A Study for proposal of welded joint strength reduction factors of modified 9Cr-1Mo steel for Japan sodium cooled fast reactor (JSFR)

Wakai, Takashi ; Onizawa, Takashi  ; Kato, Takehiko*; Date, Shingo*; Kikuchi, Koichi*; Sato, Kenichiro*

This paper proposes provisional welded joint strength reduction factors (WJSRF) of Modified 9Cr-1Mo steel applicable to the structural designing of "Japan sodium cooled fast reactor (JSFR)". JSME published a revised version of the elevated temperature design code in last year. Modified 9Cr-1Mo steel was officially registered in the code as a new structural material for sodium cooled fast reactors. The creep strength curve for the base metal of the steel was standardized by employing stress range partitioning method, same as for the welded joint. However, second order equation of logarithm stress was applied in the analysis for the base metal. In addition, the creep rupture data obtained at 700C were included in the database and data ruptured in very short term, i.e. smaller than 100 hours, were excluded from the analysis. Thus, there are some differences between the procedures to determine the creep strength curves for base metal and welded joint made of Modified 9Cr-1Mo steel. This paper discusses the most feasible procedure to determine the creep strength curve of the welded joint of the steel by performing some case studies to focus on physical adequacy and usability. Then, the strength reduction factors are provisionally proposed based on the design creep rupture stress intensities. In addition, the design of JSFR pipes were reviewed taking the proposed WJSRF into account.