Oxidation behaviour of an HTR fuel element matrix graphite in oxygen compared to a standard nuclear graphite

Moormann, R.; Hinssen, H.-K.; Kühn, Kerstin

doi:10.1016/j.nucengdes.2003.11.001

Cited by 48 publications

(21 citation statements)

References 2 publications

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“…The MG contains around 10% of incompletely graphitized resin-derived carbon because of temperature limit restriction (<2000 ∘ C) during the fabrication process of pebble fuel elements [3]. Because of its incompletely graphitized binder content, the activation energy of historic matrix-grade graphitic materials is lower than that of most modern nuclear graphite [9][10][11][12]. The activation energies of filler and binder for A3-27 were reported separately by Moormann et al [9].…”

Section: Introductionmentioning

confidence: 99%

“…Results showed that oxidation at 600 ∘ C was more damaging on strength than oxidation at 700 ∘ C, at comparable levels of weight loss which was due to the differences in the distribution of oxidation layer and mechanism of development of porosity. Whereas a block reactor core consists mainly of highly graphitized nuclear graphite and contains only a small amount of fuel element matrix graphite (MG); the active pebble-bed core consists of a large part of fuel element matrix graphite [9]. The MG contains around 10% of incompletely graphitized resin-derived carbon because of temperature limit restriction (<2000 ∘ C) during the fabrication process of pebble fuel elements [3].…”

Section: Introductionmentioning

confidence: 99%

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Oxidation Behavior of Matrix Graphite and Its Effect on Compressive Strength

Zhou

Contescu

Zhao

et al. 2017

Science and Technology of Nuclear Installations

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Matrix graphite (MG) with incompletely graphitized binder used in high-temperature gas-cooled reactors (HTGRs) is commonly suspected to exhibit lower oxidation resistance in air. In order to reveal the oxidation performance, the oxidation behavior of newly developed A3-3 MG at the temperature range from 500 to 950 ∘ C in air was studied and the effect of oxidation on the compressive strength of oxidized MG specimens was characterized. Results show that temperature has a significant influence on the oxidation behavior of MG. The transition temperature between Regimes I and II is ∼700 ∘ C and the activation energy ( ) in Regime I is around 185 kJ/mol, a little lower than that of nuclear graphite, which indicates MG is more vulnerable to oxidation. Oxidation at 550 ∘ C causes more damage to compressive strength of MG than oxidation at 900 ∘ C. Comparing with the strength of pristine MG specimens, the rate of compressive strength loss is 77.3% after oxidation at 550 ∘ C and only 12.5% for oxidation at 900 ∘ C. Microstructure images of SEM and porosity measurement by Mercury Porosimetry indicate that the significant compressive strength loss of MG oxidized at 550 ∘ C may be attributed to both the uniform pore formation throughout the bulk and the preferential oxidation of the binder.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Oxidation Behavior of Matrix Graphite and Its Effect on Compressive Strength

Zhou

Contescu

Zhao

et al. 2017

Science and Technology of Nuclear Installations

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“…The concept of ASA has been around for many decades, however due to the its low values for macrocrystalline graphite and the complex microstructures found in these materials, it has been difficult to implement in practice. This has led to a large number of kinetic studies reporting differing kinetic parameters [7][8][9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Comparative analysis of graphite oxidation behaviour based on microstructure

Badenhorst¹,

Focke²

2013

Journal of Nuclear Materials

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Two unidentified powdered graphite samples, from a natural and a synthetic origin respectively, were examined. These materials are intended for use in nuclear applications, but have an unknown treatment history since they are considered proprietary. In order to establish a baseline for comparison, the samples were compared to two commercial flake natural graphite samples with varying impurity levels. The samples were characterized by conventional techniques such as powder X-ray diffraction, Raman spectroscopy and X-ray fluorescence. The results indicated that all four samples were very similar, with low impurity levels and good crystallinity, yet they exhibit remarkably different oxidation behaviours. The oxidized microstructures of the materials were examined using high-resolution scanning electron microscopy at low acceleration voltages.The relative influence of each factor affecting the oxidation was established, enabling a structured comparison of the different oxidative behaviours. Based on this analysis, it was possible to account for the measured differences in oxidative reactivity. The material with the lowest reactivity was a flake natural graphite which was characterized as having highly visible crystalline perfection, large particles with a high aspect ratio and no traces of catalytic activity. The second sample, which had an identical inherent microstructure, was found to have an increased reactivity due to the presence of small catalytic impurities. This material also exhibited a more gradual reduction in the 2 oxidation rate at higher conversion, caused by the accumulation of particles which impede the oxidation.The sample with the highest reactivity was found to be a milled, natural graphite material, despite its evident crystallinity. The increased reactivity was attributable to a smaller particle size, the presence of catalytic impurities and extensive damage to the particle structure caused by jet milling. Despite displaying the lowest levels of crystalline perfection, the synthetic graphite had an intermediate reactivity, comparable to that of the highly crystalline but contaminated sample. The absence of catalytic impurities and the needle coke-derived particle structure were found to account for this behaviour.This work illustrates that the single most important factor when comparing unknown graphite materials from different origins is an assessment of the oxidized microstructure.This approach has the added benefit of identifying further potential processing steps and limitations for material customization.

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“…In nuclear reactors, especially, graphite acts as a moderator and reflector as well as a major structural component mainly due to its excellent thermal, physical, chemical, and mechanical properties as well as high moderating ratio [1].…”

Section: Introductionmentioning

confidence: 99%

“…Since the heat generated from the nuclear fuel is continuously delivered to the outside of the pressure container by the graphite's thermal conductivity as well. In addition the VHTR is operated at the high temperature of 900 o C or higher, the non-active Ar or He gases are used for the vapor flowing to prevent oxidation of graphite [1]. It may, however, generate critical defects such as oxidation of the graphite when an extremely small amount of impure substance exists in the non-active gases during accident [2,3].…”

Section: Introductionmentioning

confidence: 99%

Thermal Emissivity of a Nuclear Graphite as a Function of Its Oxidation Degree (2) - Effect of Surface Structural Changes -

Seo¹,

Roh²,

Kim³

et al. 2009

Carbon letters

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Thermal emissivity of nuclear graphite was measured with its oxidation degree. Commercial nuclear graphites PECA, have been used as samples. Concave on graphites surface increased as its oxidation degree increased, and R value (Id/Ig) of the graphites decreased as the oxidation degree increased. The thermal emissivity increased depending on the decrease of the R (Id/Ig) value through Raman spectroscopy analysis. It was determined that the thermal emissivity was influenced by the crystallinity of the nuclear graphite.

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Oxidation behaviour of an HTR fuel element matrix graphite in oxygen compared to a standard nuclear graphite

Cited by 48 publications

References 2 publications

Oxidation Behavior of Matrix Graphite and Its Effect on Compressive Strength

Oxidation Behavior of Matrix Graphite and Its Effect on Compressive Strength

Comparative analysis of graphite oxidation behaviour based on microstructure

Thermal Emissivity of a Nuclear Graphite as a Function of Its Oxidation Degree (2) - Effect of Surface Structural Changes -

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