| The reactor pressure vessel and the main pipe of primary loop are key equipment in the AP1000nuclear power plant (NPP), the pipe-nozzle and the safe end of which are connected by a dissimilar metal weld joint (DMWJ). Operating experience has shown that the DMWJ can remarkly affect the plant availability and safety and a few accidents had taken place in the DMWJ of the NPP. In the present, the effects of crack location, material property mismatch and geometry on the safety assessment for the safe end to pipe-nozzle have not been completely understood and accurately taken into account in failure evaluation, and no specific accurate methods esist for the integrity assessment of the complex DMWJ. In this et al, three-dimensional finite analysis models are built for a DMWJ connected the safe end to pipe-nozzle of nuclear pressure vessel, then the option3failure assessment curves (FACs) in R6are constructed with the crack locations, the complex geometry of pipe-nozzle and the four materials of DMWJ considered. Moreover, the systematic analysis is conducted to elucidate the effects of nozzle geometry and material mismatch on FACs. The LBB analysis is typically performed at the weakest property location of DMWJ, where is the interface of A508and Alloy82and the LBB evaluation diagram is constructed. In addition, the effects of applied moment and material J-resisrance curve of material on LBB behavior are investgated. The main achievements are as follows:(1) The limit moment ML of the DMWJ is mainly dominated by the plastic yield of the316L material with the lowest yield stress, and the crack locations in the DMWJ have less effect on the ML. With moving the crack location from the safe end through the DMWJ towards the thickness transition in pipe-nozzle, the FACs shift upward caused by the complex materials and strength mismatch effect in the DMWJ structure. It should be noted that the shielding effect for cracks near to nozzle would result in the larger upward shift of FACs.(2) For the cracks far from the nozzle geometry, the FACs for the safe end to pipe-nozzle is nearly the same with the simplified straight pipe; while for the cracks near to the nozzle geometry, the FACs for the simplified straight pipe are too conservative. On the other hand, for the large size crack or the total through crack, the simplified straight pipe would result in too conservative assessment results. (3) The effects of material mismatch in DMWJ on FACs are significant. For the crack in the center of weld region. the FACs is not noly related with the material property of crack tip. but also with the surrounding base material. The FACs would shift with the changes of the base material. Therefore, in the defect safety assessment for engineering application, the material property and crack location in DMWJ should be considered in detail, and the accurate FACs could be constructed based on the finite analysis.(4) The ligament instability line and the critical crack length are obtained by ductile crack stability method, and the LBB evaluation diagram is constructed. The LBB behavior can be evaluated and predicted by the location of the assessment point and crack propagation analysis. Addtionly, with the increasement of applied moment and reduction in J-resistance curve, the ligament instability line shifts downwards and the critical crack length is smaller, which result in the regions for feasibility of applying the LBB concept are lower. |
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