This paper presents a fracture assessment methods used in leak before break (LBB) assessment of sodium piping system in the Japanese sodium cooled fast reactor (JSFR). Use of thin wall pipes and compact layout of piping system are features of the design in JSFR. Since the internal pressure of piping of JSFR is low, the critical load is thermal expansion. Supposing a through wall crack (TWC) in such piping, the stiffness of the crack part will decrease, the load balance of the piping system will change from the condition without crack. The fracture assessment methods paying attention to this stiffness change at the crack part were proposed and these methods enabled rational LBB assessment. The proposed methods are much effective to loosen LBB conditions for the piping system of which the compliance is low. These methods applied to the LBB assessment of the piping system of JSFR which has the compact layout, and it was checked that the validity of these methods to loosen the LBB conditions.

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