On a leak-before-break assessment methodology for piping systems of fast breeder reactor

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. Moreover, thin-walled and large-diametric pipes have been used to reduce the number of loops, which might also result in an economical advantage. A conventional LBB assessment method is insufficient to consider these characteristics, thus an advanced method is required. In this context, in the present paper, the following points were proposed to apply the LBB assessment method to ferrite steel pipes with thin walls and large diameters: (1) The surface resistance correction factor against a flow through penetrated cracks was improved for a reasonable leakage assessment under low-pressure. (2) The R6 method was applied to an unstable fracture assessment for postulated cracks. (3) A buckling assessment was introduced in determining the critical crack length for elbows. The applicability of this proposed method has been verified through an LBB assessment on typical ferrite steel pipes.