The Leak-Before-Break (LBB) concept has an effect on the safety design of Fast Breeder Reactors (FBRs), and thus its assessment has been one of the most significant issues. In the case of a commercial-scale FBR, since the main loads are the thermal expansion and thermal transient stresses, ferrite steel with a low thermal expansion rate has been a candidate material. A conventional LBB assessment method is insufficient to consider these characteristics, thus an advanced method is required.

• The LBB assessment to ferrite pipes with thin walls and large diameters is proposed.
• The surface resistance correction factor was improved for low-pressure.
• The R6 method was applied to an unstable fracture assessment for postulated cracks.
• A buckling assessment was introduced for the critical crack length for elbows.

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