| The occurrence of control rod drive mechanism (CRDM) nozzle cracking near the J-groove weld zone in reactor pressure vessel (RPV) head was caused by stress corrosion cracking (SCC), which was the main failure model of RPV head and leads to radioactive water leakage. Welding residual stress in the J-groove weld has an essential contribution to promote the susceptibility of SCC degradation in CRDM nozzle. Currently, in the integrity analysis of CRDM nozzle, the effect of welding residual stress on service life has not been taken into consideration accurately, and there is little investigation on the distribution of stress intensity factor along the crack front and the SCC growth behavior with welding residual stress. In this paper(学位论文), the thermo-elastic-plastic finite element (FE) simulations were conducted to investigate the distributions of the welding residual stresses in the J-weld. Then the residual stress obtained by FE analysis is mapped directly to the three-dimensional (3D) FE model with a refined axial crack-tip meshes. In subsequent fracture analysis, the equivalent stress intensity factor Keq-solution along crack front was extracted after application of the operating pressure and temperature. In addition, the nature crack growth behavior, which is controlled by the actual distribution of the stress intensity factor Keq along the crack front, was discussed. The main conclusions obtained are as follow:(1) The welding residual stress in CRDM nozzle J-weld is calculated. It has been found that the welding residual stresses in the J-groove weld for both center-hole and53°side-hill CRDM nozzle are much higher on the tube outer diameter (OD) side around the region of weld than the residual stresses on the tube inner diameter (ID) side, and the hoop residual stress is much larger than the axial residual stress in the CRDM nozzle next to the J-groove weld. From the stress point view, the nozzle OD side next to the J-weld area might be degraded by SCC, and would exhibit much higher probability for axial cracking caused by the hoop stress than that for the circumferential cracking caused by the axial stress.(2) The stress intensity factor Keq-solutions for axial semi-elliptical cracks with different depths and lengths were calculated under the condition of residual stress field, operating pressure and temperature. The results show that the peak stress intensity factor Keq always appears at the crack front near the J-weld zone rather than in the center of the crack (φ=90°) due to the high tensile weld residual stress in the J-weld zone and the geometric consideration. The flaws in the CRDM nozzle near the J-weld zone will grow in a nature shape, which is controlled by the actual distribution of the stress intensity factor along the front of the crack, and do not grow in the semi-elliptical shape which is assumed by the present codes.(3) The crack due to SCC starting from the initial semi-elliptical flaw in CRDM nozzle propagates slowly in depth direction, but the crack growth rate in the length direction, which is strongly affected by the residual stress, increases significantly. However, the aspect ratio a/c (depth/length) of nature flaw development with crack propagation is slightly greater than the initial a/c=0.5, then tends to be a constant value. |
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