Development of LBB assessment method for Japanese Sodium cooled Fast Reactor (JSFR) pipes, 2; Crack opening displacement assessment of thin wall pipes made of modified 9Cr-1Mo steel

JSFR配管のLBB評価法の開発,2; 改良9Cr-1Mo鋼薄肉配管のき裂開口変位

若井 隆純 ; 町田 秀夫*; 荒川 学*; 江沼 康弘*; 浅山 泰 

Wakai, Takashi; Machida, Hideo*; Arakawa, Manabu*; Enuma, Yasuhiro*; Asayama, Tai

For sodium pipes of Japanese Sodium cooled Fast Reactor (JSFR), the continuous leak monitoring will be adopted as an alternative of volume examination of weld joints under conditions of satisfying Leak-Before-Break (LBB). Since sodium pipes of JSFR is used under the conditions that an internal pressure is very low, its thickness is small. Modified 9Cr-1Mo steel has high yield stress and its work hardening is small. In order to evaluate the coolant leak rate from a penetrated crack, crack opening displacement (COD) assessment method is required for LBB assessment of such pipes. However, COD assessment method applicable for JSFR pipes - thin wall and small work hardening material - has not been proposed yet. In this study, COD assessment method applicable to thin wall pipes made of modified 9Cr-1Mo steel was developed. The ratio of a radius (R) to thickness (t) covers 5 to 50, and the work hardening exponent of stress-strain curve expressed by Ramberg-Osgood approximation covers 5 to 25 in the proposed method. COD is classified into three components of elastic, small-scale plastic and large-scale plastic, and is evaluated as the sum of these components. By applying the proposed method, evaluation of COD of the penetrated crack in the JSFR pipes was achieved.