Temperature-Dependent Multi-Scale Pore Evolution and Nitrogen Diffusion in Nuclear Graphite
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INTRODUCTION
NUCLEAR graphite is used extensively for moderators, reflectors, and structural materials in high-temperature gas-cooled reactors (HTGRs) because of its excellent irradiation performance and adequate mechanical properties at high temperatures.[1,2] There are both nano- (2 to 500 nm) and micro- (>500 nm) pores in nuclear graphite, hence nitrogen at approximately 1 bar pressure is inevitably present in the porous structures due to processing, transporting, and storing nuclear graphite.[3–6] During HTGRs operation, 14C radiocarbons are produced by the reactions of 14N isotope and thermal neutrons, further leading to 14C release from nuclear graphite.[7] It is deduced that graphite microstructure and gas diffusion behavior have crucial effects on minimizing radiocarbons. In HTGRs, porous nuclear graphite is exposed to high temperatures, neutron irradiation, oxidation, and other deleterious environments. Among these, much effort was devoted to studying synergistic effects of temperature and other factors on graphite microstructure. Huang et al.[8,9] investigated nuclear graphite oxidation in air at temperatures ranging from 973 K
LONGKUI ZHU, ZHENGCAO LI, MINGYANG LI, and WEI MIAO are with the State Key Laboratory of New Ceramics & Fine Processing, Key Laboratory for Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China. Contact e-mail: zcli@ tsinghua.edu.cn MENGHE TU is with the Department of Reactor Engineering Research and Design, China Institute of Atomic Energy, Beijing 102413, P.R. China. HONG LI is with the Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, P.R. China. ALEX A. VOLINSKY is with the Department of Mechanical Engineering, University of South Florida, Tampa, FL 33620. Contact e-mail: [email protected] Manuscript submitted June 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
(700 C) to 1373 K (1100 C), and correlated oxidation behavior with the theory of active sites in graphite. Different oxidation mechanisms were proposed in terms of various reaction temperatures.[7,9–15] It is considered that open pores were first oxidized up to about 873 K (600 C), while the strength gradually decreased with weight loss. When the reaction temperature was close to 1123 K (850 C), the oxidation rate became slower in open pores due to the restricted oxygen diffusion rate. Then oxidation reactions ceased in open pores, and just the external graphite surfaces were oxidized above approximately 1123 K (850 C). In these processes, multi-scale graphite pores with different morphology were formed at low and high temperatures. Wang et al.[16] believe that the micro-pores nucleated and grew during low-temperature oxidation, but the ligaments between the sub-micron pores collapsed and these pores coalesced to form larger pores at high temperatures. Chen et al.[3] think that the micro-pore size became increasingly larger with oxidation of IG-110 and HSM-SC nuclear graphite at 873 K (600 C) or 1073
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