The effect of microstructure on air oxidation resistance of nuclear graphite 5 Cristian I. Contescu * , Tyler Guldan, Peng Wang 1 , Timothy D. Burchell Materials Science and Technology Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN 37932-6087, United States ARTICLE INFO Article history: Received 10 November 2011 Accepted 11 January 2012 Available online 20 January 2012 ABSTRACT Oxidation resistance in air of three grades of nuclear graphite with different structures was compared using a standard thermogravimetric method. Differences in the oxidation behavior have been identified with respect to both (i) the rate of oxidation in identical con- ditions and the derived apparent activation energy and pre-exponential factor and (ii) the penetration depth of the oxidant and the development of the oxidized layer. These differ- ences were ascribed to structural differences between the three graphite grades, in partic- ular the grain size and shape of the graphite filler, and the associated textural properties, such as total BET surface area and porosity distribution in the un-oxidized material. It was also found that the amount of strongly bonded surface oxygen complexes measured by thermodesorption significantly exceeds the amount afforded by the low BET surface area, and therefore low temperature oxygen chemisorption is not a reliable method for determining the amount of surface sites (re)active during air oxidation. The relationship between nuclear graphite microstructure and its oxidation resistance demonstrated in this work underlines the importance of performing comprehensive oxidation characterization studies of the new grades of nuclear graphite considered as candidates for very high tem- perature gas-cooled reactors. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The next generation of nuclear power plants (NGNP) is based on modular high-temperature gas-cooled rectors (HTGR) of either prismatic or pebble-bed type. Both designs use nucle- ar-grade graphite as construction materials for the moderator and the core structures. The operating temperatures are sim- ilar for the two concepts, but the irradiation level of graphite is higher for the pebble bed design. A significant challenge for construction of next generation nuclear reactors in the United States is that the H-451 graphite qualified years ago in the US is no longer available, and new grades of graphite must be qualified as replacement. An intense research program is underway in the United States for detailed characterization of physical, thermal, mechanical, electrical, and chemical properties of graphite grades considered as potential candi- dates for HTGR, and of their behavior under neutron irradia- tion and mechanical stress. A part of this research effort is characterization of graphite behavior in two oxidation re- gimes: (i) chronic oxidation caused by long term exposure to chemical impurities that may exist in the helium coolant dur- ing normal operation; and (ii) acute oxidation that might be 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.01.040 5 This submission was supported by a contractor of the United States Government under Contract DE-AC05-00OR22725 with the United States Department of Energy. The United States Government retains, and the publisher, by accepting this submission for publication acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this submission, or allow others to do so, for United States Government purposes. * Corresponding author: Fax: +1 865 576 8424. E-mail address: ContescuCI@ornl.gov (C.I. Contescu). 1 Visiting from Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China CARBON 50 (2012) 3354 – 3366 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon