Rankine-vortex model based assessment of CFD methods for simulating the effect of gas entrainment observed in the hot-pool of sodium coold fast breeder reactors

International audienceThe flow behavior in the hot pool of a compact Sodium Cooled Fast Breeder Reactor (SFR) is investigated in the MICAS facility of the CEA. Computational Fluid Dynamics (CFD) is used to assist the interpretation of the experiments by multi-scale calculations. Special attention is given to the risk of gas entrainment into the reactor core due to the formation of drain-type vortexes at the intermediate heat exchanger intakes. After theoretical considerations based on the Rankine-vortex model, the single effect gas entrainment experiment of Monji is analyzed by different CFD methods. The well-known incapacity of linear RANS models to simulate swirling flow is emphasized and limitations regarding the representation of gas cores by single-phase models are discussed. Then, the flow in the MICAS facility is analyzed by CFD using RANS and LES. The quantitative comparison between experiment and calculation shows that RANS can simulate correctly the global flow patterns in the hot pool. Small-scale drain-type vortexes have not been observed in the RANS calculations. LES can predict the onset of the formation of drain-type vortexes. However, single-phase models cannot simulate stable drain-vortexes

Plateau facility in support to ASTRID and the sfr program: an overview of the first mock-up of the ASTRID upper plenum, MICAS

International audienceThe CEA and several industrial partners are involved in the development of a 4th generation reactor cooled by sodium, ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration). It is a 600MWe pool type reactor integrating cutting edge technologies.Developments are in progress especially for the vessel and the apparatus. Experiments are needed for both validation of numerical codes and specific studies. In this way, a thermal-hydraulic loop, the PLATEAU facility, has been developed and built at the CEA Cadarache. Different mock-ups can be connected to this loop to study the different issues at various reactor conditions. Currently, 4 mock-ups are identified the internal vessel, the entire pool reactor, the link between the external and the internal vessel, a part of the internal vessel at a higher scale to study specific issues.The MICAS mock-up at 1/6 scale is dedicated to study the flow regime of the internal vessel (hot plenum), both for code validation and also engineering design development.This mock-up has been built in transparent polymer to carry out some optical measurements as laser velocimetry. Numerical studies have been engaged to determine the general flow pattern in a way to identify the area of interest for detailed phenomenological studies

Flow analysis in the upper plenum of the micas model in support of the astrid reactor program

International audienceThe sodium cooled reactor ASTRID project (Advanced Sodium Technological Reactor for Industrial Demonstration) led by the CEA has entered the basic design phase. Current developments are focused on the large equipment such as the vessels. Even if France has a great experience in the SFR technology because of the 90's EFR program, new experiments are needed both to qualify the design options and to validate the code calculations. Since experiments with sodium are more complex to carry out, most tests are conducted in models using water as simulant fluid. According to the dimensionless analysis, experiments can be designed to be representative of the phenomenology to study. The MICAS mock-up of the ASTRID upper plenum was designed regarding this methodology using the Froude number for the free surface flow issues and the Richardson number for the thermal studies. It was built at a 1/6 scale in transparent polymer for optical measurements.First experiments were dedicated to study the free surface flow and the gas entrainment. Experimental conditions, such as the water flow rate, were calculated to get the representative Froude number. From those operating conditions and the MICAS geometry, calculations were led using TRIO_CFD at CEA and Adapco StarCCM+ at AREVA. This article is devoted to present the methodology and the results of the velocity measurements in the MICAS model dedicated to validate the code calculations. A PIV system was used to measure the velocity. As this technique is based on an optical access to two perpendicular planes, its implementation on the MICAS mock-up was complex. Indeed, many immerged components, such as the pump pits and the decay heat exchangers, prevent any optical access in specific areas. Most measurements were performed around the core, the Intermediate Heat Exchanger (IHX) and the Above Core Structure (ACS). The integration of the velocity around the ACS allows calculating the flow rate crossing this structure. As the control rods in the ACS are modeled by porous media, it is an important result to fit the pressure drop coefficient in the code

MILIPOSO, Mock-up of the Diagrid and the Coupling Pipes in Support of the ASTRID SFR Program

International audienceThe CEA and several industrial partners are involved in the development of a 4th generation reactor cooled by sodium, ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration). From the 90's EFR project, the Phoenix and Super-Phoenix reactors operation feedback, the CEA has acquired a very significant experience about Sodium Fast Reactors (SFR). A new generation of simulation codes is used for the reactor design and the safety issues. Due to the physical complexity, the models need experimental validations in reduced scaled mock-up. In those purposes, a thermal-hydraulic loop, the PLATEAU facility, has been developed and built at the CEA Cadarache to study different ASTRID mock-ups under various operating conditions. Currently, 4 mock-ups are identified the MICAS model devoted to study the free surface and the flow pattern in the upper plenum, the MICONOS mock-up dedicated to the investigation of natural convection issue in the entire pool reactor, the MISHOCO model used to analyze specific issues at a higher scale in the upper plenum, and the MILIPOSO mock-up dedicated to the study of diagrid flows and the primary coupling pipes between the pump and the diagrid.This article presents the MILIPSO model the identified ASTRID reactor issues, the sizing, the design and the instrumentation. Dimensionless analysis was employed to define the reduced scale and the experimental operating conditions regarding the specific issues to investigate. This mock-up is built in transparent polymer to carry out some optical measurements such as laser velocimetry and fast video. It was designed as versatile as possible to study the effect of the dimensionless numbers on the flow pattern. Some geometrical features were simplified because of the reduced scale. The hexagonal assemblies were replaced by cylindrical tubes. To observe the right flow-rate map at the diagrid exit, different sized diaphragms were used at the assemblies outlet. The experimental program includes nominal and accidental operating conditions, cavitation studies in the primary coupling pipes, gas accumulation investigations in the diagrid. The instrumentation is based on pressure sensors at various model locations, laser Doppler velocimetry and Particles Image Velocimetry to measure respectively the velocity in the diagrid and in the primary coupling pipes

CFD analysis of the flow in the MICAS experimental facility, a water model of the hot pool of a sodium cooled fast reactor

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Analysis of the Gas Entrainment by Vortex in the Upper Plenum of the ASTRID Reactor Mock-up

International audienceThe 4th generation sodium fast neutron reactor ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) is developed by the CEA and several industrial partners. The design is in progress especially for the vessel and the equipment. From the state of art of the former sodium reactors Phenix and Super-Phenix and the EFR program (European Fast Reactor) the global and the local thermal-hydraulic issues have been listed. But the results of those previous projects are not sufficient for the current ASTRID development. New needs for experimental means were identified, especially for both validation of numerical codes and specific studies. In this way, a thermal-hydraulic loop, the PLATEAU facility, was developed and built at the CEA Cadarache research center in 2012. Since experiments with sodium are complex to carry out due to the harmfulness of this liquid metal, the tests are lead in models using water as simulant fluid. Different mock-ups can be connected to this loop to study the different issues at various reactor conditions. The first model connected to the PLATEAU facility is MICAS, mock-up of the hot plenum. This one is dedicated to study the flow regime, both for code validation and also engineering design development. It was designed at 1/6 scale and was built in transparent polymer to carry out optical measurements as laser velocimetry and fast imaging.Gas entrainment by vortex is one of the issues identified and can have an impact on the vessel design. Gas occurrence in the core induces neutron effects which may leadto core reactivity variation. Gas entrainment was first investigated at the MICAS free surface using fast imaging. A tendency regarding the geometry and the operating parameters was identified. Further investigations were focused on the velocity field around the vortices in the way to develop a gas entrainment criterion. Two types of vortex occur at the free surface those which induce gas entrainment, and those which do not. By the used of the Burgers vortex model, the downward velocity gradient and the circulation are identified as parameters influencing the gas entrainment. From the tangential velocity measured by particles image velocimetry around various types of vortex, the downward velocity gradient and the circulation were calculated using the Burgers model. Results showed that the Burgers model fit the experimental tangential velocity. Depending on the gas entrainment occurrence among the different studied vortex, a value for the gas entrainment criterion was found

Scale effects methodology applied to uprising jets in astrid

International audienceThe CEA is involved in the development of the 4 th generation of nuclear reactors, among which are the Sodium Cooled Fast Reactors (SFR). To support their design and safety, specific codes were developed and should be validated using experimental results from relevant mock-ups. Due to the complexity of building a full-sized prototype in the nuclear field, most of the experiments are performed on reduced-sized models but it may lead to scale effects. Such scale effect is under study in this paper, focusing on a critical issue in SFR reactor, which is the rising of the jet outgoing the core at low power. On this issue, a scale effects methodology is detailed, using the SFR's reduced scale mock-up MICAS as the reference scale. To achieve that, dimensionless Navier-Stokes equations under Boussinesq's approximation are considered and the Vaschy-Buckingham theorem is applied to determine the most relevant dimensionless numbers: the densimetric Froude number. Experimental campaigns have been performed to measure velocity fields. Their analysis clearly shown dependency of mean pathway of the jet on this dimensionless number. Particularly, plotting the jet angle as a function of the normalized densimetric Froude number, a change of behaviour for a threshold value of 0.45 has been observed. This result also shows negligible effects on the jet rise of the phenomena happening before jet impingement on the Upper Core Structure (UCS)

Two color LIF techniques applied on a two jets mixing flow Challenges and experimental program on a complex mock up

International audienceThere are thermal hydraulics issues of upper plenum of sodium cooled reactor ASTRID which can not be studied from past reactors feedback and numerical simulations since the calculation codes do not allow to model these problems with sufficient confidence. Thus in order to validate the numerical approaches and systems concept of ASTRID to fit the codes and simulations for validation and to better understand the complex thermal hydraulic phenomena in order to qualify the design options, we need to invetsigate temperature and velocity in this complex geometry. Laser Induced Fluorescence and Particle Image Velocimetry are optical methods used to measure temperature and velocity respectively. In all forms of LIF, a laser is used to excite fluorescent species within the flow. Typically, the tracer, an organic fluorescent dye such as Fluorescein or Rhodamine, absorbs a portion of the excitation energy and spontaneously re-emits a portion of the absorbed energy as fluorescence. In our experiments, single dye and two color LIF techniques are used to perform calibrations to study the fluorescence response of the dyes as function of main parameters: dye concentration, laser power, and temperature variation. Regarding PIV, the fluid is seeded with tracer particles then illuminated so that the particles are visible. The motion of the seeding particles is used to calculate velocity and direction of the flow being used. An experiment representative of the mixing flows in the MICAS mock up made up of two jets was designed to develop the LIF and PIV measurement process. The first jet is hot and the second cold, then optical methods for measuring simultaneously temperature by LIF and velocity by PIV is applied on the mock up. The interaction between the two jets at different temperatures and different configurations is studied regarding main dimensionless values such as Reynold's number, Richardson number and Froude number

Scale effects methodology applied to uprising jets in astrid

International audienceThe CEA is involved in the development of the 4 th generation of nuclear reactors, among which are the Sodium Cooled Fast Reactors (SFR). To support their design and safety, specific codes were developed and should be validated using experimental results from relevant mock-ups. Due to the complexity of building a full-sized prototype in the nuclear field, most of the experiments are performed on reduced-sized models but it may lead to scale effects. Such scale effect is under study in this paper, focusing on a critical issue in SFR reactor, which is the rising of the jet outgoing the core at low power. On this issue, a scale effects methodology is detailed, using the SFR's reduced scale mock-up MICAS as the reference scale. To achieve that, dimensionless Navier-Stokes equations under Boussinesq's approximation are considered and the Vaschy-Buckingham theorem is applied to determine the most relevant dimensionless numbers: the densimetric Froude number. Experimental campaigns have been performed to measure velocity fields. Their analysis clearly shown dependency of mean pathway of the jet on this dimensionless number. Particularly, plotting the jet angle as a function of the normalized densimetric Froude number, a change of behaviour for a threshold value of 0.45 has been observed. This result also shows negligible effects on the jet rise of the phenomena happening before jet impingement on the Upper Core Structure (UCS)