Development of Accident-Tolerant FeCrAl Steels Containing Al 2 O 3 Particles by Means of Internal Al Oxidation
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INTRODUCTION
THE severe accident involving light-water reactors of the Fukushima Daiichi nuclear power plant caused core melting and a hydrogen explosion on March 11, 2011. It should be noted that the source of much of the heat inducing core melting and hydrogen gas production was associated with an oxidation reaction between Zircaloy cladding and hot steam. This reaction is known to be significantly accelerated at elevated temperatures over 1473 K, providing a large amount of exothermic heat. FeCrAl steels are one of the most promising candidates for creating accident-tolerant fuel (ATF) in the light-water reactors, due to its excellent hot steam oxidation resistance that resulted from a formation of a protective Al2O3 scale on the surface of FeCrAl cladding.[1] To compensate for the neutron-reactivity penalty caused by the element Fe, which has a high neutron-absorption cross-section, reducing the cladding thickness is
HIROKI SHIBATA is with the Graduate School of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo, 060-8268, Japan. SHIGEHARU UKAI, NAOKO H. OONO, and SHIGENARI HAYASHI are with the Faculty of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo 060-8268, Japan. KAN SAKAMOTO and MUTSUMI HIRAI are with the Nippon Nuclear Fuel Development Co., LTD, 2163 Narita-cho, Oarai-machi HigashiIbaraki-gun, Ibaraki-ken, 311-1313 Japan. Contact e-mail: [email protected] Manuscript submitted July 16, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
necessary, if the uranium enrichment is kept below 5 pct.[2] FeCrAl-based oxide dispersion-strengthened (ODS) ferritic steels should be indispensable to satisfy such a requirement, since the strength level of the cladding is significantly improved by the dispersed oxide particles. Therefore, FeCrAl-ODS steels have been extensively developed by our research group. However, the FeCrAl-ODS steels contain coarser Y-Al complex oxide particles formed by the reaction of aluminum with yttria, which leads to degradation of the mechanical properties.[3] To overcome this difficulty, our group has been developing two types of new FeCrAl-ODS steels:Zr-coadded FeCrAl-ODS steels to replace Y-Al complex oxide with the fine Y-Zr complex oxide particles[4–7] as well as ceria (Ce2O3) dispersion-strengthened steel instead of yttria particles.[8] Through these studies, it has been revealed that controlling oxygen content in steels by means of mechanical alloying made it possible to form Al2O3 particles by an internal oxidation of solute aluminum, and Al2O3 particles could be useful as the dispersed oxide particles. Without introduction of the oxide particles (such as Y2O3 and Ce2O3), only solute aluminum can create not only protective Al2O3 scale on the outer surface of the cladding by external oxidation but also Al2O3 particles inside the steels by internal oxidation. In this study, the process and mechanism of Al2O3 particle formation are explored, and the possibility of mechanical property improvement is investigated by controlling oxygen content in FeCrAl steels. The mechanisms
