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Abstract
The effects of different parameters on oxidation rate are non-linear, interactive and diversified in which the change of adequacy of O2 supply is an important indicator. The influence of microstructure on oxidation rate became stronger worsening the fitting linearity to calculate the activation energy based on present method with the decreased adequacy of O2 supply due to the increase of temperature, the decrease of gas flow rate, etc. Here, we proposed a method to characterize thermal-oxidation behaviors of nuclear graphite by combining O2 supply and micro surface area of graphite. The proposed method improved the linearity and reduced the standard error of Arrhenius plots of oxidized graphite IG-110 (10 L/min reactant gas) and ET-10 (0.2 L/min reactant gas). The value of activation energy of graphite IG-110 oxidized under ASTM D7542 condition is calculated as 220 kJ/mol by this method echoing the results of previous studies with sufficient O2 supply. For the conditions with less O2 supply at low gas flow rate and/or high temperature, the change of microstructure of oxidized graphite should be obtained as an important factor influencing oxidation rate of graphite.
Highlights
Nuclear grade graphite, because of its anti-radiation performance and excellent mechanical properties, is widely used in High Temperature Gas-cooled Reactor (HTGR)[1,2] and molten salt reactor[3,4] as the material of structure, moderator, reflector and fuel element
For oxidation behaviors with high gas flow rate, the microstructure of graphite, such as surface area, was indicated to be a constant object at a certain range of Mass Loss (ML), which was independent from temperature and O2 supply[16,17]
The specimens were oxidized based on the experiment conditions (10 L/min air flow) recommended by ASTM D7542 and their pore areas were obtained based on optical microscopy examination
Summary
Because of its anti-radiation performance and excellent mechanical properties, is widely used in High Temperature Gas-cooled Reactor (HTGR)[1,2] and molten salt reactor[3,4] as the material of structure, moderator, reflector and fuel element. The studies with high gas flow rate usually had the oxidation conditions close to ASTM D754215 (originally approved in 2009) with related sufficient O2 supply (e.g. 10 L/min air flow) and a cylinder geometry specimen (e.g. D = H = 25.4 mm) based on the common sense of material engineering. The graphite oxidation rate relates with temperature, difficulty of oxidation (activation energy) and reactant supply including O2 concentration, gas flow rate and micro structure and geometry of graphite. Present studies mainly focused on the relations between oxidation rate and temperature or O2 concentration which discussed the activation energy or reaction order of graphite oxidation respectively. For oxidation behaviors with high gas flow rate, the microstructure of graphite, such as surface area, was indicated to be a constant object at a certain range of Mass Loss (ML), which was independent from temperature and O2 supply[16,17]. The adequacy of O2 supply indicated by the ratio of O2 supply to consumed O2 should be around 10 or higher to avoid the departure of the oxidation mechanism from chemical kinetic regime
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