小尺度空间内氢气流动分布特性数值研究

与核电厂安全壳大空间不同,安全壳隔间以及先进小型堆等小尺度空间中,氢气与水蒸气的混合气体流动受到壁面的限制,气流不能充分发展,可能导致氢气在某些位置积聚引发氢气风险。本文采用数值模拟与理论分析相结合的方法对小尺度空间内氢气流动分布特性进行了研究。研究发现,典型工况下小尺度空间上部形成了氢气浓度分布比较均匀的氢气浓度储备区,在中部和下部区域分别为氢气浓度过渡区和高空气浓度区;随着源项气体动量的增大,源项气体进入上部空间的能力增大,导致空间上部区域氢气浓度增大。本研究可为后续先进小型堆的氢气风险研究分析提供支持。      

基于Gasflow程序的非能动安全压水堆氢气行为计算和分析

本文利用Gasflow程序对非能动压水堆发生假想的严重事故后,安全壳内的氢气流动、分布和积聚行为进行了计算和分析,对安全壳内各房间的氢气风险进行了评价并给出了降低氢气燃烧风险的建议。计算结果表明,在发生大破口事故中,安全壳内氢气浓度较高的区域为破损蒸汽发生器隔间,内置换料水箱隔间和上部隔间,需要设置消氢系统来降低隔间内的氢气浓度。     

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。     

M310核电厂严重事故下稳压器隔间氢气风险分析

基于GASFLOW程序,选取对M310核电厂稳压器隔间内氢气风险极为不利的两种事故工况,对安全壳内氢气风险进行了分析计算。模拟结果显示:在所研究的工况条件下,卸压箱隔间、波动管隔间、稳压器隔间及穹顶区域内,只有波动管双端断裂事故在早期氢气集中释放阶段,出现了稳压器隔间内FA准则数大于1的情况,其他隔间及其他工况下所有隔间内的FA准则数和DDT准则数均不会超过1。即,所研究隔间内均可以排除燃爆转变风险。破口隔间内部氢气浓度分布主要受源项氢气浓度以及混合气体夹带作用的影响,不同位置的氢气浓度变化存在显著差别。安全壳大空间的氢气浓度呈层状结构,随着时间推移,层状结构向下推移,安全壳大空间氢气浓度分布呈均匀化趋势发展。     

严重事故下安全壳内氢气浓度场分布

利用计算流体力学程序FLuENT和GASFLOW,采用不同的湍流模型,研究了核电站严重事故下氢气在安全壳内的传输与混合过程.计算结果表明,FLUENT中的RNG k-ε模型能够较好的模拟氢气的质量扩散,动量扩散和湍流脉动特征;FLUENT中的标准k-ε模型和GASFLOW中的k-ε模型能得到工程上可以接受的计算结果;而GASFLOW中代数模型未能较好地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.同时,本文对混合气体中的水蒸汽浓度和气体的质量流速对安全壳内氢气浓度分布的影响进行了初步研究.研究表明,破口气体的密度和流速是影响氢气浓度场的重要因素;混合气体密度越小、流速越大,则有更大的浮力和初始动量作用于气体.湍流模型的选择和对浮力驱动的湍流射流的模拟是影响严重事故下氢气在安全壳内的分布模拟结果的重要因素.     

严重事故下安全壳内氢气浓度场分布影响因素的初步研究

利用计算流体力学程序FLUENT和GASFLOW，采用不同的湍流模型，研究了核电站严重事故下氢气在安全壳内的传输与混合过程。计算结果表明，FLUENT中的RNGk-ε模型能够较好的模拟氢气的质量扩散，动量扩散和湍流脉动特征；FLUENT中的标准k-ε模型和GASFLOW中的k-ε模型能得到工程上可以接受的计算结果；而GASFLOW中代数模型未能较好地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。同时，本文对混合气体中的水蒸汽浓度和气体的质量流速对安全壳内氢气浓度分布的影响进行了初步研究。研究表明，破口气体的密度和流速是影响氢气浓度场的重要因素；混合气体密度越小、流速越大，则有更大的浮力和初始动量作用于气体。湍流模型的选择和对浮力驱动的湍流射流的模拟是影响严重事故下氢气在安全壳内的分布模拟结果的重要因素。     

小型堆严重事故下安全壳内氢气行为分析

采用MELCOR程序,对小型堆破口叠加全部电源丧失的典型严重事故进行计算,并对安全壳内发生氢气燃烧、爆炸的可能性进行分析。结果表明:主管道直径3.72%的破口叠加全部电源丧失后,堆芯裸露,出现熔堆事故;同时锆水反应产生的大量氢气进入安全壳,使安全壳内氢气含量上升,在安全壳局部空间、屏蔽水箱内出现氢气燃烧。但由于小型堆安全壳净容积较小,水蒸气含量较高,氧气含量较少,不会导致氢气爆炸。     

秦山二期核电厂严重事故下安全壳内氢气浓度分布及风险初步分析

采用模块化严重事故计算工具,对秦山二期核电厂大破口失水事故(LB-LOCA)、小破口失水事故(LB-LOCA)和全厂断电(SBO)诱发的严重事故序列以及安全壳内的氢气浓度分布进行了计算分析.在此基础之上,参考美国联邦法规10CFR关于氢气控制和风险分析的标准,对安全壳的氢气燃烧风险进行了初步研究.分析结果表明:大破口严重事故导致的安全壳内的平均氢气浓度接近10%,具有一定的整体性氢气燃烧风险,小破口失水和全厂断电严重事故可能不会导致此类风险,但仍然存在局部氢气燃烧的可能.     

大型先进压水堆安全壳内水蒸气迁移的数值模拟研究

利用计算流体力学(CFD)方法,采用Uchida关联式模拟水蒸气冷凝研究大型先进压水堆一回路管道破口发生后水蒸气在安全壳内的迁移情况,分析不同冷却剂泄漏率下水蒸气的分布情况。结果表明:各冷却剂泄漏率下,水蒸气分布趋势基本一致;安全壳壁面附近水蒸气浓度随时间波动上升,且较高位置的浓度波动较小。     

严重事故下安全壳内氢气行为与风险分析

福岛事故后的核电厂安全审评过程中,国家核安全局对于严重事故下的氢气安全问题提出了更高的要求,从满足当前高标准的氢气安全要求的角度出发,有必要对安全壳内氢气行为开展更为细致深入的研究,开展氢气的三维分析,为集总参数程序的分析结果提供有益补充。本文采用一体化严重事故分析程序和流体力学程序对国产先进压水堆核电厂进行系统建模,选取大破口触发的严重事故序列,对严重事故工况下的氢气行为及氢气控制系统性能进行分析评价。首先采用一体化严重事故分析程序计算氢气产生源项、氢气产生速率和安全壳内氢气浓度分布等,评价安全壳隔间内的氢气风险。并采用计算流体力学程序,进一步对安全壳内重要隔间的氢气分布进行三维分析,研究安全壳内氢气和水蒸汽的行为,获得重要隔间内的流场、温度场、压力场、氢气分布及浓度变化等计算结果。CFD程序在计算气体分布方面要比集总参数程序更加精确和详细,通过更精细地模拟安全壳内的氢气行为,可以为集总参数程序的计算结果提供补充,为氢气控制系统的设计优化和严重事故氢气风险管理等提供有力的支持。     

水蒸气对氢气燃烧影响的数值研究

核电站严重事故下,氢气的燃烧风险是影响安全壳完整性的重要因素,而水蒸气的存在对氢气、空气混合气体的燃烧会产生重要的影响。本文采用CAST3M软件,对局部小空间内氢气的燃烧特性以及水蒸气的影响进行研究。首先对THAI装置的典型实验工况进行模拟,表明了相关燃烧模型的可用性。然后将高度为6 m、直径为2.2 m的圆柱空间作为燃烧域,对其分别计算了8%、10%、12%氢气浓度下的燃烧,并与添加25%水蒸气的相应工况进行了对比。通过对燃烧域的温度、压力以及火焰传播速度的分析,表明添加水蒸气后燃烧产生的最大压力下降,火焰的最大温度下降,火焰传播的速度下降。研究表明,水蒸气的存在对氢气的燃烧具有抑制作用,能有效降低氢气燃烧产生的后果。     

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).     

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.     

Hydrogen and steam distribution following a small-break LOCA in large dry containment

The hydrogen deflagration is one of the major risk contributors to threaten the integrity of the containment in a nuclear power plant, and hydrogen control in the case of severe accidents is required by nuclear regulations. Based on the large dry containment model developed with the integral severe-accident analysis tool, a small-break loss-of-coolant-accident （LOCA） without HPI, LPI, AFW and containment sprays, leading to the core degradation and large hydrogen generation, is calculated. Hydrogen and steam distribution in containment compartments is investigated. The analysis results show that significant hydrogen deflagration risk exits in the reactor coolant pump （RCP） compartment and the cavity during the early period, if no actions are taken to mitigate the effects of hydrogen accumulation.     

Improvement of HYCA3D code and experimental verification in rectangular geometry

Concerns about the local hydrogen behavior in a nuclear power plant (NPP) containment during severe accidents have increased with the 10CFR50.34(f) regulation after TMI accident. Consequently, investigations on the local hydrogen behaviors under severe accident conditions were required. An analytical model named HYCA3D was developed at Seoul National University (SNU) to predict the thermodynamics and the three dimensional behavior of a hydrogen/steam mixture, within a subdivided containment volume following hydrogen generation during a severe accident in NPPs. In this study, the HYCA3D code was improved with a steam condensation and spray model, and verified with hydrogen mixing experiments executed in a SNU rectangular mixing facility. Helium was used to simulate hydrogen in both the calculations and the experiments. The calculation results show good agreement with the experimental data.     

安全壳大空间内氢气分层行为的模型研究

基于蒸汽/氢气混合喷放下安全壳大空间内氢气分层行为的主导机制——惯性力、粘性力及浮升力间的相互作用,通过理论建模与实验拟合的方法,得到了预测氢气分布特性的半经验关系式,通过与环境中喷入中等蒸汽浓度及高蒸汽浓度实验数据的比较,验证了该模型的合理性,可为后期耦合安全壳内蒸汽冷凝行为影响下的氢气分布理论模型的开发提供辅助支撑;同时,通过将其应用于CAP1400缩比安全壳模型中典型氢气行为的研究发现,在容器轴向位置可能形成轻质气体积聚区、浓度梯度区及滞止区,该结果与国际基准实验（ISP47）的相关发现一致。      

安全壳模型装置内氢气分布特性及影响因素分析

采用计算流体力学方法,首先利用THAI HM-2实验对CFX分析模型的适用性进行验证,通过与实验数据的比对,表明计算结果与实验数据基本吻合,从而验证选用的模型适合对安全壳模拟装置氢气分布特性的分析。之后,建立待研究中等规模安全壳模型实验装置的三维几何模型和网格模型,采用基准工况+单因素对比的方式,分别模拟湍流浮力射流中心喷射和近壁面喷射工况以及考虑蒸汽壁面冷凝情况下安全壳模型内的氦气(氢气替代工质)流动扩散分布,讨论喷射位置因素、壁面蒸汽凝结效应对氦气分布的影响。分析结果表明,喷射位置对氦气分布的影响主要体现在壁面引流现象上,即氦气流更倾向于沿着安全壳壁面进行流动和扩散;而与安全壳壁面的换热和蒸汽的冷凝会进一步促进大空间自然对流的建立,从而较为显著地提高氦气在安全壳内的扩散和混合效果。