Analysis of hydrogen risk mitigation with passive autocatalytic recombiner system in CPR1000 NPP during a hypothetical station blackout

Hydrogen safety has attracted extensive concern in severe accident analysis especially after the Fukushima accident. In this study, a similar station blackout as happened in Fukushima accident is simulated for CPR1000 nuclear power plant (NPP) model, with the computational fluid dynamic code GASFLOW. The hydrogen risk is analyzed with the assessment of efficiency of passive autocatalytic recombiner (PAR) system. The numerical results show that the CPR1000 containment may be damaged by global flame acceleration (FA) and local detonation caused by hydrogen combustion if no hydrogen mitigation system (HMS) is applied. A new condensation model is developed and validated in this study for the consideration of natural circulation flow pattern and presence of non-condensable gases. The new condensation model is more conservative in hydrogen risk evaluation than the current model in some compartments, giving earlier starting time of deflagration to detonation transition (DDT). The results also indicate that the PAR system installed in CPR1000 could prevent the occurrence of the FA and DDT. Therefore, HMS such as PAR system is suggested to be applied in NPPs to avoid the radioactive leak caused by containment failure.     

Analysis of steam and hydrogen distributions with PAR mitigation in NPP containments

The 3-D-field code, GASFLOW is a joint development of Forschungszentrum Karlsruhe and Los Alamos National Laboratory for the simulation of steam/hydrogen distribution and combustion in complex nuclear reactor containment geometries. GASFLOW gives a solution of the compressible 3-D Navier–Stokes equations and has been validated by analysing experiments that simulate the relevant aspects and integral sequences of such accidents. The 3-D GASFLOW simulations cover significant problem times and define a new state-of-the art in containment simulations that goes beyond the current simulation technique with lumped-parameter models. The newly released and validated version, GASFLOW 2.1 has been applied in mechanistic 3-D analyzes of steam/hydrogen distributions under severe accident conditions with mitigation involving a large number of catalytic recombiners at various locations in two types of PWR containments of German design. This contribution describes the developed 3-D containment models, the applied concept of recombiner positioning, and it discusses the calculated results in relation to the applied source term, which was the same in both containments. The investigated scenario was a hypothetical core melt accident beyond the design limit from a large-break loss of coolant accident (LOCA) at a low release location for steam and hydrogen from a rupture of the surge line to the pressurizer (surge-line LOCA). It covers the in-vessel phase only with 7000 s problem time. The contribution identifies the principal mechanisms that determine the hydrogen mixing in these two containments, and it shows generic differences to similar simulations performed with lumped-parameter codes that represent the containment by control volumes interconnected through 1-D flow paths. The analyzed mitigation concept with catalytic recombiners of the Siemens and NIS type is an effective measure to prevent the formation of burnable mixtures during the ongoing slow deinertization process after the hydrogen release and has recently been applied in backfitting the operational German Konvoi-type PWR plants with passive autocatalytic recombiners (PAR).     

Three-dimensional behaviors of the hydrogen and steam in the APR1400 containment during a hypothetical loss of feed water accident

During a hypothetical severe accident in a nuclear power plant (NPP), hydrogen is generated by an active reaction of the fuel-cladding and the steam in the reactor pressure vessel and released with the steam into the containment. In order to mitigate hydrogen hazards which could possibly occur in the NPP containment, a hydrogen mitigation system (HMS) is usually adopted. The design of the next generation NPP (APR1400) developed in Korea specifies that 26 passive autocatalytic recombiners and 10 igniters should be installed in the containment for a hydrogen mitigation. In this study, an analysis of the hydrogen and steam behavior during a total loss of feed water (LOFW) accident in the APR1400 containment has been conducted by using the computational fluid dynamics (CFD) code GASFLOW. During the accident, a huge amount of hot water, steam, and hydrogen is released into the in-containment refueling water storage tank (IRWST). The current design of the APR1400 includes flap-type openings at the IRWST vents which operate depending on the pressure difference between the inside and outside of the IRWST. It was found from this study that the flaps strongly affect the flow structure of the steam and hydrogen in the containment. The possibilities of a flame acceleration and a transition from deflagration to detonation (DDT) were evaluated by using the Sigma–Lambda criteria. Numerical results indicate that the DDT possibility was heavily reduced in the IRWST compartment by the effects of the flaps during the LOFW accident.     

核电站安全壳内氢气扩散和燃烧的分析程序GASFLOW及其应用

介绍了由美国洛斯阿拉莫斯实验室(LANL)和德国卡尔斯鲁厄研究中心(FzK)共同开发的三维计算流体力学程序GASFLOW的基本数学物理模型和数值计算方法。该程序主要用于分析核电站严重事故下安全壳内氢气、水蒸气扩散分布和燃烧。列举了该程序在德国Konvio型压水堆氢气安全分析中的应用。     

Effect of spray on performance of the hydrogen mitigation system during LB-LOCA for CPR1000 NPP

During the course of the hypothetical large break loss-of-coolant accident (LB-LOCA) in a nuclear power plant (NPP), hydrogen is generated by a reaction between steam and the fuel-cladding inside the reactor pressure vessel (RPV). It is then ejected from the break into the containment along with a large amount of steam. Management of hydrogen safety and prevention of over-pressurization could be implemented through a hydrogen mitigation system (HMS) and spray system in CPR1000 NPP. The computational fluid dynamics (CFD) code GASFLOW is utilized in this study to analyze the spray effect on the performance of HMS during LB-LOCA. Results show that as a kind of HMS, deliberate igniter system (DIS) could initiate hydrogen combustion immediately after the flammability limit of the gas mixture has been reached. However, it will increase the temperature and pressure drastically. Operating the DIS under spray condition could result in hydrogen combustion being suppressed by suspended droplets inside the containment. Furthermore, the droplets could also mitigate local the temperature rise. Operation of a PAR system, another kind of HMS, consumes hydrogen steadily with a lower recombination rate which is not affected noticeably by the spray system. Numerical results indicate that the dual concept, namely the integrated application of DIS and PAR systems, is a constructive improvement for hydrogen safety under spray condition during LB-LOCA.     

氢气-空气混合气体燃烧特性试验分析

为了保证严重事故下安全壳的完整性,氢气点火器燃烧缓解措施被广泛应用于核电站内。本文在1个20m3立式圆柱罐体内进行9.28%浓度下的氢气燃烧试验,结合GASFLOW数值模拟和其他试验数据,对本次试验结果进行了综合分析。试验和模拟结果均表明:9.28%浓度下氢气完全燃烧,罐体内温度和压力快速增加;燃烧过程中罐体内高温气体通过辐射传热、对流换热和相变传热3种方式向罐体结构散热,使得罐体内温度和压力随时间逐渐降低,达到泄压和冷却的作用;燃烧过程有明显的方向性,即点燃后火焰在浮力作用下沿罐体中心线向上传播,到达顶部后转而沿罐体四周向下燃烧,燃烧初期火焰速度为11.15m/s;试验中由于内部构件的影响,火焰传播更为复杂。     

Severe accident analysis of a 1300 MWe PWR to address the hydrogen issue

The behaviour of the potentially large quantity of hydrogen generated during a severe accident has been recognised as an issue of importance since the accident at Three Mile Island. In this article, we describe a severe accident analysis for the Neckarwestheim 2 1300 MWe PWR “Konvoi” plant, performed primarily to investigate the behaviour of hydrogen in the containment, and draw conclusions regarding the need for hydrogen control systems (igniters). The Modular Accident Analysis Program (MAAP) developed by IDCOR in the United States, and the Westinghouse COMPACT multi-compartment containment code were used. The study investigated the generation, release to containment, distribution within containment and potential combustion of hydrogen produced during two severe accident sequences. Results are summarized which show that hydrogen mixing in containment is generally good and that even without hydrogen control systems, hydrogen combustion, although possible, does not threaten containment integrity.     

先进压水堆核电站氢气风险分析

核电厂在严重事故期间会产生大量氢气并释放到安全壳内,威胁安全壳的完整性。应用氢气风险分析程序GASFLOW对先进压水堆核电站在大破口失水事故叠加应急堆芯冷却系统失效导致的严重事故期间的氢气行为及风险进行分析。结果表明,当气体释放源位于蒸汽发生器隔间时,氢气流动的主要路径为"蒸汽发生器隔间—穹顶空间—操作平台以下隔间";破口隔间的氢气体积浓度分布与源项氢气体积浓度及射流形态有关,非破口区域的氢气体积浓度呈层状分布,在扩散作用下,层状分布向下推移;蒸汽发生器隔间存在着火焰加速(FA)的可能性,但基本可排除燃爆转变(DDT)的可能性,穹顶区域基本可排除FA和DDT的可能性。     

湍流模型对安全壳内氢气浓度场模拟的影响

利用计算流体力学程序FLUENT和GASFLOW研究了不同湍流模型下，氢气在安全壳内的传输与混合过程。计算结果表明：RNG k-ε模型能够得到较合理的结果，它能够较好的模拟氢气的质量扩散，动量扩散和湍流特征；FLUENT标准k-ε模型、标准k-ε模型和GASFLOW中k-ε模型能够在氢气浓度场分布上得到与RNGk-ε模型基本一致的结果，但由湍流导致的各种流动参数的波动不能在前三个模型中得到满意的模拟；GASFLOW中代数模型没能较好的模拟氢气的质量扩散和动量扩散，氢气的浓度场分布与其他模型的计算结果存在较大的差别。因此，选择合适的湍流模型，对于研究严重事故下安全壳内的氢气分布有重要的意义。     

Simulation of hydrogen deflagration and detonation in a BWR reactor building

A systematic study was carried out to investigate the hydrogen behaviour in a BWR reactor building during a severe accident. BWR core contains a large amount of Zircaloy and the containment is relatively small. Because containment leakage cannot be totally excluded, hydrogen can build up in the reactor building, where the atmosphere is normal air. The objective of the work was to investigate, whether hydrogen can form flammable and detonable mixtures in the reactor building, evaluate the possibility of onset of detonation and assess the pressure loads under detonation conditions. The safety concern is, whether the hydrogen in the reactor building can detonate and whether the external detonation can jeopardize the containment integrity. The analysis indicated that the possibility of flame acceleration and deflagration-to-detonation transition (DDT) in the reactor building could not be ruled out in case of a 20 mm² leakage from the containment. The detonation analyses indicated that maximum pressure spike of about 7 MPa was observed in the reactor building room selected for the analysis.     

秦山二期核电站氢气风险的CFD研究

利用计算流体力学(CFD)程序GASFLOW模拟了波动管大破口事故发生后7 000 s内装有22台氢气复合器的秦山二期核电站安全壳内的水蒸汽及氢气行为,得到了不同阶段的特征性流场及氢气浓度的分层情况,给出了所采用的复合器布置方案的稳定消氢速率为20 g/s,并指出了破口所在蒸汽发生器隔间内发生氢气燃烧火焰加速的可能性.同时,计算结果表明,安全壳内构筑物吸热带走了大部分从一回路释放的热量;压力变化同时受气体总质量(主要是水蒸汽质量)与温度的控制.     

严重事故氢气燃爆缓解措施的初步研究

轻水堆核电站发生严重事故时,氢气的大体积氢燃爆可能会严重威胁安全壳的完整性.氢气点火器与氢气复合器是2种严重事故下的氢气燃爆缓解设备.本文分别研究了3种氢气燃爆缓解措施,包括仅采用氢气点火器、仅采用氢气复合器和采用氢气复合器结合点火器.结果表明,采用氢气复合器结合点火器的方式可以安全、持续、有效地降低大体积氢燃爆带来的风险.     

CFD evaluation of hydrogen risk mitigation measures in a VVER-440/213 containment

In the PHARE project “Hydrogen Management for the VVER440/213” (HU2002/000-632-04-01), CFD (Computational Fluid Dynamics) calculations using GASFLOW, FLUENT and CFX were performed for the Paks NPP (Nuclear Power Plant), modelling a defined severe accident scenario which involves the release of hydrogen. The purpose of this work is to demonstrate that CFD codes can be used to model gas movement inside a containment during a severe accident. With growing experience in performing such analyses, the results encourage the use of CFD in assessing the risk of losing containment integrity as a result of hydrogen deflagrations. As an effective mitigation measure in such a situation, the implementation of catalytic recombiners is planned in the Paks NPP. In order to support these plans both unmitigated and recombiner-mitigated simulations were performed. These are described and selected results are compared. The codes CFX and FLUENT needed refinement to their models of wall and bulk steam condensation in order to be able to fully simulate the severe accident under consideration.Several CFD codes were used in parallel to model the same accident scenario in order to reduce uncertainties in the results.Previously it was considered impractical to use CFD codes to simulate a full containment subject to a severe accident extending over many hours. This was because of the expected prohibitive computing times and missing physical capabilities of the codes. This work demonstrates that, because of developments in the capabilities of CFD codes and improvements in computer power, these calculations have now become feasible.     

不同产氢速率对安全壳内氢气分布影响的数值模拟

利用计算流体力学软件（CFX），初步研究了严重事故下氢气在安伞壳空间内的流动特性，分析了不同产氢速率对安全壳内氢气分布的影响。结果表明：各种氢气释放速率情况下，氢气分布的基本趋势一致；不同的产氢速率对氢气分布的影响主要体现在氢气运动到安伞壳穹顶时所形成的涡旋小同，氢气释放速率低的序列，氢气容易滞留在穹顶，然后向下慢慢扩散，分布较均匀；氢气释放速率高的序列的氢气运动方向性强，容易向下空间运动，分布的区域集中些，分层现象明显。     

Analysis on hydrogen risk mitigation in severe accidents for Pressurized Heavy Water Reactor

Hydrogen source term and hydrogen mitigation under severe accidents is evaluated for most nuclear power plants (NPPs) after Fukushima Daiichi accident. Two units of Pressurized Heavy Water Reactor (PHWR) are under operating in China, and hydrogen risk control should be evaluated in detail for the existing design. The distinguish feature of PHWR, compared with PWR, is the horizontal reactor core surrounded by moderator in calandria vessel (CV), which may influence the hydrogen source term. Based on integral system analysis code of PHWR, the plant model including primary heat transfer system (PHTS), calandria, end shield system, reactor cavity and containment has been developed. Two severe accident sequences have been selected to study hydrogen generation characteristic and the effectiveness of hydrogen mitigation with igniters. The one is Station Blackout (SBO) which represents high-pressure core melt accident, and the other is Large Break Loss of Coolant Accident (LLOCA) at reactor outlet header (ROH) which represents low-pressure core melt accident. Results show that under severe accident sequences, core oxidation of zirconium–steam reaction will produce hydrogen with deterioration of core cooling and the water in CV and reactor cavity can inhibits hydrogen generation for a relatively long time. However, as the water dries out, creep failure happens on CV. As a result, molten core falls into cavity and molten core concrete interaction (MCCI) occurs, releasing a large mass of hydrogen. When hydrogen igniters fail, volume fraction of hydrogen in the containment is more than 15% while equivalent amount of hydrogen generate from a 100% fuel clad-coolant reaction. As a result, hydrogen risk lies in the deflagration–detonation transition area. When igniters start at the beginning of large hydrogen generation, hydrogen mixtures ignite at low concentration in the compartments and the combustion mode locates at the edge of flammable area. However, the power supply to igniters should be ensured.     

先进压水堆大破口始发严重事故下安全壳内氢气风险分析

本文采用集总参数法,在先进非能动压水堆核电厂严重事故一体化分析模型基础上,考虑先进压水堆非能动安全特性以及严重事故下采取熔融物堆内滞留(IVR)措施等特性对氢气风险的影响,开展了典型严重事故下安全壳内氢气风险分析。分别选取了冷段双端剪切断裂大破口、冷段大破口叠加IRWST重力注水有效以及ADS-4误启动三个典型大破口失水事故序列,对事故进程中的氧化温度、产氢速率以及产氢质量等特性进行了研究。选取产氢量最大的冷段大破口叠加IRWST重力注水有效事故序列,分析了氢气点火器系统的消氢效果。结果表明,堆芯再淹没过程产生大量氢气,采用点火器可有效去除安全壳内的氢气,从而降低氢气燃爆风险。     

GASFLOW程序液膜模型开发及初步验证

本文基于三维CFD安全壳程序GASFLOW开发了热构件壁面上的液膜覆盖与蒸发模型。通过选定AP1000大破口事故序列,采用耦合了液膜模型的GASFLOW程序分析了AP1000核电厂安全壳内温度压力响应及其非能动安全壳冷却系统(PCS)的性能,并与相同事故序列下WGOTHIC、MELCOR、CONTAIN等程序的计算结果进行比较。结果表明,耦合了液膜模型的GASFLOW程序可用于分析PCS的热工水力行为,其基本功能满足计算需要。     

Laser Rayleigh measurement of mixing processes and control of hydrogen combustion using quenching meshes

Two issues concerning hydrogen combustion under a severe accident scenario are addressed: (1) a laser Rayleigh scattering technique to investigate hydrogen mixing processes; and (2) the installation of metallic meshes between compartments to control and isolate hydrogen combustion within a single compartment. The Rayleigh scattering techniques are tested to determine hydrogen/air mixing processes locally and temporally as a non-intrusive probing method. To simulate mixing processes, helium is injected into a chamber filled with n-butane. Results show that helium concentration can be successfully monitored with sufficiently fast responses. Isolation and control of hydrogen burning is simulated by installing metallic meshes between compartments. Hydrogen is injected into one compartment and subsequently transported to the second compartment. Two sets of experiments are conducted with and without installing metallic meshes between the compartments. With the mixture ignited near the second compartment outlet, hydrogen combustion can be successfully contained within the second compartment with meshes, while flame propagates to the first compartment when meshes are not installed. These results demonstrate that hydrogen combustion can be controlled and isolated by installing meshes locally such that unwanted rapid pressure rise in a containment can be prevented. It also suggests the applicability of meshes for equipment survivability and protection from flame propagation by enclosing equipments with properly designed meshes.     

先进压水堆核电厂氢气控制策略分析研究

新建核电厂的设计必须做到“实际消除”早期与大量放射性释放的可能性，氢气燃爆导致的安全壳失效是必须要“实际消除”的严重事故工况之一。因此对各种消氢措施的特点进行分析研究，建立联合消氢策略评价方法，可为先进压水堆核电厂氢气控制策略选择设计评价提供支持手段。根据严重事故管理中对氢气控制策略的考虑，研究安全壳内局部位置的可燃性是相关设计评价的关键问题。根据可燃性准则、火焰加速准则、燃爆转变准则，本文使用三维CFD程序对典型严重事故工况下安全壳蒸汽发生器隔间内的可燃性及氢气风险进行模拟分析。研究结果表明，虽然喷放源项中有大量水蒸气，蒸汽发生器隔间中仍有较大区域处于可燃限值以内，合理布置的点火器能在设计中点燃并消除氢气。本研究建立的分析方法能用于对核电厂氢气控制策略选择设计的评价。     

模块式高温气冷堆进水事故后的氢气行为分析

蒸汽发生器两根传热管双端断裂是模块式高温气冷堆（HTR-PM）典型的超设计基准事故,事故可能会导致氢气在反应堆舱室内的聚集,产生爆燃甚至爆轰的风险。本文使用反应堆流体计算程序GASFLOW,模拟了两根传热管断裂后排放到蒸汽发生器舱室以及反应堆舱室内的气体的输运及分布,并利用氢气燃爆分析程序COM3D进行了舱室内的氢气燃烧分析。计算结果表明,两根传热管断裂事故排放的氢气含量很小,舱室内氢气浓度最大不超过0.1%,如此低浓度的氢气不会发生燃烧爆炸。