Material properties of stainless steels modified with addition of zirconium for supercritical water-cooled reactor

The fuel claddings of supercritical water-cooled reactors (SCWR) are designed to be irradiated by neutrons equivalent to a neutron damage level of about 15 dpa and be subject to a loaded tensile stress of up to 30 MPa because of the pellet-clad mechanical interaction at high temperatures of up to 700 C degrees. Developing cladding material that can maintain its integrity in such a severe environment is a challenge. Such a material must have high temperature strength and be resistant to corrosion and radiation damage. Austenitic stainless steels modified with the addition of zirconium, which has an effect to suppress vacancy migration, to improve radiation damage resistance have been developed. This work reports on the void swelling properties, microstructural stability, general corrosion properties, high- temperature strength, and creep rupture time of Zr-modified austenitic stainless steels. A preliminary study was conducted on the applicability of the modified stainless steels for use as SCWR fuel cladding. Void swelling was suppressed in each modified stainless steel. Some kinds of Zr-containing precipitation were observed on grains and grain boundaries in the steels after solution and aging treatments. The amount of Fe-Cr compound on the grain boundary in SUS310S+Zr increased above 700 C degrees. It is considered that the Fe-Cr compound affected the creep properties. SUS310S+Zr showed good general corrosion resistance up to 600 C degrees. SUS316L+Zr had oxide film exfoliation at 600 C degrees. The tensile properties of the modified stainless steels were about the same as SUS316. The stresses in which a creep rupture lifetime exceeds 50,000 h were 47.4 MPa for SUS316L+Zr and 36.2 MPa for SUS310S+Zr. It is suggested that a creep rupture lifetime of 50,000 h under 30 - 50 MPa of stress at 700 C degrees could be achieved. Therefore, SUS310S+Zr is considered to be a candidate SCWR fuel cladding material with high temperature strength, low creep, general corrosion resistance, and irradiation resistance, although its microstructural stability at high temperatures is a problem. SUS316L+Zr had poor general corrosion resistance. (authors)