Study of Hydrogen Evolution for the Outlet Rolled Joint of Candu Reactor Pressure Tube Under Operation Thermal Cycle

Hydrogen is presented in CANDU reactor pressure tubes component as initial protium from manufacturing as well as deuterium ingress from heavy coolant occurring during operation. Ingress is more prominent at the rolled joint connection at each end of the pressure tube such that concentration gradients cause inboard diffusion of the hydrogen. High hydrogen concentration could reduce ductility and toughness of material via delayed hydride cracking (DHC) in nuclear reactor components such as pressure tubes. A finite element-based diffusion model, accounting for the ingress and redistribution interaction of protium and deuterium interaction, was developed to study the hydrogen evolution and redistribution at outlet rolled joint region of the pressure tube. To more precisely capture the hydrogen evolution by change in temperature for the reactor thermal cycle, temperature profiles calculated by thermal-hydraulic code, along with reactor startup/shutdown history was applied. This diffusion model predicts both protium and deuterium distributions in good agreement with measurements. The modelling results shows the temperature circumferential gradient is an important factor in redistributing the hydrogen concentration in the pressure tube. Sensitivity to input parameters (such as Terminal Solid Solubility (TSS), diffusivity) is seen to affect ratchetting responses of hydride accumulation at the top of the pressure tube. Modelling results also reveal that protium redistribution causes local increase in protium concentration.