Development of modified SUS 316 stainless steel as core material for fast breeder reactors.

A modified SUS 316 stainless steel (PNC 316) for high burnup fast reactor core application has been successfully developed. The material 16Cr-14Ni-2.5Mo-0.25P-0.004B-0.1Ti-0.1Nb steel is utilized with 20% cold-worked condition. The steel exhibits excellent characteristics in creep rupture strength and swelling resistance, and is superior to any other Type 316 class stainless steels, i.e. 104 h creep rupture strength greater than 200 MPa at 650°C and swelling less than 1% at neutron fluence of 1.9×1023n/cm2 (E>0.1 MeV).Reliability of this steel has been confirmed by fast reactor irradiation of 26, 000 fuel elements and 500 fuel sub-assemblies. Postirradiation examination of the fuel elements revealed the excellent characteristics in dimensional stability, compatibility with sodium as well as fission products and radiation induced mechanical property changes, compatible with a current high burnup design target.Specifications and techniques have also been developed for the inspection of large amount of claddings, wrapper tubes and other parts for fuel sub-assemblies.