Prediction of Residual Stress Distribution for RPV CRDM Penetration Nozzles

In nuclear power plants, RPV (Reactor Pressure Vessel) upper head CRDM (Control Rod Drive Mechanism) penetration tubes has been fabricated J-groove weld geometry. Recently, the incidences of cracking in Alloy 600 CRDM tubes and their associated welds have increased significantly. The cracking mechanism has been attributed to PWSCC (Pressurized Water Stress Corrosion Cracking) and has been shown to be driven by welding residual stresses and operational stresses in the weld region. Weld induced residual stress is main factor for crack growth. Therefore exact estimation of residual stress is important for reliable operating. In this point, we have been conducting detailed welding simulation analyses for Korea Nuclear Reactor Pressure Vessel to predict the magnitude of weld residual stresses in penetration tubes. In the present work, the FE (Finite Element) simulations were conducted to investigate the effects of tube geometry (location and r_o/t) and material properties on the residual stresses in the J-groove weld for a different location of CRDM tubes. The variables of tube location included three (center-hole, intermediate and steepest side hill tube) inclination angles (Ψ). And this comparison was performed for different tube geometry (r_o/t = 2, 3, 4), different yield strength (σ_o) of tube. In CRDM tube, when increases in tube inclination angle (Ψ), axial residual stress are gradually increased, but hoop residual stresses are decreased at the nearby weld root. In effect of tube radius and thickness, when the thickness of CRDM tubes increases the residual stresses are gradually decreased at the inner surface of tube. And there is no effect of CRDM tube radius (r_o). In effect of plastic properties of Alloy 600 material in CRDM, when yield strength of the tube increases the axial residual stresses decreases but hoop residual stress increases.