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Title: Impact of air ingress on the oxidation behaviour of various graphite materials in high temperature inert-gas environments
Author: Lo, I. L.
ISNI:       0000 0004 7970 4763
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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Graphite is used in the gas-cooled nuclear reactors as both a neutron moderator and structural components, while both traditional and novel graphite materials are being studied worldwide for applications in Generation IV reactors. One of several accident scenarios for VHTRs is caused by a depressurisation accident, also known as an air ingress accident. These can occur during an air ingress accident when the fuel temperature is likely to reach 1600 °C when atmospheric air may ingress into the reactor core and the graphite components are predicted to be severely oxidised thereby changing their mechanical properties. Therefore, a comprehensive investigation of the oxidation mechanisms of graphite in various gaseous environments at elevated temperatures is essential for the reliable prediction of the behaviour of graphite under such conditions. This research study investigated of graphite oxidation behaviour under conditions where air ingresses accidentally at very high temperatures. The aim, therefore, was to investigate the oxidation behaviour of proposed graphite grades for VHTRs and to recommend the appropriate form for use in VHTRs based on levels of oxidation resistance. In addition, related characterisation examinations for graphite oxidised at different temperatures were conducted, in order to understand the detailed mechanisms of graphite oxidation corresponding to the simulation of actual graphite components. The materials selected were nuclear graphite grade IG-110, IG430, MA, MB, ATR-2E, PGA, G347A, and G458A. A dynamic oxidation system was constructed with a 3-zone furnace. The oxidation characteristics of graphite pellets in both air-rich environments and helium-containing environments at temperatures ranging from 700 to 1600 °C were investigated. The oxidation rates and activation energies were determined based on mass loss measurements in a series of oxidation tests. Although the thermal oxidation mechanism was previously considered to be the same for all temperatures higher than 1000 °C, the significant increases in oxidation rates observed at very high temperatures suggest that the oxidation behaviour of the selected graphite materials at temperatures higher than 1200 °C is different. In addition, it was discovered that, at temperatures above 900 °C, the difference in oxidation behaviour between fine grain graphite grades was less. The surface microstructure of each graphite pellet sample was characterized by Scanning Electron Microscopy, Mercury Porosimetry, Raman Spectroscopy, X-ray Diffraction, Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy, before and after oxidation. An overall comparison of surface morphology, pore structure, crystallite structure, and surface functional group was revealed. The changes in surface morphology and crystallite structure of the filler particles in the graphite materials were observed at temperatures above 1200 °C. This study also demonstrates the relationship between graphite properties and oxidation resistance, and possible intrinsic factors that contribute to oxidation at different temperature ranges, and these are discussed taking into account the dominant role played by temperature.
Supervisor: Patterson, Eann ; Yeh, Tsung-Kuang ; Tzelepi, Athanasia Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral