Graphite Dust Particles Research Articles

Modelling dust transport in DIII-D with DTOKS-Upgrade

Comprehensive upgrades to the dust transport code Dust in TOKamaks (DTOKS) that extend the plasma-dust interaction model are presented and compared with recent measurements of dust transport in DIII-D. Simulations incorporating variation in physical properties of graphite dust with temperature and size in a stationary plasma background suggest a substantial decrease in lifetimes due principally to thermal expansion. The trajectories of 53 dust grains identified from analysis of visible camera data taken across two similar shots were used to measure the dust particle velocity distributions. Dust tracks terminated mostly at the outer divertor strike point having a mean observation time of 2.1 ± 0.4 ms. Stochastic modelling of 200 graphite dust particles in the DIII-D tokamak performed with DTOKS-U using plasma simulations generated by OEDGE found similar behaviour, with particles ablating rapidly after acquiring a positive charge in the region close to the outer strike point, creating an acute source of neutral carbon atoms. The simulated mean lifetime, 11 ± 2 ms, showed approximate agreement with experimental observation when corrected by accounting for dust visibility and ignoring the longest trajectories 1.5 ± 0.2 ms. Synthetic diagnostic data generated from coupling the results of DTOKS-U with the visualisation software Calcam offers a powerful new tool for validation of simulations and predictive calculations of dust dynamics.

Evaluation of thermophoretic effects on graphite dust coagulation in high-temperature gas-cooled reactors

In high-temperature gas-cooled reactors, graphite dust particles within the reactor core and the heat transfer equipment experience large temperature gradients. Under such conditions, thermophoresis may play an important role in determining aerosol evolution. This study presents a theoretical and numerical analysis of the thermophoretic effects on aerosol coagulation within these reactors. The coagulation rates for Brownian versus thermophoretic coagulation are calculated and compared for various temperature gradients. Our results show that thermophoretic coagulation dominates over Brownian coagulation for large temperature gradients. We defined an enhancement factor to evaluate the role of thermophoretic coagulation under various reactor conditions. The enhancement factor increased dramatically with increasing temperature gradient, decreasing pressure and increasing particle diameter, but was not very sensitive to temperature change. The time evolution of the particle size distribution related to combined Brownian and thermophoretic coagulation was simulated using a log-skew-normal method of moments. The simulation results indicate that aerosol evolution can be strongly accelerated by thermophoretic coagulation under large temperature gradients.

Graphite aerosol release to the containment in a water ingress accident of high temperature gas-cooled reactor (HTGR)

Abstract As a design-basis accident, the water ingress due to the rupture of a single steam generator tube is a major threat for high-temperature gas-cooled reactors (HTGR). The rapid rise of primary pressure could reach the set pressure of safety valve and trigger the relief to the containment. For HTGR, the graphite dust particles, which usually carries fission product condensations, are also discharged to the containment during the pressure relief. The size distribution of the aerosol is crucial for the safety analysis of the containment in this accident scenario. Particularly, the high velocity flow in the safety valve may greatly change the size distribution of the graphite aerosol, due to the possible impact and fragmentation. In the present work we study graphite particle impact and fragment during pressure relief in the water ingress accident of HTGR by combining multiphase computational fluid dynamics (CFD) and finite element method (FEM). An Eulerian-Lagrangian scheme is employed to calculate the trajectory and instantaneous impact velocity of different sized particles. Then FEM with Hertz contact model is used to evaluate the outcome of the impact. CFD results show that the impact rate increases from 22% to nearly 100% as particle size increases from 0.1 μm to 1.9 μm. The critical fragment velocity is larger for smaller particle because the compressive strength limit increases due to Hall-Petch effect. By combining CFD and FEM simulation, it is shown that the possibility of fragmentation reaches 80% for particles larger than 1.5 μm. Our work suggests that this fragmentation effect needs to be considered for the initial size distribution of graphite aerosol that is discharged to the containment.

Particle Deposition by Thermophoresis Under High-Temperature Conditions in a Helium Flow

The continuous generation of graphite dust particles in the core of a high-temperature reactor (HTR) is one of the key challenges of safety during its operation. The graphite dust particles emerge from relative movements between the fuel elements or from contact to the graphitic reflector structure and could be contaminated by diffused fission products from the fuel elements. They are distributed from the reactor core to the entire reactor coolant system. In case of a depressurization accident, a release of the contaminated dust into the confinement is possible. In addition, the contaminated graphite dust can decrease the life cycle of the coolant system due to chemical interactions. On one hand, the knowledge of the behavior of graphite dust particles under HTR conditions using helium as the flow medium is a key factor to develop an effective filter system for the discussed issue. On the other hand, it also provides a possibility to access the activity distribution in the reactor. The behavior can be subdivided into short-term effects like transport, deposition, remobilization and long-term effects like reactions with material surfaces. The Technische Universität Dresden has installed a new high-temperature test facility to study the short-term effects of deposition of graphite dust particles. The flow channel has a length of 5 m and a tube diameter of 0.05 m. With helium as the flow medium, the temperature can be up to 950 °C in the channel center and 120 °C on the sample surface, the Reynolds number can be varied from 150 up to 1000. The particles get dispersed into the accelerated and heated flow medium in the flow channel. Next, the aerosol is passing a 3 m long adiabatic section to ensure homogenous flow conditions. After passing the flow straightener, it enters the optically accessible measurement path made from quartz glass. In particular, this test facility offers the possibility to analyze the influence of the thermophoretic effect separately. For this, an optionally cooled sample can be placed in the measuring area. The thickness of the particle layer on the sample is estimated with a three-dimensional laser scanning microscope. The particle concentration above the sample is measured with an aerosol particle sizer (APS). Particle image velocimetry (PIV) detects the flow-velocity field and provides data to estimate the shear velocity. In combination with the measured temperature-field, all necessary information for the calculation of the particle deposition and particle relaxation times are available. The measurements are compared to results of theoretical works from the literature. The experimental database is relevant especially for computational fluid dynamics (CFD)-developers, for model development, and model verification. A wide range of phenomena like particle separation, local agglomeration of particles with a specific particle mass, and selective remobilization can be explained in this way. Thus, this work contributes to a realistic analysis of nuclear safety.

A numerical study of particle deposition in HTGR steam generators

The present study numerically investigated the graphite particle deposition in the steam generator of a pebble-bed high-temperature gas-cooled reactor (HTGR) and discussed the effect of size and velocity of particles on turbulent and thermophoretic depositions. Turbulent closure is achieved by Reynolds stress transport equations and particle motions were simulated by a discrete particle model with rebound boundary by user-defined functions to determine deposition. The results showed that turbulent deposition plays the dominant role for large particle deposition, while thermophoretic deposition is the main factor causing small graphite dust particles to deposit on the wall. It also indicated that the particles with dimensionless relaxation time near 1 are the most deposited.

Numerical predictions of the drag coefficients of irregular particles in an HTGR

The present study analyzed the drag coefficient of simplified graphite dust particles in an HTGR using a computational fluid dynamics (CFD) model. The effects of the particle Reynolds number, flow angle and the percentage contribution of the drag coefficient are discussed. The results show that the drag coefficient for irregular particles decreases with increasing particles Reynolds number but increasing with increasing flow angle. The friction on the micron particles is the main component of the total force, while the effect of the flow angle on the drag coefficient contributes to the pressure drag. The results are correlated into accurate correlations based on the specific particles analyzed here.

Resuspension of multilayer graphite dust particles in a high temperature gas-cooled reactor

The resuspension characteristics of graphite dust particles have a significant effect on the safe operation and source analyses of HTGR. The present study presents a theoretical model for the resuspension of multiple layers of graphite dust particles based on the Rock'n'Roll model and the multilayer approach of Friess and Yadigaroglu (2001). The model is then used to analyze the influences of various physical properties such as the number of particle layers, coolant temperature and operating pressure on the resuspension characteristics. The results show that as the number of particle layers increases, the resuspension fraction of the particles decreases up to a critical number of layers after which the resuspension fraction remains constant with increasing number of layers. After the second layer, the resuspension fraction of the particles increases from zero to a maximum and then decreases, with the maximum decreasing with increasing number of layers. Larger particles and particles with higher friction velocities more easily detach from the wall and resuspend. For the same helium pressure and friction velocity, the resuspension fraction decreases with increasing temperature. For the same helium temperature and friction velocity, a higher helium pressure will give rise to the resuspension of more graphite dust particles.

Study on the resuspension of graphite dust based on the Rock'n'Roll model

The resuspension characteristics of graphite dust in a high temperature gas-cooled reactor significance impact the source terms in the reactor safety analysis. The present academic study analyzes resuspension behavior of graphite dust, including the effects of the distribution of the independent variables between the particle and substrate adhesion, the particle diameter and the time for the resuspension characteristics of graphite dust based on the Rock'n’Roll model. The results indicated that the distribution of the independent variables for the adhesion has an important influence on the resuspension characteristics of the particles and that the Biasi fitting distribution gives a better fit to the experimental data. Particles with larger mean values of adhesion between the particle and the substrate are less prone to resuspension. Hence, a larger standard deviation σa' will overestimate the resuspension fraction for a low-speed case and underestimate the resuspension fraction in the high-speed region. Graphite dust particles with larger particle diameters are more likely to be resuspended and have a greater resuspension fraction at the same friction velocity. For the conditions matching the operating parameters of HTR-10, the threshold friction velocity that enables 0.5 μm graphite dust particles to resuspend is 5.76 times larger than that of 5 μm particles and 16 times larger than that of 10 μm particles. In addition, these results also showed that the graphite dust particle fraction changes with time and can be divided into short-term and long-term effects. The critical time between the short-term and long-term effects decreases with increasing helium velocity and particle diameter.

Thermophoretic and turbulent deposition of graphite dust in HTGR steam generators

The present study calculated the graphite dust deposition in the steam generator of HTGR by using a thermophoretic deposition model and a turbulent deposition model based on the temperature and flow field distributions calculated by a computational fluid dynamics (CFD) model. The results showed that the heat flux along the heat transfer surface was not evenly distributed which affect the particle deposition. Thermophoretic deposition is the main factor causing small graphite dust particles to deposit on the surface while turbulent deposition plays the dominant role for large particle deposition. The results also indicate that the deposited particles are mainly small graphite dust particles in steam generator when the reactor is running at low power, with both small and large graphite dust particles at high reactor power levels with fewer medium size particles.

Recommended coefficients for sorption of iodine, silver, and cesium on graphite dust particles

This paper describes the work performed to find the sorption coefficients that represent well the available experimental data for cesium, iodine, and silver on dust particles. The purpose of this work is to generate a set of coefficients that may be recommended for computer code users. The following data was correlated:•I-131 on AVR dust.•Ag-110m on AVR dust.•Cs-134 and Cs-137 on AVR dust.The results are summarized as follows:•The available data can be correlated. The data scatter is about 4 orders of magnitude. Therefore the coefficients of the Langmuir isotherms vary by 4 orders of magnitude.•Sorption rates are higher at low temperatures and lower at high temperatures. This tendency has been observed in the data compiled at Oak Ridge. It is therefore surmised that the highest value of the sorption coefficients are appropriate for the low temperatures and the lowest value of the sorption coefficients are appropriate for the high temperatures. The recommended sorption coefficients are presented in this paper.•The present set of coefficients is very rough and should be a subject for future verification against experimental data.

Kinetics of dust particles around the scrape off layer in fusion devices

A kinetic model based on the balance of charge and energy over the dust particle surface around the scrape off layer (SOL) region in fusion devices has been developed; for describing the dust mass diminution, its temperature evolution and phase change process have been taken into account. The formulation has been utilized to determine the lifetime of cylindrical and spherical dust particles. A realistic situation in fusion devices, when the plasma exhibits meso-thermal flow, has been taken into account; for this purpose a rigorous approach, pioneered by Mott-Smith and Langmuir (1926 Phys. Rev. 28 727), has been adopted to derive the general expressions for the electron (ion) current on cylindrical dust surfaces and the corresponding mean energy of accreting electrons/ions in a flowing plasma. On the basis of analytical modelling the numerical results for the dust electric potential energy and the lifetime of the dust particles corresponding to a typical plasma environment near the SOL region of Mega Ampere Spherical tokamak (MAST)/Joint European Torus (JET) fusion devices have been evaluated for graphite and tungsten dust particles. The results are graphically illustrated as functions of particle size, electron/ion temperature and plasma ionization. It is seen that a large dust particle immersed in low temperature plasma can survive for long time; as an important outcome it is also noticed that the cylindrical particles of tungsten last longer than spherical particles. The findings are of relevance in characterizing and simulating the effects of a variety of dusts for experimental campaigns in large scale (ITER/Demo-like) fusion devices.

Scattering by interstellar graphite dust analog

The analysis of optical scattering data of interstellar carbonaceous graphite dust analog at 543.5nm, 594.5nm and 632.8nm laser wavelengths by using an original laboratory light scattering setup is presented. The setup primarily consisted of a laser source, optical units, aerosol sprayer, data acquisition system and associated instrumentation. The instrument measured scattered light signals from 10° to 170° in steps of 1°. The results of the measurements of the volume scattering function β(θ) and degree of linear polarization P(θ) of the carbonaceous graphite dust particles that were sprayed in front of the laser beam by using an aerosol sprayer were subsequently compared with theoretically generated Mie plots with estimated parameters.

Galactic Center Extinction: Evidence of Metallic Needles in the General Interstellar Medium

The extinction curve derived from Infrared Space Observatory mid-infrared observations of the Galactic center (GC) exhibits a surprisingly flat behavior in the ~3-8 μm region, contrary to the deep minimum expected from standard interstellar dust models consisting of bare silicate and graphite dust particles. We show that this extinction is likely caused by the presence of metallic needles in the interstellar medium (ISM) toward the GC. If the needles contribute only to the 3-8 μm extinction, they must have a long-wavelength cutoff at ~8 μm and therefore a typical length over radius ratio of ~600, smaller than the ~3 × 103 aspect ratio determined for the needles in Cas A. Homogeneously distributed throughout the ISM, they comprise only a minor mass fraction of the ISM, with a needle-to-H mass ratio of ~5 × 10-6, which is equivalent to 0.14% of the silicate dust mass. Their total ISM abundance then can be readily explained by the combined production in supernovae and O-rich stellar outflows. The GC observations show that metallic needles, in spite of their low abundance or nonuniform distribution, can be the dominant source of opacity in the 3-8 μm wavelength region. However, expelled into the intergalactic medium, their abundance is too low to cause any dimming of cosmological sources, and their length is too short to make them a significant source of submillimeter emission.