PWR Hot Leg Research Articles

An experimental investigation on CCFL characteristics during gas/low surface tension liquid counter-current two-phase flow in a small-scaling PWR hot leg typical geometry

A sharp increase in world energy demands which further results in another large progress in the development of nuclear energy establishes comprehensive developments on corresponding mitigation studies. Therefore, as a scenario of accident called LOCA is fundamentally considered, the related phenomena, i.e., the counter-current flow followed by flooding in the primary circuit of PWR, is of a great importance. The present work investigates characteristics of the flooding during a pair of gas/low surface tension liquid counter-current two-phase flow in a complex conduit representing a down-scaled of PWR hot leg typical geometry. Visual observations were obviously carried out to observe the flow phenomenology, while flow parameters were frequently varied. A typical result reveals that the gas flow rate to initiate the flooding decreases with the increase of liquid flow rate. Moreover, exhibiting locations of the onset called locus, a front flooding tends to occur during relatively low liquid flow rates while the higher liquid flow rates exhibit another flooding namely rear flooding. Accordingly, the present investigation provides a package of valuable information on a particular understanding towards the flooding characteristics to overcome the efforts on promoting safety managements on the operation of nuclear power plants..

A Brief on Optical-based Investigation towards The Interfacial Behaviors during High Viscous Liquid / Gas Countercurrent Two-Phase Flow in a Complex Conduit Representing 1/30 Down-Scaled of PWR Hot Leg Geometry

The present work briefly investigates liquid/gas countercurrent two-phase flow phenomena which can be specifically found in a mitigation during an accidental scenario in the operation of a nuclear reactor. A comprehensive knowledge on the corresponding phenomena is obviously important to avoid the failure on the cooling mechanism. Here, a pair of fluid containing high viscous liquid/gas flows through a complex conduit representing 1/30 scaled-down of PWR hot leg’s typical geometry. Furthermore, the flow structures were visually observed, while the film thicknesses are extracted by an image processing algorithm through the corresponding optical-tabulated data. The obtained results reveal that a rather sharp decrease in liquid film corresponds to the flow regime transition.

Statistical characterization of the interfacial behavior captured by a novel image processing algorithm during the gas/liquid counter-current two-phase flow in a 1/3 scaled down of PWR hot leg

This study addresses a statistical characterization of the interfacial behaviors during gas/liquid counter-current flow in a large scale of pressurized water reactor’s hot leg typical geometry on the basis of both time and frequency domain analyses. Here, the visualizations were captured by high-speed cameras. Furthermore, the image processing algorithm on the basis of pixel identification was developed to identify the time-series interfacial dynamics of the liquid holdup fluctuations, comprising the wave growth and its movement. The obtained data were further analyzed by using various advanced statistical tools comprising the probability density function, power spectral density function, and also discrete wavelet transform. Additionally, chaotic levels of the flow were obtained through the Kolmogorov-entropy analysis.Particular results reveal that a typical mechanism of flooding begins with the appearance of a wavy interface which further develops into either a roll or large wave, and later blocks the entire cross-sectional area of the conduit around the bend region. This later stage indicates the inception of flooding which is characterized by the increase of the water level close to the bend region. Next, the higher the pressure of system, the higher the frequency occurrence of slugging, while the injected gas flow rate obtained a different trend. However, the Kolmogorov entropy enables to correlate the stabilizing effect due to the fluid resistance which corresponds to either the pressure of the system or the physical properties of the fluid. In addition, there were no significant differences between the flooding mechanisms for both the air/water and steam/water flows.

A new correlation for CCFL at full scale PWR excluding the Hutze effect in UPTF data

The effect of the Hutze (a secondary ECC injection pipe within the hot leg pipe of a Konvoi-type PWR) upon CCFL was experimentally investigated at COLLIDER test facility of the Technical University Munich (TUM). COLLIDER represents a PWR hot leg with a large dimeter of 190 mm and ~¼ scale. A Hutze pipe was built into COLLIDER’s hot leg according to a German Konvoi-type PWR reactor design data. The effect of the Hutze upon CCFL are shown through comparisons with previous investigations at COLLIDER without a Hutze. Visual observations using high-speed recordings are presented at different flow conditions: supercritical conditions at low air velocities, transition into subcritical flow, and the onset of CCFL with comparison against previous recording without a Hutze. Limits of: the onset of hydraulic jumps, onset of CCFL (flooding), and most importantly CCFL characteristics are also shown and compared. The Hutze was found to induce a large impact upon these limits: two hydraulic jumps are observed prior to the onset of CCFL instead of only one (in case without a Hutze), the limits of the onset of hydraulic jumps and onset of CCFL occur at much lower air velocities, and the hysteresis effect (difference between limits of flooding and deflooding) disappears. CCFL characteristics curve is also shifted toward lower values of gas velocities and less liquid flows into the reactor vessel at same air velocities in the case with a Hutze which means a less cooling potential during the reflux condensation mode. Ohnuki correction of the Hutze effect upon CCFL characteristics of 1988 is tested and it seems to work well at small diameter but it overestimates CCFL characteristics for COLLIDER data. The previous agreement between CCFL characteristics obtained from downscaled experiments with D > 50 mm without a Hutze (including COLLIDER data and other large scale facilities) with UPTF data (at full scale) is found to happen because of the Hutze effect which shifts the CCFL characteristics downwards in Wallis diagram and not because of the correct representation of occurring CCFL phenomena. The actual CCFL characteristics for the 1:1 scale without a Hutze are expected to be higher than those reported from data with Hutze at UPTF facility. This indicates that there is still a scale effect even at a large diameter channel, and that most previous downscaled experiments have underestimated CCFL characteristics at full scale. Based upon obtained results, a correlation of CCFL characteristics at full scale excluding the Hutze effect is given. This correlation is of a special interest for the design of new PWR reactors as most new designs does not include a Hutze.

Pengaruh Rasio I/D terhadap Permulaan Flooding dan Fluktuasi Voltase Sinyal Tekanan Rezim Flooding pada Geometri Kompleks

A nuclear power plant operation requires reliability safety systems. Therefore, an accidental scenario such as the LOCA becomes a specific attention. This relates to a countercurrent flow phenomenon which permits the occurrence of a flooding regime. This study aims to investigate the effets of I/D ratios of the riser on the onset of flooding and time-series voltage fluctuations of differential pressure signals of the flooding regime on a complex geometry representing a PWR hot leg. The test section contains a combination of a horizontal, an elbow and an inclined pipes. Three I/D ratios containing 1.9 (R1), 3.9 (R2) and 8.3 (R3) were assesed. The tabulated data are expressed in the term of a flooding curve. Meanwhile, the statistical features of the time-series voltage fluctuations of the signals are presented on both PDF and PSD graphs. The results obtained show that the assesed I/D ratios do not conduct significance effects on the onset of flooding. On the other hand, there are trends which can be obtained from the time-series signals with the increase of the I/D ratio. Those trends can also be observed in the such statistical features; the PDF and PSD graphs, respectively.

Study of PWR hot leg creep rupture and RCS depressurization strategy during an SBO accident

Abstract Preventing the leakage of radioactive materials is important to nuclear safety. During a station blackout accident in pressurized water reactors, the hot leg creep rupture caused by hot leg countercurrent flow occurs before the reactor pressure vessel failure that caused by lower head rupture. The secondary fission products barrier is lost after hot leg creep rupture. An analysis for this phenomenon was done using the Modular Accident Analysis Program version 4.0.4 code. A station blackout accident for CPR1000 is simulated and the occurrence and influence of hot leg creep rupture phenomenon are analyzed in detail. After that, a sensitivity analysis of the opening of different pressurizer pilot-operated relief valves at five minutes after entering severe accident management guideline (before the hot leg creep rupture occurs) is studied. The results show that reactor pressure vessel failure time can be extended by at least 4 h if at least one pilot-operated relief valve is opened and direct containment heating phenomenon can be eliminated if at least two pilot-operated relief valves are opened.

On CCFL at a PWR Hot-Leg Pipe Geometry and Comparison Between Results in COLLIDER 1/4th- and Kobe 1/15th-Scaled Facilities

Countercurrent flow limitation (CCFL) in a pressurized water reactor hot-leg pipe geometry with a 190-mm pipe diameter was investigated experimentally and numerically at the COLLIDER test facility of the Technical University Munich in the past 3 years. This paper summarizes the most important CCFL findings learned from the COLLIDER test facility and tries to explain the reasons for obtaining different descriptions, results, and conclusions at different CCFL experimental investigations. The factors that can affect CCFL experimental results are explained in detail including some scale effects. The necessary preconditions to compare two sets of data from different CCFL experimental investigations are discussed in detail. The difference among CCFL-related limits/curves is clarified taking data at the COLLIDER as an example. The limits included the limit of the transition from a supercritical into a subcritical flow (SSTL); the onset of CCFL limit (iCCFL) inside the hot-leg pipe; the onset of CCFL limit (eCCFL) at the entrance of the steam generator; the deflooding limit (CCFLd); the CCFL characteristics curve (CCFLch), which predicts the water delivery rate after the onset of iCCFL; and the onset of hysteresis limit. It will be shown that among these limits only SSTL, CCFLch, and eCCFL are original limits while the rest are derivatives of them. In particular, it will be shown that the iCCFL limit is a combination of the SSTL and CCFLch limits. The effect of scale upon the eCCFL’s mechanism (whether a water accumulation or droplet entrainment at the entrance to the steam generator) is clarified via a comparison to a 50-mm CCFL facility at Kobe University. This paper tests the scalability of interface distribution at quasi-stationary conditions (i.e., points along the CCFLch curve) via a comparison of time-averaged interface distributions obtained at similar inlet conditions ( at the COLLIDER 190-mm and Kobe 50-mm channels. The comparison will show that interface distributions (which are directly linked to the pressure drop and interfacial momentum transfer) cannot be scaled at the bend/riser/entrance region because of the influence of the channel diameter upon occurring CCFL mechanism. Meanwhile, the water level gradient can be similar at the horizontal part, but not the relative water depth.

Counter-current flow limitation for air-water and steam-water flows in a PWR hot leg geometry

Steady state counter-current flow limitation (CCFL) experiments were conducted in a 1:3 scaled flat model of the hot leg and part of the steam generator inlet chamber of a German Konvoi pressurized water reactor. The experiments include air-water tests at 1 and 2bar as well as steam-water tests at 10, 25 and 50bar. Flooding characteristics are obtained in dependency on the dimensionless gas and liquid flow rates (Wallis-parameter). They showed a slight, but clear dependency on pressure – with increasing pressure more liquid can flow in counter-current to the gas in case of partial CCFL. Also the dimensionless gas flow rate for zero penetration slightly increases with pressure. Beside the flooding characteristics also slug frequencies were analyses basing on pressure measurements along the horizontal part of the test section. Finally, comprehensive video material was obtained which is suited for extracting quantitative data on the local flow structure. This can contribute to the further CFD-code development and validation.

Effects of Shape and Size on Countercurrent Flow Limitation in Flow Channels Simulating a PWR Hot Leg

A numerical study is presented to examine the effects on countercurrent flow limitation (CCFL) of the shape and size of hot leg models with a rectangular cross section. The CCFL was described in terms of Wallis parameters using the channel height H as the characteristic length. Numerical simulations, using the computational fluid dynamics software code FLUENT 6.3.26, were done for the air-water CCFL experiments carried out previously at Helmholtz-Zentrum Dresden-Rossendorf in a 1/3-scale hot leg model with a rectangular channel (H×W = 0.25×0.05 m2), and the results were compared with the air-water CCFL data obtained at Kobe University in a 1/5-scale hot leg model with rectangular cross section (H×W = 0.15×0.01 m2) and the results of simulations. It was found that both the height-to-width ratio and the size of the cross section affected the CCFL characteristics in the Wallis diagram. Comparison of CCFL characteristics in rectangular channels with those in circular channels showed that the hydraulic diameter Dh was a major cross-section geometry term influencing the CCFL characteristics. CCFL constants of the Wallis correlation were ~0.61 on average for the range 0.05 m ≤ Dh ≤ 0.75 m but became small for Dh ≤ 0.0254 m, and these tendencies were well reproduced by the numerical simulations.

VOF Simulations of Countercurrent Gas-Liquid Flow in a PWR Hot Leg

In order to evaluate flow patterns and CCFL (countercurrent flow limitation) characteristics in a PWR hot leg under reflux condensation, numerical simulations have been done using a two-fluid model and a VOF (volume of fluid) method implemented in the CFD software, FLUENT6.3.26. The two-fluid model gave good agreement with CCFL data under low pressure conditions but did not give good results under high pressure steam-water conditions. On the other hand, the VOF method gave good agreement with CCFL data for tests with a rectangular channel but did not give good results for calculations in a circular channel. Therefore, in this paper, the computational grid and schemes were improved in the VOF method, numerical simulations were done for steam-water flows at 1.5 MPa under PWR full-scale conditions with the diameter of 0.75 m, and the calculated results were compared with the UPTF data at 1.5 MPa. As a result, the calculated flow pattern was found to be similar to the flow pattern observed in small-scale air-water tests, and the calculated CCFL characteristics agreed well with the UPTF data at 1.5 MPa except in the region of a large steam volumetric flux.

Numerical Simulations of Counter Current Flow Experiments Using a Morphology Detection Algorithm

In order to improve the understanding of counter-current two-phase flows and to validate new physical models, CFD simulations of 1/3rd scale model of the hot leg of a German Konvoi PWR with rectangular cross section was performed. Selected counter-current flow limitation (CCFL) experiments at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) were calculated with ANSYS CFX 12.1 using the multi-fluid Euler-Euler modeling approach. The transient calculations were carried out using a gas/liquid inhomogeneous multiphase flow model coupled with a SST turbulence model for each phase. In the simulation, the surface drag was approached by a new correlation inside the Algebraic Interfacial Area Density (AIAD) model. The AIAD model allows the detection of the morphological form of the two phase flow and the corresponding switching via a blending function of each correlation from one object pair to another. As a result this model can distinguish between bubbles, droplets and the free surface using the local liquid phase volume fraction value. A comparison with the high-speed video observations shows a good qualitative agreement. The results indicated that quantitative agreement of the CCFL characteristics between calculation and experimental data was obtained. To validate the model and to study scaling effects CFD simulations of the CCFL phenomenon in a full scale PWR hot leg of the UPTF test facility were performed. Also these results indicated a good agreement between the calculation and experimental data. The final goal is to provide an easy usable AIAD framework for all ANSYS CFX users, with the possibility of the implementation of their own correlations.

104 PWRホットレグでの気液対向流の数値シミュレーション(OS1-1.数値シミュレーションの原子力への応用(1),OS・一般セッション講演)

104 PWRホットレグでの気液対向流の数値シミュレーション(OS1-1.数値シミュレーションの原子力への応用(1),OS・一般セッション講演)

VOF Calculations of Countercurrent Gas-Liquid Flow in a PWR Hot Leg

We improved the computational grid and schemes in the VOF (volume of fluid) method with the standard 𝑘−𝜀 turbulent model in our previous study to evaluate CCFL (countercurrent flow limitation) characteristics in a full-scale PWR hot leg (750 mm diameter), and the calculated CCFL characteristics agreed well with the UPTF data at 1.5 MPa. In this paper, therefore, to evaluate applicability of the VOF method to different fluid properties and a different scale, we did numerical simulations for full-scale air-water conditions and the 1/15-scale air-water tests (50 mm diameter), respectively. The results calculated for full-scale conditions agreed well with CCFL data and showed that CCFL characteristics in the Wallis diagram were mitigated under 1.5 MPa steam-water conditions comparing with air-water flows. However, the results calculated for the 1/15-scale air-water tests greatly underestimated the falling water flow rates in calculations with the standard 𝑘−𝜀 turbulent model, but agreed well with the CCFL data in calculations with a laminar flow model. This indicated that suitable calculation models and conditions should be selected to get good agreement with data for each scale.

Numerical Simulation of Size Effects on Countercurrent Flow Limitation in PWR Hot Leg Models

We have previously done numerical simulations using the two-fluid model implemented in the CFD software FLUENT6.3.26 to investigate effects of shape of a flow channel and its size on CCFL (countercurrent flow limitation) characteristics in PWR hot leg models. We confirmed that CCFL characteristics in the hot leg could be well correlated with the Wallis parameters in the diameter range of0.05 m≤D≤0.75 m. In the present study, we did numerical simulations using the two-fluid model for the air-water tests withD=0.0254 m to determine why CCFL characteristics forD=0.0254 m were severer compared with those in the range,0.05 m≤D≤0.75 m. The predicted CCFL characteristics agreed with the data forD=0.0254 m and indicated that the CCFL difference betweenD=0.0254 m and0.05 mm≤D≤0.75 mm was caused by the size effect and not by other factors.

Image-Processing-Based Study of the Interfacial Behavior of the Countercurrent Gas-Liquid Two-Phase Flow in a Hot Leg of a PWR

The interfacial behavior during countercurrent two-phase flow of air-water and steam-water in a model of a PWR hot leg was studied quantitatively using digital image processing of a subsequent recorded video images of the experimental series obtained from the TOPFLOW facility,Helmholtz-Zentrum Dresden-Rossendorf e.V.(HZDR), Dresden, Germany. The developed image processing technique provides the transient data of water level inside the hot leg channel up to flooding condition. In this technique, the filters such as median and Gaussian were used to eliminate the drops and the bubbles from the interface and the wall of the test section. A Statistical treatment (average, standard deviation, and probability distribution function (PDF)) of the obtained water level data was carried out also to identify the flow behaviors. The obtained data are characterized by a high resolution in space and time, which makes them suitable for the development and validation of CFD-grade closure models, for example, for two-fluid model. This information is essential also for the development of mechanistic modeling on the relating phenomenon. It was clarified that the local water level at the crest of the hydraulic jump is strongly affected by the liquid properties.

Numerical simulations for steam–water CCFL tests using the 1/3 scale rectangular channel simulating a PWR hot leg

Abstract In reflux condensation, steam generated in the reactor core and water condensed in a steam generator (SG) form countercurrent flow in a hot leg, which consists of a horizontal pipe, an elbow and an inclined pipe. Both countercurrent air–water and steam–water tests were previously carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) using the 1/3 scale rectangular channel simulating a PWR hot leg installed in the pressure chamber of the TOPFLOW facility. In this paper, in order to evaluate the effects of fluid properties, the steam–water CCFL (countercurrent flow limitation) tests at HZDR were simulated using the CFD software, FLUENT 6.3.26. The computational domain included the reactor vessel simulator, hot leg and SG inlet chamber in order to avoid uncertainties of boundary conditions at both ends of the hot leg. The VOF (volume of fluid) method and two-fluid (2F) model were used. In the 2F model, the combination of three correlations on the interfacial friction coefficients, which had been validated for the 1/15 and 1/5 scale tests at Kobe University, was used as a function of local void fractions. The CCFL characteristics predicted by the 2F and VOF agreed relatively well with the steam–water CCFL data at HZDR but overestimated the effects of fluid properties on CCFL. The VOF simulations were better able to reproduce the fluid properties than the 2F simulations.

Application of a new drag coefficient model at CFD-simulations on free surface flows relevant for the nuclear reactor safety analysis

This paper presents different CFD-simulations on flows which are relevant for nuclear reactor safety using a new modeling approach for the interfacial drag at free surfaces. The developed drag coefficient model was implemented together with the Algebraic Interfacial Area Density (AIAD) model ( Höhne, 2009) into the three-dimensional (3-D) computational fluid dynamics (CFD) code ANSYS-CFX. The applications considered include the prediction of counter-current flow limitations (CCFL) in a PWR hot leg, the development of hydraulic jump during the air–water co-current flow in a horizontal channel, and pressurized thermal shock (PTS) phenomena in a PWR cold leg and downcomer. For the modeling of these tasks, an Euler–Euler approach was used. This approach allows the use of different models depending on the local morphology. In the frame of an Euler–Euler simulation, the local morphology of the phases has to be considered in the drag model. To demonstrate the feasibility of the present approach, the computed main parameters of each case were compared with experimental data. It is shown that the CFD calculations agree well with the experimental data. This indicates that the AIAD model combined with new drag force modeling is a promising way to simulate the phenomena in frame of the Euler–Euler approach. Moreover the further validation of the model by including mass transfer effects should be carried out.

Numerical simulations of counter-current two-phase flow experiments in a PWR hot leg model using an interfacial area density model

In order to improve the understanding of counter-current two-phase flows and to validate new physical models, CFD simulations of 1/3rd scale model of the hot leg of a German Konvoi PWR with rectangular cross section was performed. Selected counter-current flow limitation (CCFL) experiments at the Helmholtz–Zentrum Dresden–Rossendorf (HZDR) were calculated with ANSYS CFX 12.1 using the multi-fluid Euler–Euler modeling approach. The transient calculations were carried out using a gas/liquid inhomogeneous multiphase flow model coupled with a k-ω turbulence model for each phase. In the simulation, the surface drag was approached by a new correlation inside the Algebraic Interfacial Area Density (AIAD) model. The AIAD model allows the detection of the morphological form of the two phase flow and the corresponding switching via a blending function of each correlation from one object pair to another. As a result this model can distinguish between bubbles, droplets and the free surface using the local liquid phase volume fraction value. A comparison with the high-speed video observations shows a good qualitative agreement. The results indicated that quantitative agreement of the CCFL characteristics between calculation and experimental data was obtained. The goal is to provide an easy usable AIAD framework for all Code users, with the possibility of the implementation of their own correlations.

Numerical simulation using CFD software of countercurrent gas–liquid flow in a PWR hot leg under reflux condensation

In order to improve the countercurrent flow model of a transient analysis code, countercurrent air–water tests were previously conducted using a 1/15 scale model of the PWR hot leg and numerical simulations of the tests were carried out using the two-fluid model implemented in the CFD software FLUENT 6.3.26. The predicted flow patterns and CCFL characteristics agreed well with the experimental data. However, the validation of the interfacial drag correlation used in the two-fluid model was still insufficient, especially regarding the applicability to actual PWR conditions. In this study, we measured water levels and wave heights in the 1/15 scale setup to understand the characteristics of the interfacial drag, and we considered a relationship between the wave height and the interfacial drag coefficient. Numerical simulations to examine the effects of cell size and interfacial drag correlations on numerical predictions were conducted under PWR plant conditions. Wave heights strongly related with the water level and interfacial drag coefficient, which indicates that the interfacial drag force mainly consists of form drag. The cell size affected the gas velocity at the onset of flooding in the process of increasing gas flow rate. The gas volumetric fluxes at CCFL predicted using fine cells were higher than those using normal cells. On the other hand, the cell size did not have a significant influence on the process of decreasing gas flow rate. The predictions for the PWR condition using a reference set of interfacial drag correlations agreed well with the Upper Plenum Test Facility data of the PWR scale experiment in the region of medium gas volumetric fluxes. The reference interfacial drag correlations employed in this study can be applied to the PWR conditions.

E113 VOF法を用いたPWRホットレグ内での対向流の数値シミュレーション(OS9 熱・流動(解析))

E113 VOF法を用いたPWRホットレグ内での対向流の数値シミュレーション(OS9 熱・流動(解析))