Leak-before-break analysis of shell-nozzle junction of steam generator
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Investigations on leak-before-break analysis of shell-nozzle junction of steam generator (SG) are presented here. Steam generators are integral parts of the nuclear power plants. So, to prevent the catastrophic failure of such components nowadays, leak-before-break (LBB) concept is used. There are three levels of checking LBB behavior, namely level 1, level 2 and level 3. Level 1 is inherent in the design philosophy of ASME Sec. III, which is normally followed in the pipe design. This paper describes level 2 and level 3 LBB analysis for SG shell-nozzle junction. In level 2, crack propagation analysis of surface crack at the most critical locations of SG shell-nozzle junction was carried out, showing thereby, that crack growth is insignificant during the complete one power plant life cycle. Crack propagation analysis was conducted as defined in RCC-MR code. The methodology based on Paris law, which needs evaluation of effective DeltaK (DeltaK(eff)) taking into account effect of plasticity and crack closure coefficient, was used. In level 3, through-wall leak size cracks (LSC) were postulated at the most critical locations and crack instability analysis was carried out under maximum credible loading conditions (e.g. earthquake). For crack instability analysis, various steps namely determination of leakage area and leak size crack (LSC) using leak-rate model, elastic-plastic fracture mechanics analysis (J-integral/tearing modulus approach) and limit load analysis (twice elastic slope method) were carried out. For the evaluation of critical load, elastic-plastic fracture mechanics analysis and for the evaluation of limit load, limit load analysis were carried outs Since no geometrical simplifications were possible for SG shell-nozzle junction, complete three-dimensional non-linear finite element analysis was performed. And, it has been proved that, because of postulated cracks, SG shell nozzle junction would not fail in ductile tearing and plastic collapse under maximum credible load that may act during a safe shutdown earthquake (SSE).
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