Flow transport and mixing induced by horizontal jets impinging on a vertical wall of the multi-compartment PANDA facility

In the frame of the OECD/NEA SETH project an experimental campaign has been carried out in the PANDA facility to investigate gas transport and mixing induced by a plume or a jet in the large-scale multi-compartment PANDA facility. The paper summarizes the results of the horizontal jet test series consisting of eight tests. Horizontal jets impinging on a vertical wall of one of the cylindrical PANDA containment vessels have been generated by changing various parameters, such as: type of injected fluid (steam or a mixture of steam and helium), fluid injection velocity, elevation (with respect to the containment vessel) of the injection exit, initial fluid composition in the vessels, and location of the vent line. The initial jet Froude number has been varied between 17 and 36 and in one of the test condensation occurred. The paper shows the effect of these parameters variation on the test evolution with respect to jet impingement location in the vertical curved wall and variation of impingement location as a function of buoyancy variation. Fluid mixing and stratification, characteristics of gas transport between the compartment and the effect of condensation on the overall phenomena evolution are analyzed in the paper.     

Simulation of gas mixing and transport in a multi-compartment geometry using the GOTHIC containment code and relatively coarse meshes

The recently concluded OECD SETH project included twenty-four experiments on basic flows and gas transport and mixing driven by jets and plumes in two, large, connected vessels of the PANDA facility. The experiments featured injection of saturated or superheated steam, or a mixture of steam and helium in one vessel and venting from the same vessel or from the connected one. These tests have been especially designed for providing an extensive data base for the assessment of three-dimensional codes, including CFD codes. In particular, one of the goals of the analytical activities associated with the experiments was to evaluate the detail of the model (mesh) necessary for capturing the various phenomena. This work reports an overview of the results obtained for these experimental data using the advanced containment code GOTHIC and relatively coarse meshes, which are coarser than the ones typically used for the simulation with commercial CFD codes, but are still representative of the models which are currently affordable for a full containment analysis. In general, the phenomena were correctly represented in the simulations with GOTHIC, and the agreement of the results with the data was in most cases pretty good, in some cases excellent. Only for a few tests (or particular phenomena occurring in some tests) the simulations showed noticeable discrepancies with the experimental data, which could be referred to either an insufficiently detailed mesh or to lack of specialized models for local effects.     

反应堆冷却剂系统主管道安注斜接管等温横向射流流动特性的研究

针对核反应堆冷却剂系统中的主管道安注斜接管等温横向射流问题,应用计算流体力学商用软件CFX进行等温横向射流流动的数值模拟,得出了典型运行工况下的三维流场分布.深入研究了射流与主流在不同流速比情况下等温横向射流的流动特性、影响区域及主要影响因素,在所研究的参数范围内,得出了流速比是决定等温横向射流流动特性的最重要因素,同时将数值模拟结果与流动可视化试验结果进行了比较,二者吻合良好.     

The panda tests 9 and 9bis investigating gas mixing and stratification triggered by low momentum plumes

This paper presents the results of two tests, performed in the PANDA facility in the frame of the OECD SETH project. The Test 9 and Test 9bis were characterized by the same initial Froude number (Fro ∼ 1) and were performed at constant pressure in the vessels. The vessels were initially filled with air. During the tests the plumes were generated by injecting a constant amount of steam in Vessel 1 and continuous venting of a steam-air mixture from Vessel 2. In Test 9bis the temperature of the injected steam and the initial containment wall and fluid temperatures where chosen to reach later the conditions for (wall) condensation. The on-set of condensation occurred in Test 9bis at the top of Vessel 1 when, due to the steam injection the steam partial pressure increased enough to eliminate the steam superheating. The condensed droplets propagated toward the bottom of Vessel 1, where due to the lower steam partial pressure, a portion of condensate vaporized again. The evaporation had the effect of increasing the steam partial pressure and progressively new conditions for steam condensation were obtained also at the bottom of Vessel 1. The condensation in Test 9bis had the effect of mitigating the reduction of plume buoyancy (which is a function of the air-steam concentration in Vessel 1) and for this reason the overall plume trajectory, steam-air mixing, stratification and transport between the two compartments were affected.     

Experimental interpretation and code validation based on the PHEBUS-FP programme: Lessons learnt from the analysis of the containment scenario of FPT1 and FPT2 tests

The phenomenological scenario of a severe accident is extremely complex. Its simulation requires specific models of phenomena of different nature (i.e., materials behavior, thermal–hydraulics, aerosols, etc.) and their adequate coupling in safety computer codes. Therefore, an exhaustive and extensive validation against representative databases is mandatory. PHEBUS-FP project is, beyond any doubt, one of the more valuable data sources for this purpose.The main lessons learnt from the containment simulations of FPT1 and FPT2 tests were summarized. Several safety computer codes: CONTAIN 2.0, MELCOR 1.8.5 and ASTEC 1.1 have been used. This diversity has allowed a “user-independent” cross comparison of codes and data. Overall, codes estimates have reproduced properly experimental trends and no variable has shown major discrepancies. By means of parametric studies, it has been demonstrated that the minor discrepancies found did not come from the hypotheses and approximations adopted. In addition, the analyses of codes results have assisted in the interpretation of experiments by showing potential experimental uncertainties (i.e., steam injection in FPT1) or even by crediting data from a specific measurement technique over other data sources (i.e., samplings over γ-spec in FPT1).     

Prediction of light gas distribution in experimental containment facilities using the CFX4 code

Two- and three-dimensional simulations of experiments on atmosphere mixing and stratification in a nuclear power plant containment were performed with the computational fluid dynamics (CFD) code CFX4.4. Experimental data were obtained from the Institut de Radioprotection et de Sureté Nucléaire (France), where experiments on the TOSQAN facility were performed, and from Becker Technologies GmbH (Germany), where experiments on the ThAI facility were performed. The fluid inside the vessels was modeled as a single-phase gas atmosphere, and simple models for steam condensation on structures and in the atmosphere were introduced. The purpose was to assess the applicability of the CFD approach to simulate the behavior of light gases in containments at accident conditions. The comparisons of experimental and simulated results show that, despite a tendency to simulate more intensive mixing, the proposed approach may replicate the structure of the atmosphere reasonably well.     

Investigation of Break Location Effects on Thermal-Hydraulics during Intermediate Break Loss-of-Coolant Accident Experiments at ROSA-III

The rig of safety assessment (ROSA)-III facility is a volumetrically scaled (1/424) boiling water reactor (BWR/6) system with an electrically heated core designed for integral loss-of-coolant accident (LOCA) and emergency core cooling system (ECCS) tests. Break location effects on thermal-hydraulics during intermediate LOCAs were investigated by using four experiments at the ROSA-III, the 15 and 25% main recirculation pump suction line break (MRPS-B) experiments, the 21% single-ended jet pump drive line break (JPD-B) experiment and the 15% main steam line break (MSL-B) experiment. Water injection from the high pressure core spray (HPCS) was not used in any of the experiments. Failure of ECCS actuation by the high containment pressure was also assumed in the tests.

In the MRPS-B experiments, the discharge flow turned from low quality fluid to high quality fluid when the downcomer water level dropped to the main recirculation line outlet elevation, which suppressed coolant loss from the vessel and the core. In the JPD-B experiment, the jet pump drive nozzle was covered with low quality fluid and low quality fluid discharge continued even after the downcomer water level reached the jet pump suction elevation. Low quality fluid discharge ceased after the ADS actuation. It suggestes that the JPD-B LOCA has the possibility of causing larger and more severe core dryout and cladding temperature excursion than the MRPS-B LOCA. The MSL-B LOCA was characterized by mixture level swell in the downcomer and the core. The core mixture level swell resulted in the much later core dryout initiation than that in the MRPS-B LOCA, however, ECCS actuation was also delayed because of slow downcomer water level drop.     

A numerical simulation of water jet injection behavior in fuel–coolant interaction

Water injection mode of molten fuel and coolant interaction is a key issue during the steam generator tube rupture accident in liquid metal reactors. The focus of the present study is placed on the numerical simulation of the water jet behavior falling into a pool of a denser fluid in order to get qualitative and quantitative understanding of initial premixing phase of water injection mode. A multi-phase code with the volume of fluid (VOF) method is developed. The simulation results are compared with experimental data to examine the capability of the current approach. Effects of density ratio and Froude number on cavity penetration velocity are quantitatively analyzed. The simulation results show surface waves and breakup behavior occur both at the top of the cavity during cavity collapse and at the cavity boundary. The simulation results are compared with the existing theories. At the top of the cavity, the water jet wavelength is close to the value estimated based on the Rayleigh–Taylor instability. At the cavity boundary, melt wave length is close to the value estimated based on the Kelvin–Helmholtz instability.     

基于ECC安注热混合试验的LOCUST 1.2分析与验证

在失水事故（LOCA）工况下安注系统投入使用时,蒸汽与安注冷却剂会发生流体热力学混合,热混合过程中冷腿段的冷却是直接影响堆芯再淹没与否的重要因素。中国广核集团有限公司自主研发了一款两相流热工水力系统分析软件LOCUST,可用于压水堆核电厂事故工况的分析计算。基于西安交通大学堆芯应急冷却系统（ECCS-XJTU）试验台架进行的堆芯应急冷却（ECC）安注热混合试验,本文使用LOCUST软件对ECC热混合试验进行了几何建模及计算分析。ECC热混合试验工况主要为不同流量下主管纯蒸汽与安注管过冷水的混合,蒸汽流量为25~125 kg/h,过冷水流量为100~500 kg/h。模拟计算结果和试验结果的对比分析表明：试验段出口质量流量计算值的最大相对误差在13.8%以内,混合后温度计算值的最大相对误差在8%以内,LOCUST在计算高温蒸汽和过冷水混合时的计算结果相对保守,总体上验证了LOCUST在LOCA下两相热混合安注计算的可靠性和准确性。     

不同湍流模型对氢气分布影响的数值研究

研究严重事故下安全壳内氢气分布有利于评估氢气风险。本文采用三维CFD方法对THAI装置HM2试验进行建模,并分别使用代数模型和k-ε模型模拟氢气分层形成以及破坏过程。分析结果表明,CFD模拟结果与实验数据基本符合,在模拟中可观察到氢气分层现象的形成以及水蒸气对氢气分层的逐步破坏与混合过程;在氢气注射阶段,代数模型和k-ε模型的模拟结果接近,能够反映氢气浓度分层的形成过程;在水蒸气注射阶段,代数模型基于半经验的混合长度理论,在模拟装置较复杂几何结构内水蒸气流动对氢气分层的破坏作用时并不十分理想,标准k-ε模型对装置各测点氢气浓度达到一致的时间预测与试验结果较为接近。     

Experimental study on the direct contact condensation of steam jet in the passive safety injection tank

The direct contact condensation and subsequent thermal mixing by the injected steam jet onto a quiescent coolant inside a tank were examined experimentally to simulate the phenomena in passive safety injection systems. Specifically, the influence of the steam injection velocity was studied. Even though the total flow rate of injected steam was unchanged, the pressure inside the tank increased quickly at the larger nozzle diameter. Additionally, at a larger nozzle diameter, the thickness of the thermal mixing zone decreased because the amount of direct contact condensation decreased. For the in-depth study on the role of the nozzle size for the thermal mixing, the particle image velocimetry method was used to understand the flow field of water inside the tank. The visualization results demonstrated the formation of a flow field in the coolant due to the expansion and contraction of the steam–air mixture boundary. Furthermore, the thermal mixing zone was found to be closely related to the penetration depth. Finally, a variety of penetration models were examined and compared against the experimental observation. The correlations based on the steam condensation approach under-predicted the penetration depth, whereas the approach that considers the momentum of non-condensable gas gave the reasonable prediction capability.     

铅基快堆SGTR事故下热工水力模拟及气腔扩散行为研究

本文针对铅基快堆蒸汽发生器传热管破裂（SGTR）事故,利用计算流体力学（CFD）程序对LIFUS5/MOD2台架的汽水注射进液态金属铅铋环境进行研究。研究了3种热工水力现象：铅铋环境压力上升与压力波传递,铅池液位波动和气泡夹带与铅池液位上升和蒸汽扩散。研究结果表明：CFD模型在模拟SGTR事故的压力变化和压力波传递方面具有很小的计算误差;压力波峰值会随着水侧背压的升高而增大,且局部的蒸汽腔压力会低于附近的铅池压力,抑制蒸汽爆炸发生;同时事故引起的铅铋液位上升既会引起小尺寸气泡的输运夹带,也会对铅铋环境结构件造成冲击。     

CFD simulation of steam jet-induced thermal mixing in subcooled water pool

This study examined the IRWST thermal mixing phenomena induced by a steam jet in a subcooled water pool. Due to the limitation of the current CFD code to simulate condensation, the steam condensation region model was developed to evaluate the thermal mixing phenomena. Within this region, all the steam was condensed into water, and the steam mass and energy inputs were treated as the source. This calculation was treated using single-phase CFD methods. The benchmark calculation for a thermal mixing experiment in the water tank was performed to develop an optimized 3D evaluation methodology of the thermal-hydraulic behavior in APR1400 IRWST. Steam discharge through the sparger and condensation phenomenon was modeled with the choking flow and thermal mixing model in the quenching tank using CFX11.Three types of thermal mixing experiments, local phenomena test, thermal mixing tests in cylindrical water pool and annulus water pool, were designed to provide data representative of the behavior of the prototype for CFD simulations of the thermal-hydraulic behavior in IRWST. A comparison of the calculated and experimentally measured temperature profiles showed some disagreement particularly around the sparger. The main reason for this disagreement was caused by the difference in the test and simulating conditions at the tank wall. However, moving away from the sparger, the trends of the temperature rise became similar to that in the experiment. Despite these problems, this model is the best way of evaluating the thermal mixing phenomena caused by a steam jet in a subcooled water pool.     

CFD analysis of a turbulent jet behavior induced by a steam jet discharge through a single hole in a subcooled water tank

A computational fluid dynamics (CFD) analysis for a turbulent jet flow induced by a steam jet discharged into a subcooled water pool was performed for 10 s of transients to investigate whether the currently available CFD codes can be suitably used as a tool to investigate the applicability of the existing semi-analytical correlations to a condensing jet-induced turbulent jet and to analyze the thermal-hydraulic behavior, such as global circulation and local hot spot, in a condensation pool for advanced light water reactors. As for the numerical experiment, a series of sensitivity calculations was conducted systematically to elucidate the major factors which can cause different analysis results by varying the mesh distributions, numerical models for a convection term and an eddy viscosity term. The effect of a difference in the velocity and the temperature distribution in a region between the sparger and the pool wall has not been observed in the afore-mentioned sensitivity calculations. The comparison of the CFD results with the test data shows that the CFD analysis does not accurately simulate the local phenomenon of a turbulent jet existing downstream of a steam jet. It was found that the value of the turbulent intensity at the inlet of the turbulent jet region is the most important factor because it can determine the boundary of a turbulent jet through a momentum diffusion process in a radial direction. The comparison of the CFD results with the test data shows that the CFD analysis can accurately simulate the local phenomenon of a turbulent jet existing downstream of a steam jet only when the CFD analysis reflects the physics of a turbulent jet.     

补水箱内的直接接触冷凝模型研究

补水箱是核反应堆安全系统中的重要设备。事故工况下补水箱内可发生剧烈的直接接触冷凝过程,导致补水箱内压力的迅速降低乃至振荡,影响补水箱的安全注入功能。为提高对补水箱安注行为预测的准确性,本文基于射流速度分布理论和假想管嘴分析方法,考虑液相的温度分层对传热温差的影响,结合补水箱内直接接触冷凝的一般过程,建立了针对性的冷凝传热计算方法。利用该模型对现有实验数据进行了预测,符合良好,初步验证了模型的有效性。相关研究有助于提高补水箱安注过程和相关事故安全分析的准确性。     

Multi-dimensional modelling of multiphase flow physics: high-speed nozzle and jet flows — a case study

Multi-dimensional modelling of multiphase flows has become more prevalent as computer capabilities have significantly expanded. Such analyses are necessary if the flow physics demonstrates behavior that is fundamentally different from the estimates of one-dimensional analyses. Multiphase multi-dimensional behavior may involve physical mechanisms that interact with the flow field transverse to the main fluid direction and feedback into downstream processes. Consider the physics of high-speed internal nozzle flow, downstream external jet flow and the dynamics of jet breakup. This is a prime example of a coupled problem where multi-dimensional aspects may need to be considered. This paper examines multiphase physics as an illustration of the conditions under which multi-dimensional modelling would be required. Internal nozzle flow can involve cavitation phenomena, and as the geometry becomes more abrupt or asymmetric, multi-dimensional modelling is required. High-speed simulations using our internal flow model, CAVALRY, indicate that cavitation behavior can become oscillatory as the nozzle shape is altered. This exiting internal flow emerges as a multi-dimensional external jet flow, whose downstream breakup can be noticeably influenced by the inlet conditions as well as the jet breakup mechanisms. Jet breakup models first developed for the TEXASV model are utilized in the multi-dimensional KIVA code simulations for gas–liquid flows. The simulation results suggest that similar jet breakup mechanisms are operative for a multi-fluid system. Our comparisons to particular sets of data for high-speed nozzle flow and jet breakup in a gas suggest that the approach can be extended to multiphase systems using similar concepts; i.e. TEXAS-3d.     

Experiments on basic thermalhydraulic phenomena relevant for LWR containments: Gas mixing and transport induced by buoyant jets in a multi-compartment geometry

In the frame of the OECD SETH project twenty-four experiments have been carried out on basic gas transport and mixing phenomena in the large-scale, multi-compartment PANDA facility. The experiments consist of several series based on the flow configuration driving the gas transport including plume and jet injection of saturated or superheated steam. A variety of test configurations with well-defined initial and boundary conditions have been investigated, including the parametric effects of the geometry of the injection, the composition and velocity of the injected mixture, the initial ambient composition, the vent location and the initial temperatures. The tests have been designed to be used for the validation of computer codes that are capable of analyzing the thermalhydraulic safety of the containment of light water reactors (LWRs). The results obtained form an extensive database especially valuable for the assessment of the capabilities of three-dimensional simulation tools. The present work reports on a specific test series of the aforementioned experimental program, the so-called Free-Plume Series featuring vertical injection of buoyant steam driving the gas transport. The experimental data is presented showing its quality and the trends in the experiments are analyzed. The phenomena whose prediction might be challenging for codes are emphasized. After the expiration of the confidentiality period, the experimental data is now available for the broad scientific community to be used for code validation purposes.     

Post-benchmark simulation of the THAI+ facility TH27 experiment

In order to maintain the integrity of a nuclear power plant containment and effectively manage a severe accident, it is necessary to understand phenomena occurring in the atmosphere of the nuclear power plant containment during the accident. A number of containment atmosphere mixing experiments have been performed in the dedicated experimental facilities, followed by the numerical simulations using lumped-parameter and computational fluid dynamics codes. This paper presents the THAI+ test facility experiment TH27 post-benchmark simulations performed with the lumped-parameter code ASTEC. The experiment TH27 was an initial operation test of the THAI+ facility, which has been recently constructed by expanding the experimental facility THAI with the newly constructed parallel attachable drum vessel. The experiment featured steam and helium injections and transport and mixing of gasses and steam between the two vessels, as well as wall heating and cooling of different vessels. The TH27 experiment was performed together with an international multistage benchmark, consisting of double-blind, blind, and open phases. The developed nodalization scheme and the features of the calculation are presented in the paper. The results of the calculations are compared to the experimental values for the main containment parameters – pressure, gas and wall temperatures, helium concentrations.     

Gravity-driven injection experiments and direct-contact condensation regime map for passive high-pressure injection system

A gravity-driven injection experiment of a passive high-pressure injection system with a pressurizer pressure balance line (PRZ PBL) is conducted by using a small-scale test facility to identify the parameters affecting the gravity-driven injection and the major condensation regimes. It turns out that the larger the water subcooling is, the more the injection initiation is delayed. A sparger and natural circulation of the hot water from the steam generator accelerate the gravity-driven injection. The condensation regimes identified through the experiments are divided into three distinct ones: sonic jet, subsonic jet, and steam cavity. The steam cavity regime is a unique regime of downward injection with the present geometry not previously observed in other experiments. The condensation regime map is constructed using Froude number and Jacob number. It turns out that the buoyancy force has a larger influence on the regime map transition because the regime map using the Froude number better fits data with different geometries than other dimensionless parameters. Simple correlations for the regime boundaries are proposed using the Froude number and the Jacob number.     

Analysis of Overheating Rupture in Heat-Transfer Tubes Causing Corrosive High-Temperature Reaction

In the steam generator of a liquid metal fast breeder reactor, a defect penetrating through heat-transfer tube will cause high-pressure water/steam to spout into the low-pressure sodium filling the space outside the tube, to initiate sodium-water reactions. If the leak exceeds an intermediate level (~2kg/s), the reaction jet may rupture adjoining tubes with overheating in the event of insufficient cooling available inside the tubes. Such phenomenon of overheating tube rupture presents a serious problem to the economy and safety of steam generator. With a view to clarifying the failure behavior of steam generator heat-transfer tubes under such condition a model of the phenomenon is derived through a series of tests on sodium-water reactions making use of a test loop representing the scale model of an actual fast breeder steam generator. Comparison of actual test data with analysis based on the model has yielded the following information: The failure behavior of gas-pressurized tubes fall into two categories: (a) by creep failure—occurring upon increase of cumulative damage with tube wall wastage caused by the reaction jet and (b) by ductile failure accompanied by creep—upon tube heating with the reaction jet to the extent of lowering tube wall strength below the hoop stress exerted by tube pressure. Analysis of the two categories of failure results in estimation of the percentage difference between analyzed and measured times to failure of 35–50% in the case of creep failure and of 20–50% in the case of ductile failure accompanied by creep. In practical application to steam generators in order to provide a safety margin a time factor—i.e., the safety factor indicating multiple of actual time to failure—of 3 is adopted against 1.5–2 indicated from test to be the actually applicable value.