For extension of non-creep design area and simplification of design procedures, a rational identification method of creep design area by negligible creep (NC) curves was studied. NC curves of six kinds of austenite stainless and ferrite steels for fast reactors were determined based on domestic material data. NC curves provide the relation between temperature and time that does not induce damageable creep strain under the constant stress 1.5Sm (Sm: design stress intensity). In existing Japanese design guides, non-creep design area is severely restricted by constant upper temperature limit for austenite stainless steel and ferrite steel. In the case of 316FR steel and SUS410J3, which are candidate materials of Japanese commercialized fast reactors and have excellent material property, this limit can be extended by NC curve concept considering the duration of high temperature operation. NC curves under secondary stress considering stress relaxation were also studied. However, rationalization effect was insufficient whereas evaluation process was too complex. Therefore, at the present stage, NC curves at constant stress level 1.5Sm were adopted to identify creep design area. The concept of NC curve was introduced into the interim structural design guide for commercialized fast reactors in Japan to simplify the creep design of fast reactor systems. Utilizing these curves, non-creep design becomes possible for components operated at comparatively lower temperature in normal condition.

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