Graphical representation of oxygen and carbon potentials in equilibrium with impure high-temperature inert gases
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Communications Graphical Representation of Oxygen and Carbon Potentials in Equilibrium with Impure High-Temperature Inert Gases TETSUJI NODA, MASATOSHI OKADA, AND RYOJI WATANABE In a high-temperature gas-cooled r e a c t o r (HTGR), the interaction between structural materials and imp u r i t y g a s e s in the helium coolant is considered to be a m a j o r problem. The materials are oxidized, carburized, or decarburized through high-temperature reactions with s m a l l amounts of impurities such as CO, CO2, CH4, H20, H2, and O2 which are contained in helium. Material tests in helium g a s e s containing impurities at s e v e r a l levels within the r a n g e expected in real reactors have been c a r r i e d out.z-3 Table I shows examples of s e v e r a l experimental gases.4 In a gas mixture system such as that shown in Table I, it is difficult to predict by thermodynamics the reaction behavior of metallic materials because of the varieties of impurity species. G u r r y f i r s t proposed the graphical representation of the equilibrium compositions of gas mixtures, 5 which does not require knowledge of the equilibrium concentration of each impurity at the temperature of mixture. The gas compositions are represented on the rectangular d i a g r a m s by atomic oxygen fraction, ( ) / S , and atomic carbon fraction, C-/S, where S is the sum of the number of gram atoms of oxygen, carbon, and hydrogen. The oxygen and carbon potentials as a function of temperature and o v e r a l l gas composition are shown on the graphs presented by Gurry2 Howe v e r , because of the complexity of the procedure for constructing diagrams, the application of the d i a g r a m s t o gas mixtures has been very limited. Gurry constructed the d i a g r a m s for iron/iron oxide and iron carbide/gas mixture systems by using the graphical interpolation method.5 A modified method was presented by Littlewood,6 who derived atomic fractions of oxygen and carbon from the solutions of mathematical equations and applied his method t o some metal/metal oxide/gas mixture systems. In these methods, the relationship between (~/S and C-/S were obtained indirectly and sometimes complicated calculation procedures were required. 6 In the present paper, a new method of constructing d i a g r a m s , where C/S values are calculated as a function of (~/S values, is proposed and further the application of the present method to constructing a d i a g r a m of impure helium environment is presented. If the impurity species in the i n e r t gas are assumed t o be composed of water vapor, hydrogen, methane, TETSUJI NODAis Principal Researcher, MASATOSHI OKADA is Chief ofSecond Laboratory, and RYOJI WATANABE is Division Head, Nuclear Materials Division, National Research Institutefor Metals, Ibaragi 305, Japan. Manuscript submittedFebruary 6, 1979. METALLURGICAL TRANSACTIONS B
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PH20
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[3]
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2CO = C + CO2
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PCO and H20 = H2 + 1/202
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