Study On The Stress Intensity Factor And Propagation Behavior For Weld Cracks Of Nozzle And Safe End In Nuclear Power

The nozzle safe end is a complex welding structure composed of multiple material in the primary water system of nuclear power plant, and the residual stress induced by weld process is one of main factors causing stress corrosion crack. When study the effect of weld residual stress(WRS) on the stress intensity factor and the shape of SCC:on the one hand, the accurate characterization of the WRS is the premise of structural integrity for nozzle and safe end within defects; on the other hand, there is necessary to consider the release of strain field and the redistribution of WRS field due to crack propagation. Based on above all, this paper simulates the welding process about several typical nozzle and safe end by the software of ABAQUS, and analyze the effects of WRS redistribution on the drive force(stress intensity facto) and the shape of SCC propagation, and the main work and research results obtained are as follows:(1)Use ABAQUS software to simulate the weld process of different types such as hot leg and cold leg, surge, safety, spray nozzle and safe end. On the basis of the study about the effects of the straight simplification for the nozzle transition region and reasonable welding heat input on the distribution of WRS, this paper compared the distribution of WRS on different simplified nozzle and safe end models under the average heat input. Then, the weld residual stress coefficients are provided for the nozzle and safe end diameter to thickness ratio within scope of 2 to 5.(2)Based on hot leg nozzle and safe end, study the effects of redistributed weld residual stress on the stress intensity factor along crack front. When regarding the nozzle safe end as straight pipe, there is little effects of this simplification on the K value. And the K values caused by the release of strain field and the redistribution of weld residual stress are smaller than the stress intensity factor that ignoring this effects, especially for the surface crack inner point is more obvious. Then calculate the K values among different crack under the redistributed stress field, and provide the shape factor of K used for hot leg within inner circumferential crack in weld zone.(3)Regard the initial semi elliptical crack as the research object, and analyze the SCC shape under the two condition which considering redistributed weld residual stress and ignoring this problem. In the early phase of propagation time, the SCC shapes are almost similar under this two issues; but the difference will become obviously with the increasing of SCC size, and the crack area is smaller under the redistributed weld residual stress than the area which under the initial residual stress field. In addition, compared with the SCC propagation behavior that ignoring redistributed effect, the distribution of the redistributed weld residual stress will not affect the variton of crack depth and length with time, but it can delay the SCC penetration time and decrease the extension of crack on the inner surface of the weld zone.