Session: OAC-07-01 Plant Life Extension: Aging & Life Management-1

Paper Number: 151777

151777 - Modelling of Local Hydrogen Isotope Concentration at Rolled Joints of Candu Reactor Fuel Channels and Evaluation of Potential Interaction With an Adjacent Flaw 

Abstract: 

The CANDU-PHWR (Pressurized Heavy Water Reactor) core has 380 or 480 fuel channels depending on the reactor design. One of the most critical components of a fuel channel is the cold-worked zirconium 2.5 wt% niobium (Zr-2.5Nb) pressure tube, which contains the fuel and acts as the primary pressure boundary in CANDU reactors. The pressure tubes operate in conditions of high neutron fluence, outlet temperatures of nominally 300°C, and pressures of nominally 10 to 11 MPa. The pressure tubes 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. These localized regions of high [Heq] were observed in both inlet and outlet rolled joint regions over a narrow circumferential and axial extent at about 80 mm from the ends of the pressure tube. This local area of high [Heq] is referred to as a “blip”.

The current work investigates the modelling of blip formation at the inlet rolled region and its interaction with an incident postulated blunt flaw residing at the pressure tube inside surface at the blip axial location using a comprehensive three-dimensional finite element diffusion model of the fuel channel. The model uses a generic set of inputs for this study. The ANSYS commercial finite element computer code is used to simulate pressure tube deformation and thermal behaviour for generating the stress and temperature distributions. These inputs along with the boundary conditions for [Heq] ingress, solubility limits for hydride dissolution and precipitation, and reactor Heatup/Cooldown cycles, are input into a three-dimensional finite element computer code HFE (Hydrogen Finite Element) to simulate [Heq] evolution and the accumulation of solid hydride within the pressure tube. The simulations show that the blip formation can be modelled by diffusion of [Heq] to a localized zone of lower temperature on the pressure tube outside surface created by a heat transfer path from the pressure tube outside surface to lower temperature components in the rolled joint assembly. 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.

Presenting Author: Sreehari Ramachandra Prabhu Kinectrics Inc.

Presenting Author Biography: Sreehari Ramachandra Prabhu is an Engineer/Scientist at Kinectrics Inc. He supports various high-profile projects on CANDU reactor fuel channels, including the use and development of advanced fuel channel finite element models. He holds a PhD degree in Civil Engineering from University of Waterloo, Canada, and MSc. in Safety and Reliability Engineering from University of Aberdeen, UK. He has special interest in the study of structural integrity and life extension of engineering components.

Authors:

Sreehari Ramachandra Prabhu Kinectrics Inc.
Dennis Kawa Kedward Kawa and Associates
Douglas Scarth Kinectrics Inc.
Monique Ip Bruce Power L. P.
Shawn Lowe Ontario Power Generation