Research and Development of Oxide Dispersion Strengthened Ferritic Steels for Sodium Cooled Fast Breeder Reactor Fuels

Oxide dispersion strengthened (ODS) ferritic steels is eligible for fuel pin cladding tubes to endure heavy displacement damages to 250 dpa at high temperatures up to 973 K in commercialized sodium cooled fast breeder reactor cores. Japan Atomic Energy Agency has been developing ODS steels since 1987. For ODS steels, powder metallurgy process including mechanical alloying and hot consolidation produces highly stable and fine oxide dispersoids in matrix, and provides excellent dispersion hardening. Although cold rolling process in tube manufacturing tends to cause anisotropic grain growth, alpha to gamma phase transformation for the 9Cr-ODS steel and recrystallization for the 12Cr-ODS steel can modify grain morphology and improve creep rupture strength under internal pressure. Mechanical properties of the tubes have been extensively tested in air and stagnant sodium environments to establish Material Strength Standard for fuel pin mechanical design. Also, hundreds of specimens have been irradiated in the experimental fast reactor JOYO to investtigate irradiation effect on dimensional stability and mechanical properties. Pressurized resistance welding technology has been applied to join a tube and end plugs. Both 9Cr- and 12Cr-ODS steel tubes were assembled into fuel pins, and have been irradiated since 2003 under the collaborative program in the BOR-60 in Russia.