Pressure Suppression Pool Research Articles

CFD simulation of thermal stratification and mixing in a Nordic BWR pressure suppression pool

Boiling Water Reactor (BWR) employs the Pressure Suppression Pool (PSP) as a heat sink to prevent overpressure of the reactor vessel and containment. Steam can be injected into the PSP through spargers in normal and accident conditions and through blowdown pipes in case of a loss of coolant accident (LOCA). There is a safety limit on the maximum PSP temperature at which such steam injection might cause dynamic loads on the containment structures. The performance of the pool can be affected if thermal stratification is developed when temperature of the hot layer grows rapidly while cold layer remains inactive. Simulation of pool behavior during realistic accident scenarios requires validated models that can sufficiently address the interaction between phenomena, safety systems and operational procedures. Direct modeling of steam injection into a water pool in long-term transients is computationally expensive due to the need to resolve simultaneously the smallest space and time scales of individual steam bubbles and the scales of the whole PSP. To enable PSP analysis for practical purposes, Effective Heat source and Effective Momentum source (EHS/EMS) models have been proposed that avoid the need to resolve steam-water interface. This paper aims to implement mechanistic approaches previously developed by authors for the simulation of transient thermal stratification and mixing phenomena induced by steam injection through spargers in a Nordic BWR PSP. The latest version of the EHS/EMS models using the ‘Unit cell’ approach has been validated against integral effect pool tests and applied to plant simulations. Several scenarios with boundary conditions corresponding to postulated accident sequences were simulated to investigate the possibility of stratification development and the effects of activation of different systems (e.g., blowdown pipes, high momentum nozzle) on the pool behavior.

Effect of diameter and inclination of steam injection pipe on chugging dynamics in direct contact condensation

Low mass flux of steam and high degree of subcooling are the prominent factors for occurrence of unstable direct contact condensation, commonly known as chugging, when steam is injected into a subcooled water pool. Such an arrangement is typically used in the pressure suppression pool of the primary containment vessel in a boiling water reactor. With an attempt to identify the conditions of minimum pressure oscillations during the chugging regime, the present study investigates the effect of internal diameter (0.5–1 in.) and inclination (8.75˚) of steam injection pipe on steam bubble dynamics, pressure transients and heat transfer characteristics at different steam mass flow rates (5–25 kg/hr) and submergence depths of steam pipe (1–13 cm) within the water pool. The study reveals that the condensation cycle time decreases with a decrease in the internal diameter of the pipe. Furthermore, the frequencies of fluctuations in the interfacial area of the steam bubble and pressure transients in the steam pipe exhibit an increasing trend with an increase in steam mass flow rate and a decrease in internal diameter of the steam injection pipe. The chugging frequencies in the inclined and vertical orientation of steam injection pipe lie within the range of 0.5–3 Hz. The average heat transfer coefficient increases with an increase in the internal diameter of pipe and steam mass flow rate. The amplitude of pressure oscillations has decreased significantly whereas heat transfer coefficient has increased marginally upon changing the orientation of steam injection pipe from vertical to inclined. Therefore, smaller diameter and inclined orientation of steam injection pipe are favourable for mitigation of pressure oscillations in applications of direct contact condensation.

Numerical simulation of air discharged in subcooled water pool

Turbulent jet discharges in subcooled water pools are essential for safety systems in nuclear power plants, specifically in the pressure suppression pool of boiling water reactors and In-containment Refueling Water Storage Tank of advanced pressurized water reactors. The gas and liquid flow in these systems is investigated using multiphase flow analysis. This field has been extensively examined using a combination of experiments, theoretical models, and Computational Fluid Dynamics (CFD) simulations. ANSYS CFX offers two approaches to model multiphase flow behavior. The non-homogeneous Eulerian-Eulerian Model has been used in this work; it computes global information and is more convenient to study interpenetrated fluids. This study utilized the Large Eddy Simulation Model as the turbulence model, as it is better suited for non-stationary and buoyant flows. The CFD results of this study were validated with experimental data and theoretical results previously obtained. The figures of merit dimensionless penetration length and the dimensionless buoyancy length show good agreement with the experimental measurements. Correlations for these variables were obtained as a function of dimensionless numbers to give generality using only initial boundary conditions. CFD numerical model developed in this research has the capability to simulate the behavior of non-condensable gases discharged in water.

Simulation of jets induced by steam injection through multi-hole sparger using effective heat and momentum models

Direct contact condensation (DCC) of steam in the pressure suppression pool (PSP) is used to control containment pressure in Boiling Water Reactors (BWRs) and Advanced Pressurized Water Reactors (APWRs). The competition between momentum and heat sources induced by steam injection through multi-hole spargers and blowdown pipes determines whether the pool is thermally stratified or mixed. Development of thermal stratification affects capacity of the PSP to condense steam. To enable computationally efficient modeling of the PSP transients, the Effective Heat Source (EHS) and Effective Momentum Source (EMS) models have been proposed previously. The EHS/EMS models enable simulation of the large scale pool behavior without explicit modeling of the steam water interface and DCC phenomena. One of the problems for an optimal implementation of the EHS/EMS models is the definition of the boundary conditions that impose distribution of the momentum and heat sources. In this work, EHS/EMS models are implemented using “Unit Cell” approach in ANSYS Fluent to provide detailed numerical analysis of the individual turbulent jets induced by steam injection through the sparger holes. The model is validated against data from Particle Image Velocimetry (PIV) and temperature measurements for a range of steam injection conditions in the PANDA HP5 tests. Good agreement between the test data and simulations suggests that the proposed model can provide sufficiently accurate prediction of both local and large scale phenomena induced by steam injection into the pool.

PANDA experiments within the OECD/NEA HYMERES-2 project on containment hydrogen distribution, thermal radiation and suppression pool phenomena

In this article, we summarize the experimental results obtained in the large-scale thermal-hydraulics PANDA facility (PSI, Switzerland) within the OECD/NEA HYMERES2 project. The experiments addressing hydrogen distribution allowed characterizing the flow patterns forming from jet flows impacting plates or grating platforms and the following interaction with a postulated hydrogen cloud in the containment dome. The experiments with activation of safety components allowed to compare the effects of multiple spray nozzles with a single spray nozzle or multiple cooler units with unique cooler and to assess the scalability of the related thermal hydraulics phenomena such as pressure histories, power removed, gas mixture distribution. The experiments related to thermal radiation confirmed that the presence of steam in a containment atmosphere even at very low concentration (in the order of 0.1 %) has a significant impact on the evolution of containment mixture temperature.The experiments studying pressure suppression pool phenomena provided insight on the formation and stability of thermocline under a variety of steam release conditions, e.g. from load reduction rings and from multi-hole sparger; moreover allowed to characterize containment pressure histories as function of sparger submergence in the suppression pool.

Pre-test analysis for definition of steam injection tests through multi-hole sparger in PANDA facility

Pressure Suppression Pool (PSP) is a passive safety feature in Boiling Water Reactors (BWR) and Advanced Pressurized (AP) reactors. Steam released from the primary coolant system is condensed in a large water pool to prevent containment overpressure. Injected steam induces sources of heat (buoyancy force) and momentum (inertia). The competition between the sources might result in the development of thermal stratification or mixing of the pool. Increased temperature of the top pool layer leads to higher partial pressure of steam in the containment and thus reduces pressure suppression capacity of the pool. Models with predictive capabilities are needed for the analysis of the reactor pool transients. Development and validation of the models require adequate experimental data. In this work we discuss results of the pre-test analysis that was carried out to select conditions for the tests with steam injection through sparger head and Load Reduction Ring (LRR) in a large scale PANDA facility. The aim of the tests was to obtain data on pool thermal stratification and mixing under different regimes of steam injection. Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were implemented in a computational fluid dynamics (CFD) code in order to carry out the analysis. Evolution of the pool temperature and velocity characteristics were analyzed in the scoping analysis to provide suggestions for selection of (i)pool depth, (ii)elevations of the sparger head and LRR, (iii)number of open LRR holes, (iv)layout of instrumentation, and (v)steam injection procedure for each test.

AP1000 IRWST numerical analysis with GOTHIC

The AP1000 Passive Residual Heat Removal (PRHR) system plays a significant role as it helps to remove the core decay heat, using the In-containment Refueling Water Storage Tank (IRWST) as a heat sink. The IRWST is located above the core, promoting natural circulation and allowing to be the mid-term heat-sink for the reactor core. The thermo-hydraulic pool behavior during an accident has an influence in the containment pressure and temperature, as happens in the pressure suppression pool in BWR reactors.Due to the importance of the IRWST performance on the AP1000 containment, a numerical analysis of a scaled experiment has been done using the GOTHIC code. Firstly, an IRWST scaled-down model is created, a mesh sensitivity analysis is performed, and the results obtained are compared against experimental results available in the literature. It is found the importance of the mesh discretization for the proper thermal-stratification modeling. Then, the reference model setup and mesh are applied in a full-scale prototypic model. The mesh influence on the thermal stratification is analyzed, as well as its impact on the AP1000 containment pressure. The mesh and setup for the full-scale model are selected to be implemented in a full containment 3D model in future works.

Experimental study on the scrubbing efficiency of aerosols contained in horizontal and vertically downward submerged gas jet

The scrubbing efficiency of aerosols contained in submerged gas jet in Pressure Suppression Pool (PSP) is important during the severe accident of Nuclear Power Plant (NPP). To analyze the influence of nozzle inlet pressure (from 0.12 MPa to 0.4 MPa) and gas injection direction (horizontal and vertically downward direction) on scrubbing efficiency of aerosols in non-condensable gas submerged jet, several experiments were carried out. The experimental results show that the aerosol scrubbing efficiency increases as the inlet pressure increases. Most of the aerosols in gas are retained in water when the nozzle inlet pressure is higher than 0.2 MPa, and the scrubbing efficiency is lower than 90% under low inlet pressure. The effect of gas injection direction on scrubbing efficiency is significant when the gas flow state at nozzle exit is subcritical flow, but the effect is not obvious when the inlet pressure is higher than 0.3 MPa and the gas flow state at nozzle exit is critical flow.

Direct numerical simulation of phase change in the presence of non-condensable gases

This paper describes a new numerical method for the simulation of phase change phenomena between a liquid and a vapour in the presence of non-condensable gases. The method is based on an interface-tracking approach in the framework of single-fluid modelling. The principal innovative feature represented is the capability of simulating a mixture of the condensable gas (vapour) and non-condensable gases with different densities. The formulation and subsequent discretization of the governing equations for the species transport are discussed in detail. In particular, a volume-averaged velocity field is introduced into the species transport equation in combination with a mass-averaged velocity field approach for the momentum equations. The resulting algorithm has been implemented into the incompressible Navier–Stokes solver, PSI-BOIL, which features a finite-volume approach based on a fixed, rectangular, Cartesian grid. Several verification cases have been undertaken to ensure the code modifications have been correctly implemented. These include simulation of the Stefan problem, involving evaporation and condensation in a 1D configuration, and an evaporating droplet under forced convective flow. In all cases, very good agreement has been obtained with analytical solution. A simulation of direct-contact condensation of a practical application is also presented, which serves to demonstrate the potential capability of the new approach to a wider range of engineering problems, including pressure suppression pools in nuclear reactors.

Modelling of a Nordic BWR containment and suppression pool behavior during a LOCA with GOTHIC 8.1

Boiling water reactors use the Pressure Suppression Pool (PSP) to relieve the containment pressure in case of an accident. During the event of a Loss of Coolant Accident (LOCA), drywell air and steam are injected into the PSP through blowdown pipes. This may lead to thermal stratification, which is a relevant safety issue as it leads to higher water surface temperatures than in mixed conditions and thus, to higher containment pressures. The Effective Heat (EHS) and Momentum (EMS) Source models were previously introduced to predict the effect of small-scale direct contact condensation phenomena on the large-scale pool water circulation. In this paper, the EHS/EMS models are extended by adding the effect of non-condensable gases on the chugging regime. The EHS/EMS models are implemented in the GOTHIC code to model a full-scale Nordic BWR containment under different LOCA scenarios. The results show that thermal stratification can be developed in the PSP.

Thermal stratification and mixing in a Nordic BWR pressure suppression pool

The pressure suppression pool of a Nordic Boiling Water Reactor (BWR) serves as a heat sink to condense steam from the primary coolant system in normal operation and accident conditions. Thermal stratification can develop in the pool when buoyancy forces overcome the momentum created by the steam injection. In this case, hot condensate forms a hot layer at the top of the pool, reducing the pool cooling and condensation capacity compared to mixed conditions. The Effective Heat Source and Effective Momentum Source (EHS/EMS) models were previously proposed to model the large-scale pool behavior during a steam injection. In this work, we use CFD code of ANSYS Fluent with the EHS/EMS models to simulate the transient behavior of a Nordic BWR pool during a steam injection through spargers. First, a validation against a Nordic BWR pool test with complete mixing is presented. Prediction of the pool behavior for other possible injection scenarios show that stratification can occur at prototypic steam injection conditions, and that the hot layer temperature above the injection point can be non-uniform. In cases with significant steam condensation inside the sparger pipes, the 95 °C pool temperature limit for the Emergency Core Cooling System (ECCS) pumps was reached ∼7 h after the beginning of the blowdown.

Experimental study on penetration characteristic of submerged steam jet in quiescent water

AbstractThe direct contact condensation is an effective way to rapid depressurization for light water reactor, such as pressure suppression pool, for which the characteristic of steam plume is a key parameter to evaluate the efficiency of depressurization. In this paper, a series of visualization experiments are described which investigate the characteristic of steam plume, the influence of jetting direction, air mass fraction, water temperature, diameter of jet on the steam plume were analyzed. The results show that, the dimensionless steam plume length increased with the increase of inlet pressure and water temperature for pure steam submerged jet, a correlation was set up to predict the dimensionless penetration length and the predicted errors were within the band of ±15%. Furthermore, the penetration length decreased with the rise of air mass fraction due to buoyancy force, the penetration length increased with the increase of inlet pressure, and the jet penetration length for vertical jet is longer than horizontal jet about 25% at the same inlet pressure and air mass fraction.

Frequency analysis of chugging condensation in pressure suppression pool system with pattern recognition

Abstract Direct contact condensation (DCC) phenomena in boiling water reactor (BWR) pressure suppression pool systems need to be understood to properly assess the performance of the pool as a heat sink and as a safety critical structure. Condensation oscillations in the form of chugging are challenging to predict by computational fluid dynamics (CFD) methods but safety relevant because of associated high dynamic loads on in-pool structures and the pool itself. Recently, new measurement methods for CFD validation purposes have become available. One of these techniques is visual observation using the high-speed cameras and suitable data processing method. Pattern recognition is a well suited technique for the determination of large oscillating bubble dynamics in a pressure suppression pool. In this work, the formation and collapse of the steam bubbles in chugging condensation mode are evaluated by using the pattern recognition algorithm. The pattern recognition algorithm is based on video material recorded during the direct contact condensation experiment DCC-05 of the PPOOLEX test facility. The formation speed, the shape and size of the steam bubbles and the acceleration of collapsing bubbles are estimated with the algorithm. Fast Fourier transform (FFT) is used for frequency analysis of the pattern recognized data. The frequencies found are compared to the frequency data of the pressure transducers collected during the experiment and to the previous results of the NEPTUNE_CFD simulations of the same experiment. The frequency analysis shows that the chugging frequencies of the steam bubbles range from 1 to 3 Hz, as predicted. Also the natural frequencies of the bubbles are visible around 53 Hz. Another frequency spike was observed close to the 125 Hz. This frequency is close to the mechanical resonance frequencies of the suppression pool and the blowdown pipe. Because of neither the pressure suppression pool nor the blowdown pipe are visible to the pattern recognition, the spike of the higher frequencies is most likely from the interfacial area of the bubble which resonates with the suppression pool system, affecting rapid condensation at a certain point.

Modelling of pool stratification and mixing induced by steam injection through blowdown pipes

Containment overpressure is prevented in a Boiling Water Reactor (BWR) by condensing steam into the pressure suppression pool. Steam condensation is a source of heat and momentum. Competition between these sources results in thermal stratification or mixing of the pool. The interplay between the sources is determined by the condensation regime, steam mass flow rate and pool dimensions. Thermal stratification is a safety issue since it limits the condensing capacity of the pool and leads to higher containment pressures in comparison to a completely mixed pool with the same average temperature. The Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were previously developed for predicting the macroscopic effect of steam injection and direct contact condensation phenomena on the development of stratification and mixing in the pool. The models provide the effective heat and momentum sources, depending on the condensation regimes. In this work we present further development of the EHS/EMS models and their implementation in the GOTHIC code for the analysis of steam injection into containment drywell and venting into the wetwell through the blowdown pipes. Based on the PPOOLEX experiments performed in Lappeenranta University of Technology (LUT), correlations are derived to estimate the steam condensation regime and effective heat and momentum sources as functions of the pool and steam injection conditions. The focus is on the low steam mass flux regimes with complete condensation inside the blowdown pipe or chugging. Validation of the developed methods was carried out against the PPOOLEX MIX-04 and MIX-06 tests, which showed a very good agreement between experimental and simulation data on the pool temperature distribution and containment pressure.

Thermal stratification and mixing in a suppression pool induced by direct steam injection

An experimental and numerical investigation of thermal stratification and mixing in a suppression pool is presented. Steam injected into a drywell flows through a blowdown pipe and then down to the pressure suppression pool where direct contact condensation occurs. The steam venting and condensation is a source of heat and momentum. A complex interplay between the two leads either to thermal stratification or mixing of the pool. The experiments are conducted in a scaled down PPOOLEX facility at Lappeenranta University of Technology (LUT). The corresponding numerical simulations are performed using GOTHIC with the Effective Heat Source (EHS) and Effective Momentum Source (EMS) models. The EHS/EMS models, that have been previously proposed, predict the development of thermal stratification and mixing during a steam injection into a large pool of water. The experiments exhibit the development of thermal stratification in the pool at relatively low mass flow rates and then pool mixing when the mass flow rates are increased but later thermal stratification can re-develop even at the same relatively high mass flow rates, which is due to the increasing pool temperature that shifts the condensation to a different regime. The numerical simulations quantitatively capture this complex transient pool behavior and are in excellent agreement with the transient averaged pool temperature and water level in the pool.

Experimental study on steam chugging phenomenon in a vertical sparger

Unstable direct contact condensation called ``Chugging'' that occurs in certain conditions in the pressure suppression pool of Primary Containment Vessel of Boiling Water Reactors (BWRs) was studied experimentally. The mechanisms of every phase of the chugging was described, and experimental results useful for the development and validation of more accurate CFD models were provided. The experiment was conducted with a transparent pool and a transparent polycarbonate pipe or a stainless steel pipe with inner diameter of 27mm under the conditions of the steam mass flux of 5.5–19.5kg/m2s and the pool temperature of 19–46.5°C. Pressure pulses were measured and synchronized with a high speed video camera for images acquisition. It was identified that the bubble implosion occurred while the pressure in the bubble quickly decreased. This condition might establish instability in the interfacial area which grew abruptly causing the implosion. Moreover the transparent apparatus allowed to interpret and relate internal condensations, generating pressure spikes of around 1.2MPa because of the condensation-induced water hammer. Finally, the chugging condensation regime map was created from the experimental data.

Direct contact condensation modeling in pressure suppression pool system

The pressure suppression pool of boiling water reactor as a safety system has vital importance from the nuclear reactor safety point of view and it is an interesting challenge for numerical simulations of flow with rapid phase change. This paper presents the recent analysis of computational fluid dynamics (CFD) simulations of chugging direct contact condensation (DCC) mode observed in the drywell–wetwell suppression pool (PPOOLEX) experiments of Lappeenranta University of Technology. A pattern recognition algorithm was employed to determine the bubble volume and the chugging frequency during the test. The numerical simulations were performed by using Eulerian–Eulerian two-fluid approach of the compressible flow NEPTUNE_CFD software and the OpenFOAM CFD code. The interfacial heat transfer between steam and water was modeled by using three DCC models. Flow turbulence was solved by employing two k-ε turbulence models. The significance of interfacial area modeling on the chugging DCC was tested by implementing Rayleigh–Taylor Interfacial area model. The performance of different DCC models, the effects of turbulence modeling and interfacial area modeling are presented. The sensitivity of chugging DCC to the initial conditions and to the modeled domain, and the effect of interfacial momentum transfer closure modeling on the chugging are briefly discussed. The choice of DCC model, accuracy of interfacial area modeling and the magnitude of liquid turbulence near the interface are all needed to replicate chugging in a drywell–wetwell suppression pool system. In coarse grid case, the DCC model of Coste 2004 with the Rayleigh–Taylor interface instability model of Pellegrini et al. (2015) provided best results.

Development of an aerosol decontamination factor evaluation method using an aerosol spectrometer

During a severe nuclear power plant accident, the release of fission products into containment and an increase in containment pressure are assumed to be possible. When the containment is damaged by excess pressure or temperature, radioactive materials are released. Pressure suppression pools, containment spray systems and a filtered containment venting system (FCVS) reduce containment pressure and reduce the radioactive release into the environment. These devices remove radioactive materials via various mechanisms. Pressure suppression pools remove radioactive materials by pool scrubbing. Spray systems remove radioactive materials by droplet−aerosol interaction. FCVS, which is installed in the exhaust system, comprises multi-scrubbers (venturi-scrubber, pool scrubbing, static mixer, metal−fiber filter and molecular sieve). For the particulate radioactive materials, its size affects the removal performance and a number of studies have been performed on the removal effect of radioactive materials.This study has developed a new means of evaluating aerosol removal efficiency. The aerosol number density of each effective diameter (light scattering equivalent diameter) is measured using an optical method, while the decontamination factor (DF) of each effective diameter is evaluated by the inlet outlet number density ratio. While the applicable scope is limited to several conditions (geometry of test section: inner diameter 500mm×height 8.0m, nozzle shape and air-water ambient pressure conditions), this study has developed a numerical model which defines aerosol DF as a function of aerosol diameter (d) and submergences (x).

Numerical modelling of low-Reynolds number direct contact condensation in a suppression pool test facility

In the safety pressure suppression pool systems of Boiling Water Reactors (BWRs), the condensation rate has to be maintained high enough in order to fulfill their safety function. A major part of this condensation occurs as direct contact condensation (DCC), which governs different modes varying from vigorous chugging of collapsing bubbles to mild condensation on almost flat steam–water interface. This paper discusses the Computational Fluid Dynamics (CFD) simulations of the latter, low-Reynolds number weak condensation regime. The numerical simulations were performed with two CFD codes, NEPTUNE_CFD and OpenFOAM, in which the DCC phenomenon was modelled by using the Eulerian two-fluid approach of interpenetrating continua without interfacial tracking. The interfacial heat transfer between steam and water was modelled by using the DCC models based on the surface renewal and the surface divergence theories. Flow turbulence was solved by employing the standard k–∊ turbulence model. The CFD results of this study were validated against the test results of the POOLEX facility of Lappeenranta University of Technology. In the reference test STB-31, the condensation phenomena were limited to only occur on a stable steam–water interface by very low steam mass flux applied and thermal insulation of the blowdown pipe. The simulation results demonstrated that the surface divergence model predicted the condensation phenomena quite accurately both qualitatively and quantitatively while the surface renewal model overestimated it strongly.

Approach and Development of Effective Models for Simulation of Thermal Stratification and Mixing Induced by Steam Injection into a Large Pool of Water

Steam venting and condensation in a large pool of water can lead to either thermal stratification or thermal mixing. In a pressure suppression pool (PSP) of a boiling water reactor (BWR), consistent thermal mixing maximizes the capacity of the pool while the development of thermal stratification can reduce the steam condensation capacity of the pool which in turn can lead to pressure increase in the containment and thereafter the consequences can be severe. Advanced modeling and simulation of direct contact condensation in large systems remain a challenge as evident in commercial and research codes mainly due to small time-steps necessary to resolve contact condensation in long transients. In this work, effective models, namely, the effective heat source (EHS) and effective momentum source (EMS) models, are proposed to model and simulate thermal stratification and mixing during a steam injection into a large pool of water. Specifically, the EHS/EMS models are developed for steam injection through a single vertical pipe submerged in a pool under two condensation regimes: complete condensation inside the pipe and chugging. These models are computationally efficient since small scale behaviors are not resolved but their integral effect on the large scale flow structure in the pool is taken into account.