Formulation of high-temperature strength equation of 9Cr-ODS tempered martensitic steels using the Larson–Miller parameter and life-fraction rule for rupture life assessment in steady-state, transient, and accident conditions of fast reactor fuel.

• A single high-temperature strength equation expressing the mechanical strength in different deformation and rupture modes was derived for 9Cr-ODS TMS cladding tubes. • This equation can predict the rupture life of the cladding tubes under various stresses and temperatures over time. • The reason why the equation can be applied to different deformation and rupture modes is considered to be the effect of the fine-grain matrix of 9Cr-ODS TMS. • The study suggested the thermal activation process is dominant even for the high-temperature deformation modes exceeding yield stress in the 9Cr-ODS TMS with the fine-grained matrix. This paper discusses the applicability of Straalsund et al.'s technique for combining the Larson–Miller parameter (LMP) and life-fraction rule to form a single high-temperature strength equation for 9Cr-oxide-dispersion-strengthened (ODS) tempered martensitic steels (TMS). It uses the extensive dataset on creep rupture, tensile, and temperature-transient-to-burst tests of 9Cr-ODS TMS cladding tubes in the α-phase, α/γ-duplex, γ-phase matrices, which are accumulated by the Japan Atomic Energy Agency so far. The technique is adequately applicable to 9Cr-ODS TMS cladding tubes. A single high-temperature strength equation expressing the mechanical strength in different deformation and rupture modes (creep, tensile, temperature-transient-to-burst) is derived for 9Cr-ODS TMS cladding tubes. This equation can predict the rupture life of the cladding tubes under various stresses and temperatures over time. The applicable range of the high-temperature strength equation is specified in this study and the upper limit temperature for the equation is found to be 1200 °C. At temperatures higher than 1200 °C, the coarsening and aggregation of nanosized oxide particles and the γ to δ phase transformation are reported in previous studies. The high-temperature strength equation can be well applied to the creep and tensile strength in the α-phase matrix, the creep strength in the γ-phase matrix and the temperature-transient-to-burst strength in both phases except for the low equivalent stress (43 MPa) at temperatures exceeding 1050 °C. The mechanism of the notable consistency between creep and tensile strength in the α-phase matrix is discussed by analyzing the high-temperature deformation data in the light of existing deformation models. The study suggested the thermal activation process is dominant even for the high-temperature deformation modes exceeding yield stress in the 9Cr-ODS TMS with the fine-grained matrix. [ABSTRACT FROM AUTHOR]