Research On Residual Stress And Crack Propagation In J-bevel Of Nuclear Reactor Pressure Vessel Head

Reactor Pressure Vessel(RPV)is the core equipment of a nuclear power plant.The head of the pressure vessel is connected to CRDM socket through a J-bevel weld.The J-bevel weld zone belongs to dissimilar metal welding.The service condition of RPV is at pressure of17.1MPa and a temperature of 343?,and with the corrosive boric acid working medium.Under corrosive working media and harsh environmental conditions,J-bevel weld is prone to stress corrosion cracking and cause serious nuclear accidents.Therefore,the research on the residual stress field and crack propagation under the residual stress field in the J-bevel weld zone is of great significance,which provides a guarantee for the safe operation of nuclear power facilities.In this study,CRDM nozzle mock-up was investigated to complete several experiments:(1)Based on ANSYS software,the welding process of J-bevel weld(43.7°sample)was numerically calculated by APDL language,and the temperature field and stress field during welding were obtained.The distribution rules of residual stress on the outer wall of J-bevel weld and the inner wall of pipe are investigated.Then the blind hole method was used to measure the residual stress of the test coupons.The numerical simulation results were compared with the experimental test results,and the stress distribution laws of the two were consistent.There is an extreme point of stress at the welding toe where it is verified the reliability and accuracy of the numerical simulation.(2)Based on the life and death element technology to simulate the drilling process,the strain release coefficients(6?,b?of the weld material are calibrated.The finite element calibration value is in good agreement with the recommended value in ASTM E837-13a with the maximum error is only 0.21%.Through the blind hole method,the residual stress of the J-bevel specimens with different tube angles of 0°,25.9°and 43.7°are measured.The results show that with the increase of the inclination of the tube,the residual stress near the weld area of the external wall of the J-bevel weld and the internal wall of the tube generally increases.(3)With the consideration of the actual service conditions,after the simulation of the J-bevel welding is completed,a pressure of 17.1 MPa and a temperature of 343?are applied to simulate the residual stress distribution in the actual working environment.Compared with the state prior to the start up operation,the residual stress level of the inner and outer wall welds of the simulated part is reduced after the work.The maximum decrease in the residual stress on the outer wall is 60%and the maximum decrease in the residual stress on the inner wall is 37%.(4)An axial semi-elliptical crack was established on the inner wall of the J-bevel weld.The stress-strain field of the crack tip of different sizes and the stress intensity factor K_I of the crack front under the residual stress field were investigated.In the range of about 23mm from the crack tip,the residual stress field will be reallocated,and the residual stress field with cracks beyond this range is basically the same as the original stress field(without cracks).The residual stress field directly affects the stress-strain field of the crack growth tip,and the stress-strain field distribution of the crack-growth tip is consistent with the residual stress field distribution trend under the residual stress field.The deepest point of the crack front(?=90°)is not the maximum stress intensity factor K_I value,whilst maximum value will always appear at the spot near the J-bevel weld.As the initial crack angle?increases,the stress intensity factor K_I at the front edge of the crack decreases.The axial crack(?=0°)has the largest K_I whilst the circumferential crack(?=90°)has the smallest K_I,indicating that the axial crack is more prone to propagation than circumferential cracks.Finally,this article proposes a series of measures to prevent stress corrosion cracks in the weld zone.