High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO 2 as Impurities

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TRODUCTION AND BACKGROUND

OVER the past few decades, the very high temperature reactor (VHTR) has emerged as a leading advanced nuclear reactor concept. High efficiency of electricity production (>50 pct) and its high service lifetime (>60 years), combined with a broad range of process heat applications, such as hydrogen production, distinguish it from other ‘‘Generation IV’’ nuclear reactor systems.[1–3] In this concept, helium gas with outlet temperature up to 1273 K (1000 C) will pass through an intermediate heat exchanger, where it will transfer heat to a secondary coolant. The helium inevitably contains parts per million (ppm) level of CO, CO2, H2, H2O, and CH4 as impurities, which arise mainly from reactions between the hot graphite core and in-leakage of O2, N2, and water vapor from seals, welds, and degassing of reactor materials such as fuel, thermal insulation, and in-core structural materials.[4,5] Typical DEEPAK KUMAR, Graduate Student, RAGHAVENDRA R. ADHARAPURAPU, Postdoctoral Researcher, and GARY S. WAS, Professor, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] TRESA M. POLLOCK, Professor, is with the Materials Department, University of California, Santa Barbara, CA 93106. Manuscript submitted June 14, 2010. Article published online January 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

concentrations of helium impurities in various gascooled reactors are shown in Table I. Depending on the impurity concentration, temperature, and alloy composition, the impurities react with the metallic surfaces of the heat exchanger, resulting in oxidation, oxide reduction, carburization, and decarburization. These oxidation processes can degrade the mechanical properties of the alloy; e.g., oxidation reduces the load-bearing cross section of the component and internal oxide precipitates can act as preferential crack initiation sites and decrease the creep and fatigue life of the alloy.[6] A significant reduction in the creeprupture ductility of alloy 800H,[7] alloy 617,[8] and Hastelloy XR[9] was reported in carburizing environment in comparison to pure helium and air environments. A coarse and semicontinuous film of carbides forms along the grain boundaries during carburization, which may act as preferential crack initiation and propagation paths and could severely decrease the operating life of the alloys. Grain boundary migration and sliding was identified as the dominant creep deformation mechanism in the candidate alloys, such as alloy 617 at 1273 K (1000 C),[10,11] and the dissolution of carbides due to decarburization may lead to significant loss of the creep strength. Therefore, a detailed knowledge of the oxidation mechanisms and rates of microstructure degradation is important to estimate the lifetime of the component and define VOLUME 42A, MAY 2011—1245

Table I. Typical Concentration of Impurities in Impure Helium[4] Reactor Dragon AVR PNP HHT HTGR-SC AGCNR

H2 (ppm)

H2O (ppm)

CO2 (ppm)

CO (ppm)

CH4 (ppm)