the mixture is considered to be steam inerted. This criterion is ϕ H2O < 65%,

⎧ ⎨ϕ ⎩H

2

⎡ 8 ⎤ ≥ 4% + max ⎢0, (ϕ H2 O − 30%) ⎥ ⎣ 35 ⎦

(10)

where ϕH2 O is the steam volume fractions and ϕH2 is the hydrogen volume fraction. If Condition (10) is met, the chemical kinetics equation, dϕO2 1 dϕH2 O 1 dϕ H 2 − =− = = k (T )ϕ H ϕO = ω ′ 2 2 2 dt dt 2 dt (11) is solved with the function k defined by ⎛ E ⎞ k (T ) = Cf ⋅ exp ⎜ − ⎟ ⎝ R ⋅T ⎠

(12)

in which the temperature in the reaction rate constant is equal to 2000 K, k is the reaction rate coefficient, Cf is the frequency factor, R is the universal gas constant, and E is the activation energy, J/mol.

Tsinghua Science and Technology, October 2006, 11(5): 549-558

552

2.2

Recombiner model

There are several recombiner models in the GASFLOW code including the NIS type, Siemens type, Siemens-FR-90/1 type, and GRS type. This analysis used the Siemens type recombiner box. (1) Calculate the time-dependent hydrogen and oxygen volume fractions, ϕH2 ,IN (t ) and ϕO2 ,IN (t ) at the recombiner inlet. (2) Check the species composition from ϕO ,IN (t ) ⎤ ⎡ ϕIN = min ⎢ϕH2 ,IN (t ), 2 2 (13) ⎥ η (t ) ⎦ ⎣ (3) Determine both the recombination rate, R′(t ) , and the recombination efficiency, η (t ) , using the volume fraction as a percentage. (4) Compute a reference velocity as R′(t ) U 0 (t ) = AINη (t ) ρ H2 ,IN (t )

(14)

where AIN is the flow cross-section area of the recombiner box inlet. (5) Solve the time-dependent velocity, U(t), flowing into the recombiner box, ⎤ R ′(t ) dU (t ) 1 1⎡ = [U 0 (t ) − U (t ) ] = ⎢ − U (t ) ⎥ dt τ τ ⎣⎢ AINη (t ) ρ H 2 ,IN (t ) ⎦⎥ (15) where τ is the time constant. (6) Compute chemical kinetics with change of species concentrations with 1 ηU (t ) AIN cH 2 ,IN (t ) (16) ω′ = 2 V

3

Applications

3.1 Evaluation of hydrogen deliberate ignition risk

Dry hydrogen is injected into a closed room with the same configuration as the steam generator room of Daya Bay NPP, as shown in Fig. 2. The room initially contains dry air at 1 bar (1 bar =100 kPa) and 300 K. For the most severe conditions, dry hydrogen is assumed to be released into the room without steam, which tends to reduce the hydrogen ignition risk. Dry hydrogen enters the room from the floor at the same initial conditions: 1 bar, 300 K, 200 g/s. Three cases

were analyzed: dry hydrogen distribution without mitigation, deliberate ignition at an early stage, and deliberate ignition at a late stage.

Fig. 2 Single closed room for deliberate hydrogen ignition

3.1.1 Hydrogen distribution without mitigation Dry hydrogen enters the room without any mitigation measures and without self-ignition, with the mixture temperature at 800 K during the whole process. The flame acceleration and detonation potential, as characterized by the expansion ratio σ index , the hydrogen

cloud dimension D, and the DDT index R in the closed room, were evaluated for times of 0-600 s. The results in Fig. 3a show that from 0 to 20.1 s, the hydrogen cloud dimension D increases from 0 to 692.3 cm; from 20.1 s to 550 s, D is constant at 693 cm; and after 550 s, D decreases rapidly. As shown in Fig. 3c, from 33 s to 192 s, the average σ index is larger than 1, which indicates that flame acceleration may exist. At 18.5 s, DDT index, R, is larger than 1, which indicates a potential for DDT, and at 76 s, R reaches its peak value of 102 (Fig. 3d). As mentioned earlier, the hydrogen cloud dimension, D, is calculated based on the grid cells whose hydrogen concentration is between 4% and 75%, the lower and upper flammability limits. When the hydrogen is initially released into the containment, the hydrogen volume fraction in the cells near the hydrogen source will be larger than 4%. As the hydrogen enters the containment, the hydrogen cloud dimension, D, increases rapidly. At 407 s, the average hydrogen volume fraction in some cells is larger than 75%, so D starts to decrease. 3.1.2 Early ignition With the early ignition, the hydrogen is ignited as soon as the local hydrogen concentration is above 4%. The analyses were performed with four igniters at different

XIAO Jianjun (肖建军) et al：Safety Implementation of Hydrogen Igniters and …

553

Fig. 3 Results for hydrogen distribution in a single room without mitigation measures

locations in the room (Fig. 2). From 0 to 20 s, 4 kg of hydrogen is released into the room, with about 3.41 kg removed by the igniters. Igniters at higher locations, such as Igniter 1, should cause an early ignition because buoyancy causes hydrogen transport into the upper part of the room. The dotted lines in Figs. 4a and 4b show that the hydrogen cloud dimension, D, in the room rises rapidly at the beginning to 6.92 m at 6.9 s with the detonation cell size λ being 9.89 m at this moment. Therefore, the corresponding DDT index, R, is 0.1 at this time, as shown in Fig. 4c. The expansion ratio index decreases from 1.5 to 0.43 as ignition occurs, and then fluctuates around 0.43 (Fig. 4d). The analysis indicates that neither DDT nor flame acceleration will occur with early ignition. 3.1.3 Late ignition Ignition at 50 s is considered to be late ignition. From 0 to 60 s, 12 kg of hydrogen was released into the room, with about 9.76 kg removed by the igniters. The solid lines in Figs. 4a and 4b show that at 50 s the hydrogen cloud dimension, D, is 6.92 m, with a

detonation cell size, λ, is 0.013 m, and the DDT index, Rindex , reaches 76. At these conditions, hydrogen ignition will create a strong detonation. As shown in Fig. 4c and Fig. 4d, from 0 to 50 s, the expansion index, σ index , is equal to 1.24, while the DDT index, Rindex , rises from 1 to 76 in about 30 s. The results indicate that flame acceleration and DDT are inevitable if no mitigation measures are taken at an early stage. Though the hydrogen concentration may rise to 75% (Fig. 3b), the upper flammability limit, a real NPP containment cannot possibly reach the hydrogen self-inert limit. So, hydrogen must be removed at an early stage to reduce the hydrogen combustion risk as much as possible. 3.1.4 Comparison between early and late ignitions The average volume fraction of the mixture gas at the early and late stages are shown in Figs. 5a and 5b. After ignition, steam is generated by the hydrogen combustion. The steam effectively increases the detonation cell size, λ, which reduces the DDT index, Rindex . Therefore, the possibility of detonation transition decreases as shown in Figs. 4b and 4c. Early and late

554

Tsinghua Science and Technology, October 2006, 11(5): 549-558

Fig. 4 Results for hydrogen behavior in a single room at early and late ignitions

Fig. 5 Results for hydrogen behavior in a single room at early and late ignitions

XIAO Jianjun (肖建军) et al：Safety Implementation of Hydrogen Igniters and …

ignitions create large differences in the temperature and pressure loads (Figs. 5c and 5d). With early ignition, the temperature of Igniter 1 rises quickly from 340 K to 2500 K, with smaller increases at the other igniter temperatures. There are no obvious pressure peaks with early ignition. With late ignition, the four igniter temperatures rise from 340 K to 2500 K, while the pressure rises from 1.6 bar to 9.3 bar in a very short time. The results indicate that high-temperature and high-pressure loads which occur with late ignition will greatly threaten the room integrity, but that the location of Igniter 2 is better than the other igniters because it more effectively removes the hydrogen without an extremely high temperature peak. 3.2

Hydrogen mitigation measures with igniters and recombiners

The room with volume 464 m3 initially contains dry air at 1 bar and 300 K. Hydrogen enters the room from 0 to 15 s at a release rate of 500 g/s. Automatic hydrogen igniters and recombiners are installed in the room as shown in Fig. 6. The three cases were: igniters only, recombiners only, and a combination of recombiners and igniters[9].

Fig. 6 Closed room for the study of hydrogen mitigation measures

Fig. 7

555

3.2.1 Igniters only The study in Section 3.1 indicates that hydrogen igniters can remove hydrogen quickly and safely if the location and ignition time are selected reasonably. However, hydrogen igniters are not effective if the hydrogen concentration is lower than the flammability limit[10]. The steam concentration also affects the effectiveness of the igniter, since the hydrogen flammability limit increases as the steam concentration increases. If the steam volume fraction is above 65%, hydrogen ignition will be totally inerted by the steam. Figure 7 shows the relationship between the steam volume fraction and the flame acceleration ratio, and the deflagration-to-detonation transition. The two ratios decrease with increasing steam concentration which indicates that steam can effectively control hydrogen flame acceleration and DDT. Figure 8 shows that hydrogen is removed by the igniters alone in a very short time, with the hydrogen mass, hydrogen volume fraction, flame acceleration ratio, and the DDT ratio then remaining constant. However, the igniter cannot remove the hydrogen if the hydrogen concentration is below the flammability limitation. Hydrogen will accumulate in the room, and high locally hydrogen concentrations may occur for some conditions, such as steam condensation. 3.2.2 Recombiners only The hydrogen removal rate of a catalytic recombiner is less than that of a hydrogen igniter; but a hydrogen recombiner can effectively control hydrogen deflagration risk if the hydrogen release rate is low. In addition, a recombiner, which differs from an igniter, can remove hydrogen even when steam has suppressed the hydrogen ignition risk[11].

Effect of steam volume fraction on the flame acceleration ratio and DDT

Tsinghua Science and Technology, October 2006, 11(5): 549-558

556

Fig. 8 Hydrogen conditions in the closed room

Fig. 9

Average temperature and pressure in the closed room

Large quantities of hydrogen (release rate at 2 kg/s) will be released into the containment under severe accidents. In these cases, the hydrogen can not be removed by recombiners alone. Hydrogen will accumulate in the containment, and once the hydrogen concentration reaches the flammability limitation, deflagration and detonation may occur. As show in Fig. 9, when hydrogen is released into the room at 500 g/s, if only recombiners are used, from 0 to 330 s, the average hydrogen concentration is higher than the hydrogen flammability limitation, with a peak value of

drogen flammability limitation, with a peak value of 22%. After 330 s, the hydrogen concentration slowly decreases to below the flammability limit. However, the flame acceleration ratio and DDT are larger than 1 at later times which indicates that the risk of hydrogen turbulent deflagration, even detonation, exists with high hydrogen release rate if only recombiners are used. 3.2.3 Combination of igniters and recombiners Recombiners combined with igniters can safely and

XIAO Jianjun (肖建军) et al：Safety Implementation of Hydrogen Igniters and …

effectively control the hydrogen risk. At the early stage of hydrogen release, the igniters can safely burn the hydrogen to reduce the hydrogen concentration to or below the flammability limit. Recombiners can then remove the hydrogen to keep the concentration below the flammability limit, and reduce the flame acceleration ratio to 0.64 and DDT to 0.1, as shown in Fig. 8. Therefore, the combination of recombiners and igniters can effectively reduce the hydrogen concentration, so as to avoid the flame acceleration and the deflagrationto-detonation transition[12,13]. As shown in Fig. 9, when both recombiners and igniters are used, the average temperature and pressure in the room increase to 1700 K and 5.1 bar in the early stage, because the igniters burn large quantities of hydrogen. The temperature and pressure are not high enough to compromise the integrity of the room. After this brief period of high temperature and pressure, the temperature and pressure drop quickly, so that at about 200 s they are lower than the average temperature and pressure when only igniters were employed.

4

with late ignition, hydrogen detonation transition will quickly occur with high local thermal and pressure loads which will threaten the integrity of the containment. This paper also compares three hydrogen mitigation measures: igniters only, hydrogen recombiners only, and a combination of recombiners and igniters. Using igniters only does not remove the hydrogen effectively when hydrogen concentration is less than the flammability limit or when the steam concentration is too high. Using recombiners only cannot deal with the scenarios of high hydrogen release rates, so the risk of hydrogen deflagration and detonation increase at later times. The combination of recombiners and igniters can reduce hydrogen concentrations to below the flammability limit early in the accident, with the hydrogen safely, effectively, and continuously removed by recombiners at the middle and late stage. References [1]

NNSA. Technology policy about a few important safety problems in the design of a new nuclear power plant.

Conclusions

Large quantities of hydrogen will be generated in a hypothetical severe accident in a nuclear power plant. The hydrogen accumulation in the containment may lead to turbulent deflagration or detonation, which may threaten the integrity of the containment. This paper evaluates the risk of deliberate hydrogen ignition which includes determination of the hydrogen/steam source location and release rate, analysis of the 3-D hydrogen distribution, application of the σ and λ criteria to evaluate the flame acceleration and detonation potential at the time of ignition, optimization of the igniter system, and modeling of the diffusion flame and the deflagration progress. The σ and λ criteria are largely based on experimental research. For a postulated accident, hydrogen will accumulate in the upper region of the room because of buoyancy. Reasonable location of the igniter system and selection of the initial ignition time are critical to effective hydrogen removal and control of the hydrogen concentration and the high local thermal and pressure loads. Hydrogen can be removed by a slow diffusion flame, with flame acceleration and DDT excluded. With early ignition, the hydrogen will be eliminated by slow combustion without high thermal and temperature loads, but

557

Beijing: NNSA, 2002. (in Chinese) [2]

Xiong Benhe. Progression of severe accident research on nuclear power plants worldwide and preliminary consideration of severe accident strategies for LingAo phase II. Chinese Journal of Nuclear Safety, 2002, 3: 47-54. (in Chinese)

[3]

Breitung W. A mechanistic approach to safe igniter implementation for hydrogen mitigation. In: Proceedings of the OECDNEACSNI Workshop on the Implementation of Hydrogen Mitigation Techniques. Winnipeg, Manitoba, 1996.

[4]

Travis J R, Spore J W, Royl P, et al. GASFLOW: A computational fluid dynamics code for gases, aerosols and combustion. Volume 1: Theory and computational model. FzK and LANL, 2001.

[5]

Travis J R, Spore J W, Royl P, et al. GASFLOW: A computational fluid dynamics code for gases, aerosols and combustion. Volume 2: User manual. FzK and LANL, 2001.

[6]

Reynolds W C. The element potential method for chemical equilibrium analysis: Implementation in the interactive program STANJAN. Department of Mechanical Engineering, Stanford Universtiy, Palo Alto, CA, 1986.

[7]

Breitung W, Royl P. Procedure and tools for deterministic analysis and control of hydrogen behavior in severe acci-

Tsinghua Science and Technology, October 2006, 11(5): 549-558

558 dents. Nuclear Engineering and Design, 2000, 202: 249[8]

and overpressure protection of the containment for future

Sherman M P, Tieszen S R, Benedick W H. The effect of

boiling water reactors. Nuclear Engineering and Design,

obstacles and transvers venting on flame acceleration and

2000, 197(3): 281-299.

transition to detonation for hydrogen-air mixtures at large [9]

[11] Omoto A, Moriya K, Karasawa H. Hydrogen management

268.

[12] Heck R, Kelber G, Schmidt K, Zimmer H J. Hydrogen re-

scale. NUREG CR-5275, 1989.

duction following severe accidents using the dual recom-

Breitung W. SOAR on flame acceleration and deflagra-

biner-igniter concept. Nuclear Engineering and Design,

tion-to-detonation transition in nuclear safety at the

1995, 157(3): 311-319.

group’s final meeting at FzK. Karlsruhe, August, 1999.

[13] Morfin F, Sabroux J, Renouprez A. Catalytic combustion

[10] Rohde J, Chakraborty A K, Huttermann B. Hydrogen be-

of hydrogen for mitigating hydrogen risk in case of a se-

haviour and mitigation in water-cooled nuclear power re-

vere accident in a nuclear power plant: Study of catalysts

actors. In: Proc. Commission of the European Communi-

poisoning in a representative atmosphere. Applied Cataly-

ties, Bruxelles, 4-8 March, 1991. EUR 14039 EN, CEC,

sis B: Environmental, 2004, 47(1): 47-58.

Luxembourg, 1992: 224.

President of Duke University Awarded Honorary Doctorate of Tsinghua Professor Richard Brodhead, President of Duke University, was awarded Tsinghua University Doctorate on June 29，2006. Tsinghua President Gu Binglin attended the awarding ceremony held in the main building and awarded the diploma to Professor Brodhead. Tsinghua Vice President Chen Jining hosted the ceremony. After the ceremony, Professor Brodhead delivered a speech titled “Toward a New University: Research and Education in a Global Society.” “As we continue to train students in specialized disciplines, we must also work to cross-train them, to introduce them to other bodies of knowledge and methods of analysis and to challenge them in the arts of intellectual synthesis,” said Professor Brodhead. He noted that together with supplying more range to the expertise the teachers hope to impart, it is also important to promote the sharing or pooling of understandings, and teach the students how to combine their bits of knowledge with those of others in the service of larger goals. Tsinghua President Gu Binglin met and had a talk with Professor Brodhead prior to the ceremony, exchanging ideas on cooperation between the two universities. Richard H. Brodhead became Duke’s ninth president on July 1, 2004, after a 32-year career at Yale University. In addition to serving as president, he is a professor of English at Duke. （http://news.tsinghua.edu.cn）