Study On Simulation For Expanding And Welding Tube-to-Tubesheet And Joint Performance For The AP1000 Steam Generator Of Nuclear Power

By using the three-dimensional finite element simulation, the process for expanding and welding tube-to-tubesheet and the joint performance at room temperature and operating conditions have been investigated for the AP1000 steam generator of nuclear power. The main research work and the results obtained are as follows:(1) The process for expanding and pulling-out tube-to-tubesheet joints at different expansion pressure have been simulated for AP1000 and AP600 respectively. Residual contact pressure at the interface of joints, and the residual stress at the transition zone were calculated. The relations between expansion pressure and residual contact pressure, the wall reduction, as well as pull-out strength were obtained. The results show that, with increasing the expansion pressure, the residual contact pressure and the wall reduction both increase linearly; and the pull-out force also increases.(2) By using the thermo-mechanical numerical method, the effects of the overlaying layer on tubesheet, heat source radius and welding parameters on welding residual stress have been investigated. Distribution of welding residual stress was obtained and the influence of welding parameters on the residual stress was examined. It has been found that there is high stress concentration in the weld root after welding. Along the radial path on the tubesheet surface, the radial and circumferential residual stresses decrease after an initial increase in a range of 5mm and eventually tend to be zero. The overlaying layer can effectively weaken the welding residual stress. It was also found that the welding residual stress decreases with increasing heat source radius and decreasing welding current.(3) Tube-to-tubesheet joint performance under operating condition has been studied, and the influence of temperature loading, pressure loading and coupling of these two factors on residual contact pressure and pull-out strength were investigated. The results show that the working pressure loading can effectively raise the residual contact pressure, pull-out strength and the tightness of joints; while the working temperature loading can severely degrade the joint performance thus reduce the service safety and reliability of joints. The joint performance under coupling condition depends on temperature loading combined with pressure loading. In order to avoid performance degradation of joints and leakage accidents, primary fluid temperature should be controlled not to be too high and primary fluid pressure not to be too low.