Ingress and Redistribution of Protium and Deuterium in Candu Reactor Pressure Tube Rolled Joints

Abstract

Hydrogen isotopes are present in CANDU® reactor pressure tubes as initial protium from manufacturing and through ingress 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. Redistribution by diffusion is also affected by temperature and stress gradients.

Ingress and diffusion modelling is applied to demonstrate that hydrogen equivalent concentration, [H_eq], remains within safe limits to ensure pressure tube integrity over the reactor operating interval. Previous modelling assumed that protium was only present from manufacturing and that ingress only applied to deuterium; redistribution by diffusion was based on [H_eq] without distinguishing diffusion of each isotope. Higher than initial concentrations of protium may be caused by redistribution and/or ingress of protium. A diffusion model accounting for the ingress and redistribution interaction of both isotopes is applied to the rolled joint region of pressure tubes. The model includes solubility and hydride precipitation effects relevant to thermal cycles of reactor operation. Results are discussed in consideration with observed axial and circumferential variations in concentration distributions.