Material Properties of Oxide Dispersion Strengthened (ODS) Ferritic Steels for Core Materials of FBR; Mechanical Strength Properties of Sodium Exposed and Nickel Diffused Materials [Interim Report]

Kato, Shoichi ; Sakurai, Tadashi*; Yoshida, Eiichi

An oxide dispersion strengthened(ODS) ferritic steel have excellent resistance to swelling and superior creep strength, they are expected to be used as a long-life cladding material in future advallced fast reactor. In this study, sodium environmental effect on the ODS steel developed by JNC were clarified through tensile test after sodium exposure for maximum 10,000hrs and creep-rupture test in sodium at elevated temperature. The exposure to sodium was conducted using a sodium test loop constituted by austenitic steels. For the conditions of sodium exposure test, the sodium temperatures were 923K and 973K, the oxygen concentration in sodium was below 2ppm and sodium flow rate on the surface of specimen was less than 110 sup-4 m/s. Further the specimen with the nickel diffused was prepared, which is simulate to nickel diffusing through sodium from the surface of structural stainless steels. The main results obtained were as follows;(1)The results showed excellent sodium-resistance up to a high temperature of about 973K in stagnant sodium conditions, and its considered that the effects of sodium environment on tensile properties were negligible. In case of stagnant sodium condition, creep-rupture strength in sodium was equal to the in argon gas, and no sodium environmental effect was observed. The same is true for the creep-rupture ductility. (2)The tensile properties of nickel diffused test specimens at high temperatures simulating microstructure change were equal to that of the thermal aging process specimens. These tensile tests suggest that sodium environmental effects can be ignored. However, the effect of nickel diffusion on creep strength are not clear at present and experimental investigation are being conducted. (3)The coefficient of nickel diffusion in the ODS steel can be estimated based on the results of nickel concentration measurement. This value is larger than that of the diffusion coefficient for typical alpha-Fe steel at temperature below 973