| The operating experience of nuclear power plant shows that the stress corrosion cracking (SCC) in high temperature and high pressure of water in the primary loop is the main failure mode of the dissimilar metal welded joint of the reactor pressure vessel safe-end nozzle. The accurate calculation of stress intensity factors for the cracks in weld metal are needed for analyzing the SCC crack growth, life prediction, LBB(Leak Before Break) analyses and evaluation of defect safety accurately. In this paper, a three-dimensional finite element model was built for the safe end in the Generation III AP1000nuclear power. The inner-surface circumferential cracks with different sizes in the weld metal are simulated by using the finite element method.The distribution of stress intensity factor along the crack front were calculated with considering welding residual stresses and at different bending moments. The natural shapes of SCC cracks were simulated and compared with the hypothetical semi-elliptic crack growth shape in codes. Then the growth paths of SCC cracks for different initial crack sizes were analyzed with the LBB failure analysis diagram of the safe end. The growth trends and LBB behavior of the SCC cracks were predicted. The main results obtained are as follows:(1) The weld residual stress (WRS) has a great influence on the distribution of the stress intensity factors along the inner-surface circumferential crack front, and the influence is more larger for the deeper the crack. The maximum value of K generally appears near the inner-surface where the peak value of WRS exists, and does not appear at the deepest point of crack which is assumed by the present codes. The values of the stress intensity factor along the crack front increase with increasing the crack depth, crack length and the bending moments.(2) Based on the FEM results for cracks with different sizes subjected to various bending moments combined with WRS and the fitting expression, the interpolation calculation for K values along the front for a given crack under any load can be realized by a purpose-written VB program, which lays a foundation for natural growth analysis of the SCC crack in the welding region in the AP1000safe end.(3) With considering the effect of WRS, the shapes of SCC natural crack growth, which is controlled by K distribution along the crack front are quite different from the assumption of semi-elliptic crack growth in the existing codes. The SCC crack growth rate near the inner surface is larger than that of the semi-elliptic crack growth, and that around the deepest point is smaller than that of the semi-elliptic crack growth rate. Therefore, the influence of WRS should be taken into account when the shape of SCC crack growth in welding region safe end, the interval of time from crack growth to crack penetration and the crack growth length are calculated. Besides, the K distribution along the crack front, which is relevant to the geometry of safe end, material and loads, should be calculated by3-D FEM method. The WRS accelerates the SCC crack growth and increases the inner surface crack length.(4) Due to the high fracture toughness and the high resistance to crack growth of nuclear materials, the critical size is quite large for the crack instability propagation after the SCC penetration, and the Lbb is more likely to occur. For a given crack depth, the corresponding initial limit crack length for producing rupture (LBB does not occur) was determined. The safety margin of LBB can be extended by increasing the fracture toughness of nuclear material and reducing the bending moment. |
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