ABSTRACT Leak-before-break (LBB) is an important concept that could confirm design and integrity evaluation of nuclear power plant piping. For the LBB analysis, the detective leakage rate should be calculated for a through-wall cracked pipes. For this calculation, the crack opening displacement (COD) calculation is essential. Recently, sodium faster reactor (SFR) which has thin-walled pipes with Rm/t ranged 30-40 was introduced and then the investigation of these thin walled pipes and elbows has received great attention in the LBB evaluation. In this context, the threedimensional finite element (FE) analyses for thin elbows with circumferential crack under in-plane bending are carried out to investigate the elastic COD values. Finally, the solution for elastic COD which can cover sufficiently thin elbow is successfully addressed
