Resuspension Of Graphite Dust Research Articles

Experimental and mechanistic study of dispersed micrometer-sized particle resuspension in a square straight duct with rough walls

The resuspension of graphite dust is an important phenomenon in the release of radioactivity and the safety of nuclear reactors during severe accidents. In this study, a visualization experimental platform is constructed to study effects of particle size, flow velocity, and wall roughness on the resuspension characteristics of graphite particles. A statistical model of particle resuspension applicable to monolayer dispersed particles is developed based on the moment equilibrium of the particles and the flow field characteristics, as calculated by the large-eddy simulation framework. The results show that particle resuspension can be divided into short- and long-term resuspension stages. Most particle resuspension occurs during the short-term stage. With increases in flow velocity and particle diameter, the aerodynamic or adhesion force acting on the particles increases, and corresponding particle resuspension fraction increases. The influence of rough walls on particle resuspension is related to both the force on the particles and the arm ratio between the wall morphology and the particle diameter. A comparison with the experimental results demonstrates that the particle resuspension model developed in this study accurately predicts the impact of flow velocity, particle size, and wall roughness on particle resuspension.

Adhesion Force Measurements: Graphite Cluster Interacting with VHTR Steel

Graphite dust is generated in the reactor core during normal operation of very high temperature reactors (VHTRs). This dust is transported throughout the reactor circuit and plates-out at different locations. The resuspension of graphite dust is believed to be a major contributor to the nuclear source term. The adhesion force is an important parameter governing the resuspension of the dust. The present study employed an atomic force microscope to measure the adhesive force between a reactor-grade graphite cluster “particle” and VHTR structural materials including Inconel 617 and Hastelloy X in an air glove box. Results for a reactor-grade graphite (MLRF-1 from SGL Carbon Ltd.) cluster particle interacting with Inconel 617 and Hastelloy X samples are reported under four different surface conditions including as received, and after 5, 10, and 15 min of oxidation. These forces were also predicted using the Johnson-Kendall-Roberts theoretical model with the estimate of the work of adhesion. The measured values depend on oxidation times but are in general a factor of about 20 lower than the predicted values. With surface roughness taken into account, the predicted values differ from the measured values by factors of 2 and 4 at the maximum for Hastelloy X and Inconel 617, respectively.

AFM measurements of adhesive forces between carbonaceous particles and the substrates

Graphite dust is carbonaceous particles generated during operation of High Temperature Gas-Cooled Reactors (HTR). Graphite dust resuspension is the key behavior associated with HTR source term analyses and environmental safety assessment. The adhesive force is the key factor that determines the resuspension rate. The present study used an atomic force microscope (AFM) to measure the adhesive force between a single carbonaceous particle and the substrate. The measurements were performed on mica, graphite IG110 and Inconel 800H. The prepared “probe cantilevers” were mesocarbon microbeads (MCMB), fuel element debris from HTR-10 and graphite NBG18. The equivalent work of adhesion was derived from the measured adhesive force and calculated based on substrate profile approximation and the JKR theoretical model. The measured work was smaller than the ideal work of adhesion, most likely due to the rough particle morphology and the rough substrate surface. Additionally, a shape factor imposes a constraint on the lateral deformation of the particles. Furthermore, surface roughness could reduce the adhesive force some depending on the particle size. Once the particle was too small to be trapped into a trough, the adhesive force would not be further reduced.

Graphite dust resuspension in an HTR-10 steam generator

Graphite dust has an important effect on the safety of high-temperature gas-cooled reactors (HTR). The flow field in the steam generator was studied by the computational fluid dynamics (CFD) method, with the results indicating that the friction velocity in the windward and the leeward of the heat transfer tubes is relatively low and is higher at the sides. Further analysis of the resuspension of graphite dust indicates that the resuspension fraction reaches nearly zero for particles with a diameter less than 1μm, whereas it will increases as the helium velocity in the steam generator increases for particle size larger than 1μm. Moreover, the resuspension fraction increases as the particle size increases. The results also indicate that resuspension of the particles with sizes larger than 1μm exhibited obvious differences in different parts of the steam generator.

Graphite dust resuspension in a depressurization accident of HTR

Graphite dust has a significant effect on the safety of high temperature gas-cooled reactors. The present study experimentally analyzes the resuspension of graphite dust in a depressurization accident. A series of aerodynamic verification tests in an experimental system with gas flow at the sound speed at the orifice agree well with theoretical results. The results showed that the friction velocity decreases with time during the depressurization while the resuspension fraction increases when augmenting the initial pressure. The experiment results show good agreement with the predictions of the revised Rock ‘n’ Roll model by Biasi et al., indicating that this model accurately predicts the resuspension of graphite dust in a HTR depressurization accident.