Evaluation of stability of precipitates under irradiation in 316FR steel used as fast reactor structural material

Toyota, Kodai ; Onizawa, Takashi  ; Wakai, Eiichi*  

316FR steel, a modification of 316 austenitic stainless steel, will be used as a structural material in the sodium cooled fast reactor (SFR), one of the initiatives being developed in Japan to achieve carbon neutrality in order to combat global warming. To withstand the high-temperature operating environment of the SFR, the alloy design of the 316FR steel has been optimized to have high creep strength for a long time with controlled precipitation by optimizing the alloy composition. In order to clarify that 316FR steel can maintain its properties under the high temperature (around 550C) irradiation environment of the SFR, the authors mainly conducted in-situ observations under electron beam irradiation at high temperatures to investigate in detail the irradiation effects on the precipitates (mainly carbides), which are characteristic of 316FR steel. As a result, it was found that the precipitates in 316FR steel are more stable than those in type 304 stainless steel under irradiation without coarsening at grain boundaries or within grains. The characteristics and attractiveness of 316FR steel, the results obtained, and the mechanism of creep behavior under irradiation are also explained.