Development of numerical evaluation method for fluid dynamics effects on jet breakup phenomena in BWR lower plenum

The jet breakup phenomena of the molten cores during a severe accident are affected by some complicated structures, such as control rod guide tubes, instrument guide tubes, and core support plate, in the lower plenum of the boiling water reactors (BWRs). A multi-phase computational fluid dynamics approach combined with experiments is considered to be the best way to estimate the jet breakup phenomena in the BWR lower plenum, and a numerical analysis method has been developed based on the interface tracking method code TPFIT (Two-Phase Flow simulation code with Interface Tracking). The analysis method developed was applied to single-/multi-channel experiments for verification and validation in this study. Furthermore, results from the numerical analysis were compared to the experimental results obtained using the multi-phase flow visualization technique using a high-speed camera and the particle image velocimetry method. As a consequence, it is found that the simulation method developed in this study can qualitatively simulate the jet breakup phenomena in the complicated structure.     

Development of Transuranium Element Recovery from High-Level Radioactive Liquid Waste

In the present study, the jet breakup characteristics of molten material is experimentally investigated in nonboiling condition using Wood's metal to isolate the key features of jet breakup phenomenon from the conjugated nature of melt breakup and steam generation. The experimental apparatus consists mainly of melt generating furnace and melt crucible equipped with variable nozzle diameter, a rectangular water tank of 350×350×800mm equipped with temperature controlling heater and thermocouples. The jet diameters were 10 and 20 mm and the jet velocity was varied by pressurizing the melt container. Wood's metal of 70°C melting temperature was used. Visualization of jet breakup provided characteristics of jet breakup in water. The range of jet velocity was 2.2–5.5 m/s. The debris were collected and sieved and it was shown that the debris sizes of 1.0–2.8 mm had the largest mass fraction, up to 50%. In the present experimental conditions, the Kelvin-Helmholtz instability is considered the most probable cause of jet breakup.     

Simulation of melt jet breakup and debris bed formation in water pools with IKEJET/IKEMIX

The objective of the development of the code system KESS is simulating the processes of core melting, relocation of core material to the lower head of the reactor pressure vessel (RPV) and its further heatup, modelling of fission product release and coolability of the core material. In the scope of the code development, IKEJET and IKEMIX were designed as key models for the breakup of a molten jet falling into a water pool, cooling of fragments and the formation of particulate debris beds. Calculations were performed with these codes, simulating FARO corium quenching experiments at saturated (L-28) and subcooled (L-31) conditions, as well as PREMIX experiments, e.g. PM-16. With the assumption of a reduced interfacial friction between water and steam as compared to usually applied laws, the melt breakup, energy release from the melt and pressurisation of the vessel observed in the experiments are reproduced with a reasonable accuracy. The model is further applied to reactor conditions, calculating the relocation of a mass of corium of 30 t into the lower plenum, its fragmentation and the formation of a particle bed.     

Particulate debris formation by breakup of melt jets: Main objectives and solution perspectives

In the present editorial article, questions about modeling of melt breakup are emphasized which seem not yet to be finally solved in the contributions to the present issue. These questions concern the basic instability and stripping approaches as well as the determination of the decisive fluid flow along the jet and thus the feedback of breakup with the produced mixture. Concerning the validation, understanding of material influences is emphasized by evaluations about jet lengths and diameter relations for FARO and PREMIX experiments. Evaluations on the formation of particulate debris require to consider breakup as well as crust formation on resulting drops of melt depending on the mixture properties, especially the steam content in the mixture. Sufficient breakup as well as quenching appear only to be possible with not too high steam contents in the mixture. A simplified modeling is envisaged, adapted to experimental results which nevertheless captures the major effects over a sufficient bandwidth of conditions.     

Fragmentation of a molten corium jet falling into water

During a severe accident of a pressurized water nuclear reactor, a large mass of corium could pour into the vessel bottom as a compact jet. When the corium mass reaches the water at the bottom of the vessel, an intense fragmentation may occur. This could lead to a significant mixing of corium and water, likely to cause a steam explosion which could damage the structures. An analytical study has been established in order to quantify the corium jet fragmentation. This study consists mainly in modeling the vapor flow surrounding the jet as well as the instability which occurs at its interface. In comparison with previous studies, this model pays particular attention to the jet-produced particles which interact with the vapor flow. A complete model has been set up in order to calculate the jet breakup length and the generated particles’ diameter under each specific situation characterized by initial conditions. This model mainly relies upon results from boundary layer theory and linear instability calculations. The full model’s results are compared to existing experiences in this field and a final correlation of the results is established. A good agreement is obtained on the jet breakup length, however the predicted particle diameter tends to be too large. This last result could be explained by a secondary breakup of the particles in water and by a large uncertainty in the vapor flow.     

Simulation of containment jet flows including condensation

The validation of a CFD code for light-water reactor containment applications requires among others the presence of steam in the different flow types like jets or buoyant plumes and leads to the need to simulate condensation phenomena.In this context the paper addresses the simulation of two “HYJET” experiments from the former Battelle Model Containment by the CFD code CFX. These experiments involve jet releases into the multi-compartment geometry of the test facility accompanied by condensation of steam at walls and in the bulk gas. In both experiments mixtures of helium and steam are injected. Helium is used to simulate hydrogen. One experiment represents a fast jet whereas in the second test a slow release of helium and steam is investigated. CFX was earlier extended by bulk and wall condensation models and is able to model all relevant phenomena observed during the experiments. The paper focuses on the simulation of the two experiments employing an identical model set-up. This provides together with other validation exercises the information on how well a wider range of flowing conditions in a full containment simulation can be covered with a single set of models (e.g. turbulence and condensation model). Some aspects related to numerical and modelling uncertainties of CFD calculations are included in the paper by investigating different turbulence models together with the modelling errors of the differencing schemes applied.     

System Code Calculation and Experimental Verification for Double-hole Steam Jet Condensation Heat Transfer Process Based on ADS1 3 Scaled Model

The steam direct contact condensation of high-temperature steam in sub-cooled water is an important way to reduce the temperature and pressure in the primary circuit in the third generation of advanced pressurized water reactors such as AP1000 and CAP1400 in the event of accidental overpressure. Based on the system codes of RELAP5 and COSINE, the process of saturated steam injecting into large volume sub-cooled water through a double-hole nozzle was modeled, calculated and analyzed. The temperature distributions along the axial direction of the high-temperature steam ejected from the nozzle were obtained. At the same time, the visual experiments of steam jet condensation were performed. The thermocouple matrix and high-speed camera were used to measure the key thermal-hydraulic parameters to obtain the temperature distributions along the steam plume and the flow patterns of the steam jet, which were used to verify the accuracy of the system code to simulate the process of steam spraying and condensation. The results show that the system code RELAP5 can basically simulate the general trend of ADS steam condensation process under the simplified model. The average error of the simulation results is 2.97% compared with the experimental results. In addition, the COSINE code was used to further modify and improve the model of the spraying condensation process. Considering the influence of the overall flow in the water tank on the condensation characteristics, the simulation results fit well with the experimental results, with an average error of 1.89%. However, the actual double-hole spraying process is complex and has obvious three-dimensional characteristics, so the relevant condensation heat transfer model in the system code still needs to be further improved to simulate its local condensation characteristics more accurately.     

基于ADS1～3缩比模型的双孔蒸汽喷放冷凝传热系统程序计算与实验验证

高温蒸汽在过冷水中喷放直接接触式冷凝是AP1000、CAP1400等三代先进压水堆一回路在事故超压情况下重要的降温降压途径。本文基于系统程序RELAP5、COSINE对饱和蒸汽通过双孔喷洒器喷入大容积过冷水中进行直接接触冷凝这一过程进行建模、计算、分析,获得高温蒸汽从喷口喷出后沿轴向的温度分布。同时开展蒸汽喷放冷凝可视化实验,采用热电偶矩阵和高速摄像机等对关键热工参数进行测量,以获得蒸汽汽羽的温度分布和喷放流型等,用于验证系统程序对蒸汽喷放冷凝过程模拟的准确性。结果表明,采用RELAP5程序基本能模拟简化条件下的ADS蒸汽喷放冷凝总体变化规律,模拟结果与实验结果相比平均误差为2.97%。此外,采用COSINE程序对喷放冷凝过程模型进行了进一步修正和改进,考虑水箱内整体流动对喷放特性的影响,模拟结果与实验结果吻合较好,平均误差为1.89%。但由于实际双孔喷放过程较为复杂,并且存在明显的三维特性,所以仍需对系统程序中相关冷凝传热模型进行完善,以更精确地模拟其局部冷凝特征。     

Experimental study on breakup and fragmentation behavior of molten material jet in complicated structure of BWR lower plenum

To estimate the state of reactor pressure vessel of Fukushima Daiichi nuclear power plant, it is important to clarify the breakup and fragmentation of molten material jet in the lower plenum of boiling water reactor (BWR) by a numerical simulation. To clarify the effects of complicated structures on the jet behavior experimentally and validate the simulation code, we conduct the visualized experiments simulating the severe accident in the BWR lower plenum. In this study, jet breakup, fragmentation and surrounding velocity profiles of the jet were observed by the backlight method and the particle image velocimetry (PIV) method. From experimental results using the backlight method, it was clarified that jet tip velocity depends on the conditions whether complicated structures exist or not and also clarified that the structures prevent the core of the jet from expanding. From measurements by the PIV method, the surrounding velocity profiles of the jet in the complicated structures were relatively larger than the condition without structure. Finally, fragment diameters measured in the present study well agree with the theory suggested by Kataoka and Ishii by changing the coefficient term. Thus, it was suggested that the fragmentation mechanism was mainly controlled by shearing stress.     

Simulation of melt jet breakup experiments by JASMINE with an empirical correlation for melt particle size distribution

We modified JASMINE code, a fuel–coolant interaction simulation code developed at Japan Atomic Energy Agency (JAEA), to extend the applicability for ex-vessel melt coolability assessment. The modification included addition of a melt particle size distribution model based on an empirical correlation and a simple non-local radiation heat transfer model, improvement in the treatment of melt particle generation, and re-agglomeration of settled particles. The modified code was tested by simulating melt jet breakup experiments, namely selected cases of ALPHA/GPM series with alumina–zirconia mixture and steel melt by JAEA, and FARO experiments with urania–zirconia mixture by Joint Research Center Ispra. Simulation results showed that the code reproduces the experimental results well for the cases with a deep subcooled water pool where the melt breaks up completely. On the other hand, significant underestimation of heat removal from the melt and overestimation of agglomeration of settled melt was encountered for conditions with a shallow or saturation temperature water pool. The melt agglomeration behavior in the simulation was sensitive to model parameters on the agglomeration criterion and heat transfer depending on conditions.     

Development of unstructured mesh-based numerical method for sodium–water reaction phenomenon in steam generators of sodium-cooled fast reactors

When pressurized water or vapor leaks from a failed heat transfer tube in a steam generator of sodium-cooled fast reactors, a high-velocity and high-temperature jet with sodium–water chemical reaction may cause wastage on the adjacent tubes. For safety assessment of the steam generator, a computational fluid dynamics code called SERAPHIM calculating compressible multicomponent multiphase flow with sodium–water chemical reaction has been developed. The original SERAPHIM code is based on the finite difference method. In this study, unstructured mesh-based numerical method for the SERAPHIM code was developed to advance a numerical accuracy for the complex-shaped domain including multiple heat transfer tubes. Numerical analysis of an underexpanded jet experiment was performed as part of validation of the unstructured mesh-based SERAPHIM code. The calculated pressure profile showed good agreement with the experimental data. To investigate the effect of the introduction of the unstructured mesh and to confirm applicability of the numerical method for the actual situation, water vapor discharging into liquid sodium was analyzed. The calculated behavior of the reacting jet agreed with the previous experimental knowledge. It was demonstrated that the proposed numerical method could be applicable to evaluation of the sodium–water reaction phenomenon.     

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.     

Numerical Analysis of Jet Breakup Behavior Using Particle Method

A continuous jet changes to droplets where jet breakup occurs. In this study, two-dimensional numerical analysis of jet breakup is performed using the MPS method (Moving Particle Semi-implicit Method) which is a particle method for incompressible flows. The continuous fluid surrounding the jet is neglected. Dependencies of the jet breakup length on the Weber number and the Froude number agree with the experiment. The size distribution of droplets is in agreement with the Nukiyama-Tanasawa distribution that has been widely used as an experimental correlation. Effects of the Weber number and the Froude number on the size distribution are also obtained.     

The DEEPSSI project, design, testing and modeling of steam injectors

The DEEPSSI project is a steam injector research programme. Among thermal-hydraulic passive systems, the steam injectors (also called “condensing ejectors” or “steam jet pumps”) are very interesting apparatus with very specific characteristics (high velocity, very low pressure). The envisaged reactor application is the Steam Generator Emergency FeedWater System (EFWS) of Pressurised Water Reactors (PWRs). The heart of this project is the development and the testing of an innovative steam injector design. Three experimental facilities are involved: CLAUDIA in France, IETI in Italy and IMP-PAN in Poland. In these facilities, different design options have been tested and some significant improvements of the initial design have been obtained.In addition to the experimental studies, the development of a steam injector computational model has been undertaken in order to model industrial systems based on steam injectors. The one-dimensional module of the system code CATHARE2 has been chosen to be the basis of this model. The first results obtained have confirmed the capabilities of CATHARE2 to describe the steam injector thermal-hydraulics.     

Experimental validation of the helical steam generator model in the TASS/SMR code

The transients and setpoint simulation/system-integrated modular reactor (TASS/SMR) code has been used to identify the safety margin of a 65-MWt advanced integral reactor and to evaluate its design performance. Although, the code has been verified by using simplified and analytical problems as well as a reliable system code, its validation has not been fully established. This paper deals with a validation of the TASS/SMR code by using two kinds of separate effect tests related to heat transfer at a helically coiled steam generator. The heat transfer experiments were performed by using a full-scale prototype of the steam generator cassette of the advanced integral reactor and a scaled-down steam generator cassette. Analytical results show that the TASS/SMR code predicts the thermal hydraulic parameters, including the system pressure and fluid temperature at the primary and secondary sides of the steam generator cassette, and the heat transfer rate through the steam generator cassette well. The validation results in this study show that the TASS/SMR code is applicable for heat transfer calculations related to the helically coiled steam generator of the advanced integral reactor.     

Condensation regime diagram for supersonic/sonic steam jet in subcooled water

This study was designed to determine the behaviour of the flow patterns of supersonic and sonic steam jet condensation in subcooled water. The effects of steam mass flux, water temperature and pressure ratio on the flow pattern were discussed. The results indicated that the flow pattern was not only affected by steam mass flux and water temperature, but also affected significantly by pressure ratio. The expansion and contraction ratios of the steam plume were influenced significantly by pressure ratio, and a correlation for predicting the expansion and contraction ratio was given based on the theoretical expansion ratio of sonic steam jet. At relatively low steam mass flux and high water temperature, the transition of flow pattern from stable jet to unstable jet was observed, and a criterion of stable-unstable jet transition was given. Moreover, the three-dimensional condensation regime maps, including stable steam plume shapes and unstable jet, for sonic and supersonic steam jet under different conditions were discussed. Four different shapes of the steam plume occurred in sonic steam jet regime map, and six different shapes of the steam plume occurred in supersonic steam jet regime map. The regime maps were also validated against the previous experiments.     

Breakup and fragmentation behavior of molten material jet in multi-channel of BWR lower plenum

To estimate the current status of reactor pressure vessel of Fukushima Daiichi nuclear power plant, it is important to clarify the breakup and the fragmentation behavior of molten material jet in boiling water reactor (BWR) lower plenum by a numerical simulation. To clarify the effects of complicated structures on jet breakup and fragmentation behavior, we conducted visualized experiments simulating the severe accident in the BWR by using the multi-channel experimental apparatus. In this study, the jet falling behavior, the jet breakup length, the fragmentation behavior and internal/external velocity profiles of the jet are observed by the backlight method and the particle image velocimetry. It is clarified that the complicated structures prolong the jet breakup length or make the fragments fall together to the lower plenum similar to the bulk state. In addition, it is clarified that strong shearing stress occurs at the crest of interfacial waves at the side of the jet. Finally, the fragment diameters measured in the present study well agree with the theory based on the shering stress by changing the coefficient term. Thus, it is suggested that the fragmentation mechanism is controlled by shearing stress and the fragment diameter can be estimated by adjusting the characteristic value.     

Simulation of basic gas mixing tests with condensation in the PANDA facility using the GOTHIC code

The paper reports the results of the assessment of the GOTHIC code using the data of four basic tests with condensation performed in the PANDA large-scale facility. Three of these experiments featured vertical injection, and in one the transient response due to a high-momentum horizontal injection (jet) was investigated. The injected fluid was either saturated steam or a superheated mixture of steam and helium, and the fluid initially present in the vessels was pure air. The simulations were carried out using a detailed three-dimensional representation of the vessels. In general, the results of the simulations, which used a relatively coarse mesh, were in good agreement with the data. Limitations in modelling local phenomena controlled by complex flow patterns (e.g. heat transfer in the region of an impinging jet) and the need for refined meshes to reproduce certain aspects of the transients (e.g. erosion of the interface between layers of different gas composition) were also identified. Finally, the analyses indicated that in two tests the role played by re-vaporisation of the condensate film was unexpectedly large, and this effect should be more carefully considered in the containment analyses and future model developments.     

Experimental investigation of spray induced gas stratification break-up and mixing in two interconnected vessels

To analyze the effect of containment spray on gas mixing and depressurization, two experiments (ST3_1 and ST3_2) were performed with two interconnected vessels. These experiments were conducted in the frame of the OECD/SETH-2 project using the PANDA facility. The vessels were preconditioned such that a helium-rich layer is formed in the upper section of the first vessel, henceforth referred to as Vessel-1. In the case of the first experiment (ST3_1), the remaining volume of Vessel-1 and the entirety of the second vessel, Vessel-2, were filled with pure steam. For ST3_2, the second experiment presented here, pure steam was replaced with a steam-air mixture instead. Water was injected from the top of Vessel-1 with a spray nozzle projecting downwards. Transient behavior of system pressure, as well as global redistribution of gases is investigated. The results reveal that spray activation is very effective in containment system depressurization. Additionally it is found that the depressurization occurs at a higher rate for the systems containing more steam and less non-condensible gas. The depressurization rate gradually slows down, however, as the steam concentration decreases due to condensation, and non-condensible gases spread over the vessel system. It is also observed that the spray activation initiates the breakup of the helium-rich layer. The composition of the gas atmosphere plays a crucial role in determining the initiation time of the breakup; the presence of large amounts of non-condensible gas such as air delays the beginning of the helium layer breakup by approximately 200 s. The downward component of spray momentum causes the entrainment and the recirculation of the ambient gas atmosphere. Together with the entrainment and condensation effect, spray activation influences the gas mixture density in Vessel-1 and this generates a driving force for inter-compartment flow. As a result of this, an increase of helium-rich gas mixture is observed in the regions far away from the spray, i.e., in Vessel-2.     

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.