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.     

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.     

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。     

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。     

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。     

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.     

Analysis of strategies for containment venting in case of severe accidents

During a core melt accident, a pressurization of the containment has to be expected, which could lead to a failure of the containment due to overpressurization. This failure mode is expected to be the most likely one for large dry containments under accident conditions. Also during a core melt accident, a large quantity of hydrogen may be generated, giving the potential of a loss of containment integrity due to violent hydrogen combustion. Timely venting of the containment atmosphere can prevent overpressurization and may perhaps make the hydrogen situation in the containment less severe. This paper discusses the thermodynamic consequences of different vent strategies for a large German PWR during core melt accidents.     

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.     

A study on evaluating a passive autocatalytic recombiner PAR-system in the PWR large-dry containment

A systematic step-by-step framework for analyzing hydrogen behavior and implementing passive autocatalytic recombiners (PARs) to mitigate hydrogen deflagration or detonation risk in severe accidents (SAs) is presented. The procedure can be subdivided into five main steps: (1) modeling the containment based on the plant design characteristics, (2) selecting the typical severe accident sequences, (3) calculating the hydrogen generation including in- and ex-vessel period, (4) modeling the gas distribution in containment atmosphere and estimating the hydrogen combustion modes and (5) evaluating the efficiency of the PAR-system to mitigate the hydrogen risk with and without catalytic recombiners, according to the safety criterion. For the Chinese 600MWe pressurized water reactor (PWR) with a large-dry containment, large break loss-of-coolant accident (LB-LOCA) is screened out as the reference severe accident sequence, considering the nature of hydrogen generation and the probabilistic safety assessment (PSA) result on accident sequences. The results show that a certain number of recombiners could remove effectively hydrogen and oxygen, to protect the containment integrity against hydrogen deflagration or detonation.     

The design and evaluation of a passively cooled containment for a high-rating pressurized water reactor

An integrated pressurized water reactor (PWR) containment was conceptualized that allows heat to be rejected passively to the environment. The proposed containment is based on the demonstrated Ebasco Waterford 3 design. The secondary concrete shell was equipped with inlet and outlet vents that create an air-convection annulus. These vents also permit the submersion of the lower part of the primary containment into an external water pool. An internal water pool located at the bottom of the lower containment was added to increase in-containment heat storage. The performance of the proposed passively cooled containment was evaluated using a subdivided volume code, version 3.4e; the relative novelty of subdivided volume analyses for containment performance evaluation requires experimental verification of principal code predictions. Two experiments were carried out; one to test the performance of the external moat, and one to verify the code’s ability to predict thermal-stratification inside the containment. To improve the subdivided-volume simulation of convection-related parameters, a modeling technique (boundary layer flow approximation) was devised. Finally, the behavior of the proposed containment was evaluated for the worst-case large break loss of coolant accident and the worst-case main steam line break accident. Peak pressures remained below 0.45 MPa during both transients; internal wall pressure differences, equipment qualification temperatures, pressure restoration time also remained below design limits. The mitigation capability of hydrogen recombiners was also evaluated.     

Analysis of hydrogen flame acceleration in APR1400 containment by coupling hydrogen distribution and combustion analysis codes

This study was conducted as part of the construction of an integrated system to mechanistically evaluate flame acceleration characteristics in a containment of a nuclear power plant during a severe accident. In the integrated analysis system, multi-dimensional hydrogen distribution and combustion analysis codes are used to consider three-dimensional effects of the hydrogen behaviors. GASFLOW is used for the analysis of a hydrogen distribution in the containment. For the analysis of a hydrogen combustion in the containment, an open-source CFD (computational fluid dynamics) code OpenFOAM is chosen. Data of the hydrogen and steam distributions obtained from a GASFLOW analysis are transferred to the OpenFOAM combustion solver by a conversion and interpolation process between the solvers. The combustion solver imports the transferred data and initializes the containment atmosphere as an initial condition of a hydrogen combustion analysis. The turbulent combustion model used in this study was validated by evaluating the F22 test of the FLAME experiment. The coupled analysis method was applied for the analysis of a hydrogen combustion during a station blackout accident in an APR1400. In addition, the characteristics of the flame acceleration depending on a hydrogen release location are comparatively evaluated.     

Efficacy assessment of independent severe accident mitigation measures in OPR1000 using MELCOR code

In-vessel and ex-vessel mitigation strategies have been revisited to improve the severe accident management (SAM) for operating nuclear power plants. Because independent mitigation measures tend to produce positive and adverse effects simultaneously, it is necessary to investigate the efficacy of individual measures by means of proper quantification. Thus, in the present study we investigated the overall efficacy of existing SA mitigation strategies prepared for the Optimized Power Reactor 1000 MWe (OPR1000) by means of MELCOR 1.8.6 code. The numerical evaluation showed that the Mitigation-01, feeding water into the steam generators, is the most effective among the other mitigations. In addition, Mitigation-02, reactor coolant system depressurization, could not mitigate the SA sufficiently when applied individually. Among the four ex-vessel mitigation strategies, execution of containment spray was effective in removing most of the aerosol fission product but also intensified hydrogen combustion by increasing the partial hydrogen pressure owing to steam condensation. Mitigation-07, operation of passive autocatalytic recombiners (PARs), could reduce the hydrogen concentration, though the catalytic reaction was predicted to increase the containment pressure. In conclusion, this study suggests that mitigation measures should be carefully selected, and that counteracting measures should be prepared to minimize potential adverse effects.     

双层安全壳压水堆LOCA事故的放射性后果分析审查

压水堆冷却剂丧失设计事故后果分析主要涉及释放源项、大气弥散和场外辐射后果三个方面。针对具有双层安全壳的压水堆冷却剂丧失设计基准事故,简述了放射性后果分析方法,计算了场外放射性后果,并对放射性后果审查中应当注意的几个问题进行讨论：（1）事故源项。通常可分别考虑为释放到内层安全壳的源项,以及释放到环境的源项。前者主要取决于堆芯裂变产物的积存量,后者不仅与释放到内层安全壳的裂变产物有关,而且与自然的和工程的清除过程以及外层安全壳的缓解作用密切相关。（2）大气弥散因子的计算。根据环境资料的获取情况,可以按照NRCR．G．1．4给出的确定论方法进行保守的估计,也可以按照NRCR．G．1．145描述的概率论方法进行估算。（3）场外放射性后果。主要考虑隔离区边界和低人口区外边界的个人剂量,包括全身剂量和甲状腺剂量。     

Fluid dynamic analysis of a catalytic recombiner to remove hydrogen

Catalytic reacting surfaces in recombiners are a reliable way to remove hydrogen as well as other burnable gases like CO in a passive way from the containment atmosphere of a nuclear power plant (NPP) during an accident. Industrial mature designs are ready to be installed in large dry containments to act as a mitigation measure preferably in the case of severe accidents. Experiments have been carried out to study manifold aspects of recombiners like the efficiency of hydrogen removal, start-up conditions, poisoning, oxygen starvation, steam and water impact and others. Mostly the global behaviour of a given device in a larger environment has been investigated in order to demonstrate the effectiveness and to facilitate the derivation of simplified models for long-term severe accident analyses. There are a number of reasons to look inside a recombiner to understand the interaction of chemistry and flow. This can help in understanding the dependencies of non-measurable variables (e.g. reaction rate), of local surface temperatures and more. It also offers possibilities to increase the chemical efficiency by optimising the geometry properly. Computational fluid dynamics (CFD) codes are available to be used as development tools to include the specifics of catalytic surface reactions. The present paper describes the use of the code system CFX (CFX 4.1 Flow Solver User Guide. 1995, Computational Fluid Dynamics Services, AEA Technology plc, Oxfordshire, UK) for creating a recombiner model. Finally its comparison with existing test data is discussed.     

Innovative devices for hydrogen removal

In case of a severe accident in light water reactors (LWR) a high amount of hydrogen, up to about 20 000 m_n³, might be generated and released into the containment. The mixture, consisting of hydrogen and oxygen, may either burn or detonate, if ignited. In case of detonation the generated shock wave may endanger the integrity of the containment or safety-related systems. Consequently, an effective removal of hydrogen is required. Hydrogen and oxygen react exothermally at catalytically acting surfaces, already at room temperature, and this is used in catalytic recombiners. It is recommended to combine recombiners with spark or catalytic igniters, in order to cover a broader spectrum of accident sequences. In this contribution, state of the art of hydrogen removal devices are reviewed and the possibilities for innovative methods, making use of the phenomena arising in the containments, using further components will be illustrated accordingly.     

Thermodynamic consequences of leakages in a double containment during severe accidents

For a large nuclear power plant under normal operating conditions a leakage rate for the containment of 0.25 vol.%/day is admissible. During a successfully controlled LOCA leakages of the containment will be released through filters by the annulus^* air exhausting system into the environment. During a core melt accident a pressurization of the containment has to be expected, which could lead to a failure of the containment due to overpressurization. When openings in the containment steel shell occur before a catastrophic failure, a depressurization into the annulus takes place. The area of the openings determines the depressurization rate and the thermodynamic conditions in the annulus. Furthermore the behaviour of the components being necessary for accident mitigation is influenced too. This paper discusses the thermodynamic consequences of leaks in the containment shell of a German PWR during a core melt accident. The results of those calculations are the necessary boundary condition for the estimation of fission product retention in the annulus.     

Ignitors to mitigate the risk of hydrogen explosions — A critical review

Risk analyses and the accident at the Three Mile Island plant (TMI-2) have shown that the formation of large amounts of hydrogen during severe accidents poses a real danger to the containment integrity. Proper means must be taken to prevent the occurance of global or sometimes even local detonations. The applicability of ignitor systems to protect large dry PWR containments is critically discussed. Although already adopted for Mark III-BWR and Ice-condenser PWR-containments the reliability of spark plug ignitor systems to protect large dry containments from the possible consequences of a local or global detonation is neither proven experimentally nor analytically. An experimental study of possible post-accident inertisation procedures deems necessary and may yield a more convincing mitigation procedure.     

严重事故管理导则中的氢气风险判断及事故模拟分析

建立严重事故管理导则中用于判断氢气燃烧、超压风险以及安全壳降压时氢气风险的判断工具。用一体化事故分析程序对全厂断电事故进行模拟计算,用该氢气风险判断工具对不同事故阶段的氢气风险进行分析。结果表明:在全厂断电始发的严重事故下,没有氢气复合器且没有安全壳喷淋时,安全壳大气在一段时间内会被水蒸气惰化,不会发生燃烧,但如果应急电源恢复,重新启动安全壳喷淋时,有可能引起氢气燃烧甚至造成安全壳超压;在增加氢气复合器后,没有造成安全壳超压的风险,并且判断结果是保守的。     

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。     

基于CFD的安全壳局部隔间非能动氢气复合器布置方案研究

非能动氢气复合器用于压水堆核电厂严重事故条件下安全壳内氢气的消除。通过计算流体力学(CFD)方法能够给出事故条件下非能动氢气复合器周围三维流场和温度场的分布。基于CFD程序根据非能动氢气复合器消氢公式,计算非能动氢气复合器进出口的气体流量和气体组分,并作为非能动氢气复合器的边界条件,开展三维空间内非能动氢气复合器消氢速率和氢气分布情况研究。结果表明：简化的非能动氢气复合器模拟方案能很好地模拟非能动氢气复合器样机的消氢效果;对安全壳内局部隔间开展非能动氢气复合器消氢效果研究发现,在相同环境条件下,非能动氢气复合器布置在较高位置与布置在较低位置相比,布置在较高位置时,非能动氢气复合器具有更高的消氢速率,隔间整体氢气浓度较低,但是非能动氢气复合器布置在较高位置时出现隔间底部局部氢气聚集的情况。