Implementation and validation of a particle resuspension model in a CFD code – Application to an air ingress scenario in a vacuum toroidal vessel

International audienceDuring a loss of vacuum accident (LOVA), dusts that will be present in the future tokamak ITER are likely to be re-suspended, inducing a risk for explosion and airborne contamination. Evaluating the particle resuspension in case of a LOVA is therefore a major issue and it can be investigated by using a CFD code. To this end, this article presents the implementation of a resuspension model in a CFD code (ANSYS CFX), its validation with experimental tests conducted by IRSN and lastly its application to an air ingress in a vacuum toroidal vessel with a volume comparable to ITER one.In the first part of the article, the implementation of the Rock’n’Roll model in ANSYS CFX for constant friction velocities and its adaptation to non-constant friction velocities will be detailed. This model, developed by Reeks and Hall [1], describes the kinetics of resuspension phenomenon by taking into account the equilibrium between the different forces acting on the particle deposited on a rough wall, such as the airflow forces and the adhesive forces. The second part of the article will be devoted to the validation of the model implementation. As one of the key parameters in the resuspension phenomenon is the friction velocity, CFD simulations will be compared to experimental resuspension tests from IRSN, on the BISE facility used by Peillon et al. [2] for the steady state case and on a dedicated facility for the transient case, allowing to validate both forms of the Rock’n’Roll model. Finally, the paper presents the simulations obtained on the particle resuspension for an air ingress scenario in a large vacuum vessel for which friction velocity field was already studied by Gelain et al. [3]. This case is particularly complex because the initial pressure is close to the vacuum, so the particle behaviour is different from that at atmospheric pressure. Further, a competition between airflow forces and gravitational force occurs, due to the vacuum, potentially restricting the resuspension, and the pressure influence also has to be taken into account in the particle transport and deposition [4].Three particle diameters were studied allowing to show the evolution of the resuspension with this parameter and to calculate dust resuspension rates and fractions during the air ingress

Particle behavior close to the lower wall of a tokamak type geometry during a loss of vacuum accident

International audienceThe behavior of tungsten and beryllium particles close to the wall of a tokamak type geometry (i.e. a toroidal vacuum vessel) during a loss of vacuum accident (LOVA) is explored in this paper.Values of the flow field and temperature of the air surging into the torus were calculated by Computational Fluid Dynamics (CFD) simulations from an initial low pressure of 1000 Pa by Gelain et al., Fusion Engineering and Design, 100:87–99, 2015. A new CFD calculation is performed here for an initial low pressure of 500 Pa. The aerodynamic forces on particles in rarefied flow are derived from the calculated friction velocity and temperature in the lowest wall region. The focus is on spherical particles, but these values of forces are complemented for comparison with an estimate of the force on an elongated particle following Sentman (1961). Typical particles with diameters of 1, 2, 5, 10, 20 μm are considered. The possibility for particles which are detached from the lower wall to be entrained away the wall is explored. A particle may be detached by the flow field provided it escapes the adhesion force with the wall, which occurs only for a rough wall and large particles. It is then entrained away from the lower part of the wall provided its weight is smaller than the aerodynamic force, which occurs even at low pressure for all sizes of studied beryllium particles and only for specific sizes and a pressure above 1500 Pa for tungsten particles. This key influence of particle weight on the particle dynamics in rarefied gas flow is the focus of the paper. As a result, only particles of intermediate diameters may be resuspended and transferred away from the wall during the whole pressurisation sequence of the vessel. Using the data for the initial low pressure of 500 Pa, these particle diameters are 5, 10 μm for tungsten and 5, 10, 20 μm for beryllium

Influence of the beta energy decay spectrum and particle size on the aerosol specific self-charging rate of radioactive aerosol

International audienceNuclear accidents, such as Chernobyl or Fukushima, led to release of radioactive aerosols into the environment, which are measured all over the world. During this long-range transport, aerosols undergo an electrical self-charging due the radioactivity they carry. The specific self-charging rate for particles containing β emitter radionuclides is always considered equal to the specific activity of the particles (Bq/particle) (Clement & Harrison, 1992; Gensdarmes et al., 2001). This assumption is supported by the fact that radionuclides usually considered (Kim et al., 2017), such as 137Cs, 132Te or 131I, have a high mean energy β decay which leads to an electron path length in matter larger than the particle diameter.This study aims to quantify the influence of this assumption by performing calculations of the specific self-charging rate of particles considering the full energy spectrum of β emitter radionuclides. The specific self-charging is treated as the electron escape probability from the particle. Calculations of electron transport in particle matter are realized with Geant4 toolkit (Agostinelli et al., 2003).The particles studied are single spheres with diameters ranging from 20 nm to 200 µm. The simulations are realized for pure iron particles (density 7.87 g/cm3) in vacuum environment (i.e. no interaction of electrons with matter outside the particle). The β emitter radionuclide considered for the calculation, 132Te, is selected from the case studied by Kim et al. (2017). We assume that the radionuclide is homogeneously distributed in the whole particle and random electron emission occurs. The maximum energy of β decay of 132Te is 240.1 keV. The mean energy of β decay is roughly equal to one third of the maximum energy. To calculate characteristic path length of electron in matter, usually a lower value, equal to a quarter of the maximum energy, is considered in order to take account of the higher coefficients of linear energy transfer for low energy electrons of the spectrum (Gensdarmes et al., 2001). The escape probability of electrons is defined for each particle diameter by the ratio of the electrons that exit the particle by the total number of generated electrons. 105 electrons are generated for each diameter and energy considered. An energy size bin of 1 keV is considered for calculation with the full spectrum.The assumption of escape probability equal to 1 for β emitter radioactive aerosol studied here leads to an overestimation of specific particle self-charging rate for diameters below 20 µm. The calculations based only on the mean energy could not give accurate results; the discrepancy lies between 10 % and 25 % in comparison to that obtained with the full spectrum for diameters between 1 µm and 10 µm. Specific attention has to be paid for other radionuclides with low energy spectrum like tritium

Implementation of a Particle Resuspension Model in a CFD Code: Application to an Air Ingress Scenario in a Vacuum Toroidal Vessel

International audienceDuring a loss of vacuum accident (LOVA), dust particles that will be present in the future tokamak ITER are likely to be resuspended, inducing a risk for explosion and airborne contamination. Evaluating the particle resuspension/deposition and resulting airborne concentration in case of a LOVA is therefore a major issue and it can be investigated by using a CFD code. To this end, this article presents the implementation of a resuspension model in a CFD code (ANSYS CFX) and its application to an air ingress in a vacuum toroidal vessel with a volume comparable to ITER one. In the first part of the article, the Rock’n Roll model and its operational version with the Biasi’s correlation is presented. The second part of the article will be devoted to the implementation of the Rock’n’Roll model in ANSYS CFX for constant friction velocities and its adaptation to non-constant friction velocities. Finally, the paper presents the simulations obtained on the particle resuspension for an air ingress scenario in a large vacuum vessel. This case is particularly interesting and non-intuitive because as the initial pressure is reduced, the particle behavior is different from that at atmospheric pressure. Further, a competition between airflow forces and gravitational force occurs, due to the low pressure environment, potentially restricting the resuspension, and the pressure influence also has to be taken into account in the particle transport and deposition (Nerisson, 2011). Three particle diameters were studied allowing to show the evolution of the resuspension with this parameter and to calculate dust resuspension rates and airborne fractions during the air ingress

Coulombic force contribution to nano scale aerosol capture by a wire grid: Quantitative comparison of experiments and simulations

International audienceFibers of filter media and aerosol particles both typically have some distributions of electric charge. Attractive coulombic interactions between charged particles and fibers enhance aerosol filtration efficiency; however, these coulombic interactions are difficult to characterize due to the complex, nonuniform fiber charge, diameter and spacing of typical filter media. We develop an experimental filter assembly to control these coulombic interactions. The filter assembly consists of three sequential electrically isolated metallic grids-high voltage is applied to the middle grid and the adjacent outer grids are electrically grounded such that the electric potential distribution in the filter assembly is well-defined. We test this filter assembly with aerosols of controlled diameter (ranging between ~70-500 nm) and electric charge (1e-), and systematically increase the applied voltage to the middle grid to enhance coulombic interactions and consequently the filter efficiency. We develop particle trajectory simulations to model our experimental filter assembly such that all parameters in the simulation have direct basis from the experimental system. We find excellent quantitative agreement between the filter efficiency measured in experiments and predicted from simulations without any fitting parameters. The precise control and accurate simulation of coulombic interactions demonstrated in this study indicate that the experimental and simulation methodologies developed here may be applied to uncover fundamental insights into the role of coulombic forces in more complex phenomena such as filter clogging

Production d'aérosols réprésentatifs d'opérations de démantélement pour des études de toxicologie

International audienceThis work describes a method to produce and to characterise aerosols from cutting operations that represent decommissioning process within nuclear facility containing tritium. The result obtained makes it possible to calculate the airborne release fraction and aerosol size distribution in the case of 316 stainless steel.Ce travail propose une méthode pour produire et caractériser un aérosol représentatif d'une opération de découpe lors du démantèlement d'une installation nucléaire contaminée en tritium, en vue de définir des simulants afin de réaliser des études de toxicologie. Les résultats obtenus ont permis de déterminer des fractions mises en suspension et la granulométrie de l'aérosol, pour la découpe de tube en acier inoxydable 316

CFD modelling of particle resuspension in a toroidal geometry resulting from airflows during a loss of vacuum accident (LOVA)

International audienceDuring a loss of vacuum accident (LOVA), dusts that will be present in the future tokamak ITER are likely to be resuspended. Such dusts may present a risk of explosion and airborne contamination. Hence, an assessment of the amount of particles which may be resuspended in case of LOVA is a major safety issue in ITER and more widely for all the fusion reactors. This article presents the implementation of the Rock’n’Roll model for the particle resuspension in ANSYS CFX, its validation with experimental data and its application for a LOVA accident in a toroidal geometry.The validation results show a good agreement between numerical and experimental results concerning the amount of resuspended particles as well as the kinetics of resuspension.The final application for modelling a LOVA in a toroidal geometry with different aerosol sizes shows a good behaviour of the model and a good agreement of the results with the expected physics

Capture de particules micrometriques par mousse aqueuse seche -approche experimentale

International audienceWithin the framework of studies conducted by the CEA on the capacity of aqueous confinement to mitigate detonation effects, an experimental campaign has been done on the confinement, by aqueous foam, of micrometric metal particles, dispersed by explosive. The results obtained from the different configurations have been compared in order to quantify the capture rate as a function of confinement size, particle size and explosive charge mass. They confirm the foam's efficiency in capturing particles, and specifically the larger ones.Dans le cadre d'études conduites par le CEA sur la capacité d'un confinement aqueux à atténuer les effets liés à la détonation d'une charge explosive, une campagne expérimentale portant sur le confinement, par mousse aqueuse, de particules métalliques micrométriques, dispersées par explosif, a été menée. Les résultats obtenus à partir des différentes configurations ont été comparés afin de déterminer le taux de capture en fonction de la taille des confinements, de la taille des particules et de la masse de la charge explosive. Ils confirment l'efficacité de la mousse pour capturer les particules, et, spécifiquement, les plus grosses

Contribution of IRSN R&D to reducing the risks of airborne pollutants dispersion during the dismantling of nuclear facilities

International audienceThis paper presents a general overview of IRSN research activities for assessing and reducing the risksof airborne dispersion of radioactive contamination during the dismantling operations of nuclearfacilities. Four main topic areas are addressed: aerosol source term, contamination transfers, behaviorof containment equipment and contamination metrology. Firstly, the various experimental and numericalmeans involved are presented

Evaluation of coulomb force contribution on the filtration of aerosols using a well-characterized metallic grid

National audienceWe present an experimental and numerical methodology through which we can compare the filtration efficiencies taking into account a precise description ofthe electrostatic force between a spherical particle and a fiber.Nous présentons une méthodologie expérimentale et numérique grâce à laquelle nous pouvons comparer les efficacités de filtration en prenant en compte une description précise de la force électrostatique entre une particule sphérique et une fibre