Evaluations of macro structure and strength of ODS steels heated to ultra-high temperature simulating accident situations

Tanno, Takashi  ; Fujita, Koji; Yano, Yasuhide  ; Otsuka, Satoshi   ; Nakashima, Hideharu*; Mitsuhara, Masatoshi*; Onuma, Masato*; Toyama, Takeshi*; Kaito, Takeji 

By applying oxide dispersion-strengthened (ODS) steel, which has excellent high-temperature strength, to the fuel cladding tube, it is expected that the risk of fuel pin rupture up to the ultra-high temperature range including accidents will be reduced and the safety of the plant will be improved. Oxide dispersoids as a reinforced phase in ODS steels are considered to be more stable than carbonitrides and intermetallic compounds used as reinforced phases in other heat-resistant alloys. It is important to actually ensure that the metallographical structure and strength are maintained up to the ultra-high temperature. In this study, in order to evaluate the performance limit of ODS steel against ultra-high temperature, extreme heat treatments simulating accident situations were carried out, and then the macrostructure and strength were evaluated. Regarding the macrostructure, it was generally stable up to 1250 C and the crystal grains were fine, but it was coarsened at 1300 C or higher. On the other hand, it was found that the hardness decreased at 1250 C or higher. This theme was supported by JPMXD0219214482, a nuclear power system research and development project of the Ministry of Education, Culture, Sports, Science and Technology.