Study On The Defect Assessment Methods For The Dissimilar Weld Joint In The Safe End Of The AP1000 Nuclear Pressure Vessel

In this paper, three-dimensional finite element analysis models are built for the welded safe end of AP1000 nuclear pressure vessels. Through finite element analysis (FEA) of fracture mechanics for the circumferential surface cracks with different sizes in the weld metal, the defect assessment methods for the dissimilar weld joint in the safe end are studied. The simplified defect assessment method, advanced failure assessment diagram (FAD) method and leak-before-break (LBB) assessment method are constructed elementarily. And these methods are related to the complex structure of the safe end, the geometry of welding groove, the complex materials at weld joint region of the dissimilar metal and the crack sizes. The accuracy and complexity of the above-mentioned three methods are raised gradually.The simplified defect assessment method is constructed according to fully-plastic limit load criteria. The stress ratios are calculated from the loading condition; the flaw lengths are inspected or calculated at the end of service period. Allowable flaw depths for a given final flaw length are obtained by interpolation in the table which we constructed. By comparing the flaw depth at end of service period with the maximum allowable flaw depth, the safety of the defect could be assessed. In the advanced FAD assessment method, we constructed FACs, limit loads and stress intensity factors which are related to the complex structure of the safe end, the complex materials at weld joint region of the dissimilar metal and the crack sizes. The accurate FAC related to crack sizes should be selected for defect assessment. In the LBB assessment method, the LBB curves and the LBB assessment figures are constructed, and they both are related to the complex structure of the safe end, the principal load and the J-resistance curves (material toughness) of the materials at weld joint region. Based on the defect sizes and loading condition, we can assess whether the defects in the dissimilar metal weld joint in the safe end will fail in LBB mode.