The lifetime of a fast reactor fuel pin is most strongly determined by the internal creep rupture strength of the cladding induced by the internal pressure of the fission gas at a temperature of around 700 °C. For 9Cr-ODS steel cladding, internal creep rupture data at 650, 700, and 750 °C are shown in Figure 15.30 Additionally, the best fit lines for hoop stress versus rupture time at each temperature are shown by solid lines. These results confirmed that creep rupture strengths in the hoop and longitudinal directions of cladding are almost the same, due to their equi-axed grains. The corresponding creep rupture curves for HT931 and austenitic PNC31632 are also presented for comparison. PNC316 is a typical austenitic clad­ding developed by JAEA in the fast reactor program. Notably, superior performance in rupture time is shown in 9Cr-ODS steel cladding. The slope of PNC316 is steeper, and there is a cross-over before 1000 h at 750 °C and before 10 000 h at 700 °C. The stress condition of the fast reactor fuel pin gradually increases due to the accumulation of fission gases and reaches around 120 MPa at its final service milestone of 75 000 h at 700 °C. In this stress range, it is obvious that 9Cr-ODS steel cladding is of advantage.

The ultimate tensile strength (UTS) of 9Cr-ODS ferritic cladding in the hoop direction as measured in a temperature range from room temperature to 850 °C, is shown in Figure 16, together with the corresponding data for the ferritic-martensitic stain­less steel (PNC-FMS)19 that is conventionally used as fast reactor fuel cladding. The strength of 9Cr-ODS steel is superior to that of conventional PNC-FMS. The uniform elongation that takes place from room temperature to 900 °C is also shown in Figure 16. In the temperature range from 400 to 700 °C at which a fast reactor is commonly operated, the measured uniform elongation exhibits adequate ductility. This advantage of superior elongation in the produced clad­dings can probably be ascribed to the pinning of dis­locations by oxide particles, which retard recovery and sustain work-hardening.