Integrated Catalyst System for Removing Buildup-Gas in BWR Inert Containments During a Severe Accident

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1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1084 Integrated Catalyst System for Removing Buildup-Gas in BWR Inert Containments During a Severe Accident Kenji Arai *, Kazuo Murakami, Nagayoshi Ichikawa, Mika ahara, Ryoichi Hamazaki and Hirohide Oikawa oshiba Corporation 4-1, Ukishima-cho, Kawasaki-ku, Kawasaki, Japan A hydrogen removal system by catalysis has been being developed as a severe accident countermeasure for a boiling water reactor (BWR) in which the containment is inerted with nitrogen gas. he system is a hybrid catalyst system composed of a hydrogen-oxygen recombining catalyst and an ammonia synthesis ruthenium catalyst. he former contributes to preventing the atmosphere from exceeding the flammable limit by recombining the hydrogen and oxygen. he latter contributes to reducing the build-up gas by removing the hydrogen and the nitrogen. he reaction heat of the hydrogen-oxygen recombination promotes the ammonia synthesis. he paper describes catalyst performance tests to determine the catalyst material, the catalyst carrier material and the catalytic promoter, and also describes the catalytic system performance under a typical severe accident scenario with a MAAP analysis. KEYWORDS: severe accident, hydrogen removal, catalyst system, ammonia synthesis, ruthenium catalyst I. Introduction A hydrogen removal system by catalysis has been being developed as a severe accident countermeasure for a boiling 1), 2) water reactor (BWR). he system is a hybrid catalyst system composed of a hydrogen-oxygen recombining catalyst and an ammo nia synthesis catalyst. Why ammonia? A BWR primary containment vessel is inerted with nitrogen gas in order to prevent the atmosphere being over the flammable limit following an accident in which oxygen gas is produced by radiolysis of water. Even in a severe accident, the containment atmosphere can be, therefore, kept below the flammability limit by both the inerting and the flammability gas control system which recombines hydrogen with oxygen. However, a massive amount of hydrogen which is generated with a metal-water reaction and/or MCCI (molten core concrete interaction) in a severe accident cannot be removed and the pressure load to the containment vessel is maintained. he hydrogen removal system with the hybrid catalyst is aiming at removing the massive hydrogen gas as well as keeping the containment atmosphere below the flammable limit. his paper describes the concept of the integrated catalyst system for removing hydrogen, a catalyst performance test to determine the catalyst material and the analysis for the system performance during a severe accident. II. Hydrogen Removal System with Hybrid Catalyst In the hybrid catalyst system, the ammonia synthesis catalyst is placed above the hydrogen-oxygen recombination catalyst. igure 1 shows a typical arrangement of the catalysts. he ammonia synthesis catalyst combines hydrogen with nitrogen which is loaded in the containment for inerting in order to reduce the containment pressure. Ruthenium (Ru) catalyst is used since it can synthesize ammonia under a low-pressure and low-temperature condition which is typical to the BWR severe accidents. he hydrogen-oxygen recombining reaction by the recombination catalyst is an exothermic chemical reaction which preheats the hydrogen-oxygen-nitrogen mixture gas for promoting the ammonia synthesis reaction as well as removes oxygen for preventing the flammable gas combustion. A plate type configuration is preferable for the hydrogen-oxygen recombination catalyst since it generally causes a rapid and large temperature rise in reaction gases because of its excellent thermal conductivity. 3) It was reported that passive auto-catalytic recombiners using hydrogen-oxygen recombining catalysts can bring the gas temperature above 550K. 4) Outflow (H20, NH3, Reaction Heat) Inflow (H2, O2, N2) 1st stage heatup promotes 2nd stage reaction 2nd Stage Catalyst: (3/2H2+1/2N2 -> NH3) 1st Stage Catalyst: (H2+1/2O2 -> H2O) ig. 1 Concept of integrated catalytic recombiner * Corresponding author, el , ax , 1 kenji2.arai@toshiba.co.jp

2 Evaporator Mixer P Reaction Chamber catalyst Condenser Gas Chromatograph N 2 O 2 H 2 low Meter P Pressure gage hermocouple Off-gas reatment Sys. ig. 2 Catalyst test loop Since both of the recombination and the ammonia synthesis reactions are exothermic reactions, the hybrid catalyst system causes a natural circulation of the gas in the containment space and requires no active devices to perform its function. he hybrid catalyst system can be, therefore, a passive system. Both the depressurization of the containment vessel and the prevention of the combustion following a severe accident can be realized by the hybrid catalyst system. he chemical reactions are expressed by the following equations, respectively; H2 + 1/2 O2 -> H2O E8 [J/kmol-H2] (1) 3/2 H2 + 1/2 N2 -> NH E7 [J/kmol-H2]. (2) III. Catalyst Performance est he Ru catalyst performance test has been conducted to investigate the ammonia synthesis performance under typical severe accident conditions and to determine the catalyst carrier material and the catalytic promoter. 5) he test loop for the catalyst performance test is shown in ig. 2. Nitrogen (N2), oxygen (O2), hydrogen (H2) gases and steam can be supplied to the reaction chamber. he catalyst performance was, therefore, investigated under a forced flow condition. he reaction chamber contains pellet type catalysts in a fixed bed configuration. he hydrogen removal rate is measured by detecting the ammonia synthesis rate with using a gas chromatograph. he gas temperature is measured at five locations including the inlet, outlet and inside of the reaction chamber, as shown in ig. 2. In the first phase of the test, catalyst screening tests were conducted to determine the catalyst material, catalyst carrier material and catalytic promoter. Based on the screening test results, Ru-Cs/Al2O3 and Ru-Ba/Al2O3 catalysts were selected for the ammonia synthesis catalyst. In the second phase, the performance of the Ru catalyst was investigated. Major test conditions are summarized in able 1. he temperature range was determined considering the temperature rise with the H2-O2 recombination reaction heat in the 1-st stage catalyst. he dissociation of nitrogen molecular bond determines ammonia synthesis rate. Hence the dependence of the ammonia synthesis rate on the gas composition was investigated as well as the gas temperature. able 1 Major test conditions for catalyst test Gas pressure Inlet gas temperature MPa K Gas composition (H2/N2 ratio) he typical test results are presented in ig. 3. he hydrogen removal rate with ammonia synthesis can be correlated with the nitrogen mass flux to the catalyst which suggest the dissociation of nitrogen molecular bond determines the reaction rate. he data are reasonably correlated with the following Arrhenius type equation: x DR( H 2 ) K exp( E / ) M N 2 = (3), where DR(H2) is hydrogen removal rate, is catalysis temperature, MN2 is nitrogen mass flux and K, E, x are constants. he estimations using the above equation are shown in ig. 3 with the solid lines which provide a good fit to the test data. It is also shown that the reaction rate becomes pronounced where the temperature is above 573 K. In the test case using the hybrid catalysts composed of hydrogen-oxygen recombination catalyst (Pd) and ammonia synthesis catalyst (Ru), hydrogen-oxygen-nitrogen gas mixture was used. igure 4 shows the average gas temperature history in Pd and Ru catalysts when the inlet gas composition is 4% H2, 1.8% O2 and 94.2% N2. he inlet 2

3 gas velocity was maintained constant such that it approximately corresponds to the natural circulation velocity measured for the auto-catalytic recombiner. 4) he gas temperature was kept below 300 K at the catalyst inlet and was kept supplied to the catalyst. he gas temperature was increased without time delay by the reaction heat of the H2-O2 recombination reaction and reached around 600K in a half hour. he average gas temperature in the Ru catalyst which is placed downstream of the Pd catalyst followed the gas temperature history in the Pd catalyst and became above 550 K in a few hour. It is shown that the gas temperature in both catalysts kept increasing. rom these results, the ammo nia synthesis can be smoothly initiated and promoted by the H2-O2 recombination heat. IV. System Performance Analysis A MAAP analysis was conducted to evaluate the performance of the hydrogen removal system with the hybrid catalyst and the plant behavior following a severe accident. Based on the test results, an analysis model for the hybrid catalyst system has been developed and incorporated into the severe accident analysis code MAAP. Additional analysis models to account for the phenomena related to the flammable gas behavior have been incorporated to MAAP, as shown in ig Analysis Model (1) lammable gas generation model he original MAAP accounts only hydrogen generation by metal-water reaction and MCCI. he additional flammable gas generation by water radiolysis was incorporated considering both recombination and inhibition effect by hydrogen and iodine in water phase. 6) Hydrogen and oxygen generation by radiolysis is calculated based on decay heat fraction (considering β/γ ratio) and distribution of each P species. Effective generation rate (G-value) is a function of hydrogen and iodine concentration dissolved in water. (2) Integrated catalyst model he analysis model for the hybrid catalyst system evaluates Eq. (3) the gas temperature rise in the catalyst region, the resultant natural circulation flow caused by the reaction heat and the hydrogen removal rate by the hybrid catalyst. he H2-O2 recombination catalyst is assumed to be a plate type configuration considering the preferable heat convection characteristics. he ammonia synthesis catalyst is assumed to be a pellet type catalyst in a fixed bed configuration since it suppresses the natural circulation flow and causes a larger temperature rise in the catalyst region. he analysis model accounts for the effect of the catalyst configuration on the ig.3 NH3 catalyst test result natural circulation flow. ig.4 emperature history in hybrid catalyst 3

4 ig. 5 Phenomena related to flammable gas behaviour 2. Analysis Conditions A prototypical large ALWR plant with passive containment cooling system (PCCS) was selected for the long term analysis, and dedicated version of MAAP with PCCS model 7) was applied. Low pressure core damage sequence was selected as a representative scenario resulting ex-vessel termination of accident progression. Analysis conditions are summarized in able 2. he MAAP nodalization for the containment is shown in ig. 6. Rated hermal Power able 2 Major analysis condition 3926 (MW) Drywell / Wetwell Vol (m 3 ) / 6000 (m 3 ) Rated PCCS Capacity 50 (MW) Initial Gas Concentration N2 96%, O2 4% Catalyst volume 0.5 (m 3 ) 3. Analysis Result A comparison for the containment pressure response to the severe accident is illustrated in ig. 7, between the analysis cases with and without the hydrogen removal system. Due to the fuel cladding oxidation, a large amount of hydrogen is produced and causes a pressure rise in the reactor containment. In the case with the integrated catalyst system, the containment pressure can be reduced by about 200kPa and be kept around the design pressure (~ 400kPa). he partial pressure of each gas component is shown in ig.8. As is clearly seen, about two thirds of excessive hydrogen is depleted. In addition, the reduction in N2 partial pressure contributes to the reduction of the containment pressure. he ammonia (NH3) produced from the catalytic reaction of hydrogen and nitrogen, transfers into the suppression pool, and contributes to keep the pool water as low ph. he process has additional merit that the basic condition of pool water suppresses the release of volatile iodine to vapor phase, thus prevents the degradation of catalyst by iodine, and eventually reduces the leakage of fission product to environment. Steam Supply PCCS Main Steam eed RPV ailure RPV W/C/S/H/ S/RV S/H/ Break W/S/H/ Upper Drywell Condensate Return W/ V/B S/H/G/ Gas Vent S/H/G/ Wetwell Lower Drywell Connecting Vent W/C/S/H/G/ Main Vent W/S/H/G/ Suppression Pool W:,C:,S:Steam,H:Hydrogen,G:Gas(N2,CO2,CO),:ission Products ig. 6 MAAP nodalization 4

5 ig. 7 Long erm Containment Pressure ransient References 1) K. Arai et al. (2000). Excessive Hydrogen Removal System for Severe Accident, Annual Meeting of AESJ, N47, [in Japanese]. 2) M. Harada, M. ahara and K. Arai (1999). Hydrogen Depletion System, US patent ) C. ukuhara and A. Igarashi (2000). Plate-ype Catalytic Reaction System for Promotion of hermal Conductivity, J. of Catalyst Society of Japan, Vol. 42, No. 1 [in Japanese]. 4) K. Kobayashi et al. (2002). Application of Passive Autocatalytic Recombiner (PAR) for BWR Plants, J. of Atomic Energy Society of Japan, Vol. 1, No. 1 [in Japanese]. 5) K. Murakami et al. (2003). Development of Hydrogen Removal System for Severe Accident, 2003 Annual Mtg. of the Atomic Energy Society of Japan, L60 [in Japanese]. 6) M. ahara and H. Oikawa (1997). Investigation of Catalytic CS Performance Considering Plant System Behavior, 5th International Conference on Nuclear Engineering, Nice, rance, ICONE ) M. Akinaga and H. Oikawa (1998). Evaluation of Passive Containment Cooling System Performance during Severe Accidents, 6th International Conference on Nuclear Engineering, New Orleans, USA, ICONE ig. 8 Containment Partial Pressure ransient V. Conclusions A hybrid catalyst system composed of the H2-O2 catalyst and ammonia synthesis catalyst has been developed to remove the hydrogen produced during a severe accident in a BWR containment inerted with N2 gas. he catalyst performance test has been conducted to select the ammonia synthesis catalyst and to investigate the performance of the catalyst. It was shown that the performance is closely related to the N2 gas mass flux to the catalyst since the dissociation of the N2 molecular bond determines the ammonia synthesis rate. he Arrhenius type equation to predict the hydrogen removal rate has been developed for the ammonia synthesis catalyst. Based on the test results, the analysis model for the hybrid catalyst system has been developed and incorporated to the severe accident analysis code MAAP. he MAAP analysis result for a typical severe accident scenario has shown that the hybrid catalyst system has an substantial effect to reduce the containment pressure by removing the H2 and N2 partial pressures in the containment. 5

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