Hydride orientation near pressure tube-end fitting rolled joints

Abstract

The connection between Zr–2.5%Nb pressure tubes and modified martensitic stainless steel-403 end fittings in Pressurized Heavy Water Reactors (PHWRs) at the rolled joint is a critical site marked by enhanced hydrogen diffusion and elevated tensile residual stresses. The application of circumferential stress above a threshold leads to the formation of radial hydrides. Formation of radial hydride leads to a drastic reduction in fracture toughness and increase in Delayed Hydride Cracking (DHC) velocity. A comprehensive understanding of residual stress, hydride orientation and radial hydride fraction in this region is essential for the assessment of pressure tube structural integrity. In this study, we employ 3D nonlinear finite element analysis to estimate the residual stresses near rolled joints for both 220 MWe and 700 MWe pressure tubes. Additionally, we pioneer an experimental investigation into hydride orientation near roll joints through internal pressurization of hydrided pressure tube and end fitting assemblies. The study delves into the influence of residual stresses due to rolling process on hydride orientation and radial hydride fraction, providing crucial insights for the evaluation of the structural integrity of pressure tubes.