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Characterization of graphite particles (dust) produced by abrasion that would occur in a pebble bed reactor is of interest for reasons of safety, operation, and maintenance. To better understand this abrasion and particle generation, we have built a test apparatus to produce particles by sliding abrasion in a 1% to 5% relative humidity air environment. We have used a commercial-grade graphite in our experiments and have generated size distributions for the abraded particles. We have also fit lognormal functions to those size distributions (for use in computer codes); determined particle shapes; measured temperature and humidity during the tests; measured and calculated wear rates; and measured the surface roughness of both pretest and posttest samples, particle surface areas, pore volumes, and pore volume distributions of particles produced during abrasion of graphite surfaces under different loadings and sliding speeds. The experiments showed that as loading (analogous to pebble depth in the reactor) and sliding speed increase, so do the wear rates and numbers of particles produced, while swface roughness decreases, increases, and then decreases. Brunauer-Emmett-Teller measurements show that abrasion increases surface area from 0.583 m²/g in the bulk material to 555 m²/g in material abraded at high loading and high sliding speed. Wear rates range from 0.005 to 0.991 g/m per contact site. The size of the particles observed was <4000 nm. In all, our research shows that pebble abrasion is a complex process that is not constant during operation and thus should be considered for future work. [ABSTRACT FROM AUTHOR]

Abstract: Adsorption of iodine on graphite is of great interest for operation and safety of high temperature nuclear reactors. Graphite can adsorb significant amounts of iodine and retain it for a long period of time. Significant amount of work on this subject has been done in the past. Various types of adsorption apparatus have been designed and data were collected. The types of graphite used in past studies are not available anymore, and as a consequence the data are not applicable for the new type of commercial nuclear grade graphites. However, the past experimental systems, data, and their analysis are useful to design a better experimental system, collect more accurate data, and, provide better understanding of the adsorption process and data. In addition the existing data can be used to generate a framework to understand the types of adsorption processes taking place. In this work, we have conducted an exhaustive literature review and further analyzed the data. Four adsorption isotherms; the Langmuir, the Freundlich, and the two isotherms proposed in the International Atomic Energy Agency (IAEA) Tecdoc-978 were used to correlate the available equilibrium adsorption data. For most of the data, the simple Langmuir and the Freundlich isotherms provided a reasonable fit of the data. The Polyani's potential theory was also used to check the consistency of the data and as indicated by the theory, most of the data set provided a single characteristic curve. The isosteric heats of adsorption calculated using the literature data suggested that iodine-adsorption on graphite could be a chemisorption process. [Copyright &y& Elsevier]

In high-temperature gas reactors, graphite particle adhesion and resuspension from structural surfaces play a role in source term estimations. This paper describes measurements of the adhesion force between an irregular graphite cluster (henceforth called a graphite particle) and Hastelloy X samples having different surface conditions. An atomic force microscope (AFM) was used. The graphite particle was attached to the AFM probe and then brought directly into and out of contact with the surface in air; the adhesion force was obtained from the resultant force curve. The adhesion forces of the graphite particle with Hastelloy X (as received, polished, and different oxidations) and mica surfaces were determined. From the resulting adhesion forces, the work of adhesion W12 (energy per unit area) was calculated. Although the values of the measured pull-off (adhesion) forces were found to be of the same order of magnitude, they differed by surface condition depending where on the sample the adhesion force was measured. The theoretical value of the adhesion force was calculated using the theory of Johnson, Kendall, and Roberts. When compared to the values calculated from this theory, the measured values were lower by a factor of 100 in some cases and 1000 in others. This difference may be due to the approximation of the irregular graphite cluster probe as a perfect graphite particle sphere and to not taking into consideration asperities on the surface of the particle probe. Additionally, covalent bonds may form between the surface elements and the graphite particle because of the applied load. In this paper, the effects of oxidation on the adhesion of graphite particles to the mica and Hastelloy X surfaces are also discussed. [ABSTRACT FROM AUTHOR]

Very high temperature reactors (VHTRs) and high temperature gas-cooled reactors (HTGRs) can develop extreme temperatures in excess of 900°C that make them theoretically very efficient, potentially in the range of 45% to 50%. The high temperatures, however, can also lead to a corresponding increase in fission product transport out of the fuel, which is potentially a source term-related safety issue. The aim of this work was to develop a repeatable, accurate, and cost-effective process to measure the diffusion coefficients of fission products in graphitic VHTR materials, particularly those materials used in the fabrication of TRISO [tristructural isotropic] fuel pellets'. Specifically, this work has focused on the diffusion of silver in graphite. We constructed graphite cells that could be filled with a silver diffusant in the form of silver flakes, silver powder, or a preloaded, silver-laden graphite powder. The cells were hermetically sealed and heated to temperatures comparable to those that will be found in VHTRs. After being kept at various amounts of time and temperature, these cells were imaged using microtomography and electron microscopy. Concentration profiles were measured by sectioning the heat-treated cells and analyzing them using neutron activation analysis. Estimated diffusion coefficients for silver in a commercial grade of graphite are reported, but the method is easily adapted to any grade of graphite material including nuclear grades and to a variety of other fission product species. [ABSTRACT FROM AUTHOR]

Abstract: One of the most severe accident scenarios anticipated for VHTRs is an air ingress accident caused by a pipe break. Graphite oxidation could be severe under these conditions. In this work, the oxidation rate of NBG-18 nuclear-grade graphite was studied thermogravimetrically for different oxygen concentrations and with temperatures from 873 to 1873K. A semi-empirical Arrhenius rate equation was developed for the temperature range of 873–1023K. The activation energy of NBG-18 was 187kJ/mol and the order of reaction was 1.25. The penetration depth of oxidant was about 3–4mm for NBG-18 oxidized at 973K. Increased porosity and changes in external geometry became more prominent at higher temperatures from about 1173 to 1873K. The surface of oxidized NBG-18 was characterized by SEM, EDS, FTIR and XPS. [Copyright &y& Elsevier]

Graphite particle generation by interpebble abrasion and by abrasion of pebbles with the containment vessel during operation of a pebble bed reactor is an issue of interest in the safety analysis of this class of very high temperature reactor. To understand particle generation, we have constructed an apparatus to generate graphite particles from preformed graphite hemispheres under rotational/spinning abrasive loading. We have initially used commercial-grade graphites in our experiments and have generated size distributions for the abraded particles, determined particle shapes, and measured the particle surface areas, pore volumes, and pore volume distributions of particles produced during abrasion of graphite surfaces under different conditions. The size distributions were studied using an Aerodynamic Particle Sizer™ and a Scanning Mobility Particle Sizer™. Most of the particles observed were in the range from 18.1 to 600 nm in diameter. The scanning electron micrographs showed that the particles tend to be irregular in shape and porous in nature. We have also conducted Brunauer-Emmett-Teller surface area and pore volume measurements that have verified the highly porous nature of the particles. The calculated surface area and open porosity for our initial measurements of the particles from this particular grade of commercial graphite were found to be 626 m² g-1 and 68%, respectively. In addition, the average surface roughness of fresh samples was 0.966 Ra µm at the point of contact. [ABSTRACT FROM AUTHOR]

• The flows of Helium and Xenon under pressures of 1.47 × 104 Pa for IG-11 were in the free molecular regime for IG-11. • The permeability constant for Helium in IG-11 was 6.259 × 10−6 m2 s−1. • The permeability constant for Xenon in IG-11 was 1.614 × 10−6 m2 s−1. • The ratio of the Helium and Xenon permeabilities in IG-11 did not follow fully the inverse square root of their molecular masses. Understanding the flow and diffusion (transport) of gases through nuclear graphite is of interest in both the prismatic and pebble bed high-temperature gas reactors (HTGRs), for normal operation and under accident conditions, for example, as related to graphite oxidation. Both the laminar and turbulent gaseous transport are of interest. It has also been noted that nuclear graphite can have pore sizes that can possibly lead to non-continuum transport, as the pore size can be comparable to the gaseous mean free path. We report measurements and analyzes of experiments performed with IG-11 graphite in the pressure range of 10 Pa to 14,700 Pa, 293 K, using Helium, and research grade xenon (99.999%), respectively. These values are in fair agreement with those reported for different graphites and gases by other investigators, but are also higher by six orders of magnitude than those reported for Kr permeability in a developmental very low permeability graphite-HXT-90. Our measured ratio of the Helium and Xenon permeabilities is about 4, different from m Xe m He ≈ 5.72 that corresponds to diffuse reflection at the graphite surface for both gases. Simple analysis indicates that the two gases have slightly different accommodation coefficients (about 25%) with the graphite surface. However, there could also be other reasons, such as experimental uncertainties or the need for a more detailed analysis. [ABSTRACT FROM AUTHOR]