Development of high-chromium steel for the sodium-cooled fast reactor in Japan and creep–fatigue assessment of the steel

This paper presents the provisional material specification and characteristics of the high-chromium (Cr) steel for the sodium-cooled fast reactor (SFR) in Japan and creep–fatigue assessment of the steel. Based on the mechanical test and metallurgical examination results, it was clarified that tungsten (W) should be diminished to achieve better ductility and toughness. Then the provisional specifications of the high-Cr steel for SFR components were proposed. Material characteristics, e.g. cyclic stress–strain relationship and creep strain curve, were also presented based on the material test results. Using these characteristics, a creep–fatigue strength assessment of the steel was performed. Conservative predictions were obtained and it was clarified that such conservatism resulted from over estimation of creep damage caused by too large initial stress at the beginning of stress relaxation. For precise assessment, it is essential to develop an appropriate model accounting for the effect of creep softening due to strain hold.