4.08.7.1 Sodium Compatibility

It is essential to evaluate the environmental effects of sodium on the mechanical strength properties of ODS steels to ensure their structural integrity throughout their design life-time in SFR. ODS-steels basically display superior compatibility with sodium. For 9Cr — ODS steel (M93) and 12Cr-ODS steel (F95), which are potential cladding materials for SFR, their UTS at 700 °C after exposure to sodium in a stagnant state is shown in Figure 30.53 Both show almost constant strength after exposure to sodium, and it was confirmed that there is no degradation up to 10 000 h. For conven­tional ferritic steel without Y2O3, a clear strength reduction occurs above 600 °C due to decarburization phenomena in sodium. ODS steel does not show such a clear strength reduction because the fine Y2O3 oxide particles remain stable in steel, thereby maintaining the strength of the steel.

Figure 31 shows the results of creep-rupture tests with internally pressurized specimens in a stagnant sodium environment.54 The creep-rupture strength of 9Cr-ODS steel (M11) in sodium is equal to its strength in air, and no impact from a sodium environ­ment was observed. However, under a flowing sodium condition of 4.5 ms, the element nickel penetrates the surface of ODS steel cladding, where an increase in nickel concentration and decrease in chromium con­centration were observed at 700 °C. These results suggest that the effects of a sodium environment can be ignored under stagnant conditions; however, as fuel cladding is utilized in an environment with a high flow rate of sodium, the effects of the microstruc­ture change associated with nickel diffusion into the cladding surface need to be considered.53