Modelling of Local Hydrogen Isotope Concentration at Rolled Joints of CANDU Reactor Fuel Channels and Evaluation of Potential Interaction With an Adjacent Flaw

The pressure tubes in CANDU-PHWR (Pressurized Heavy Water Reactor) are subject to various ageing related degradation mechanisms, including ingress of deuterium, which is a hydrogen isotope, through a corrosion reaction with the heavy water coolant. Pressure tube structural integrity is dependent on the hydrogen equivalent concentration, Heq, which accounts for both hydrogen and deuterium, and increases with reactor operating time. In recent Heq measurements, high Heq levels were observed at local areas of some pressure tubes that were in service for long operating times. This local area of high Heq is referred to as a “blip”. This paper investigates the modelling of blip formation at the inlet rolled region and its interaction with an incident postulated blunt flaw residing on the pressure tube inside surface at the blip axial location using a comprehensive three-dimensional finite element diffusion model of the fuel channel and a set of generic inputs. The analysis results indicate that the presence of a blip has a minimal effect on the build-up of hydrides at a postulated blunt flaw-tip of a flaw residing at the pressure tube inside surface at the blip axial location over long operating times.