Session: MF-01-01 Application of Fracture Mechanics in Failure Assessment

Paper Number: 123315

123315 - Delayed Hydride Cracking Initiation Evaluation of a Fretting Flaw in a Candu Reactor Zr-Nb Pressure Tube With Consideration of Finite Length of the Flaw and Flaw Shielding Effect 

In-service evaluation of a flaw that is detected in a Zr-Nb alloy pressure tube in a CANada Deuterium Uranium (CANDU) nuclear reactor includes an analysis to demonstrate that delayed hydride cracking will not initiate from the flaw.  The flaw can be assessed either as planar or volumetric with a blunt root radius. A flaw caused by fretting due to flow-induced vibration was detected by ultrasonic examination in a Zr-Nb alloy pressure tube in a CANDU nuclear reactor.  The flaw was replicated to obtain details of flaw geometry.  The flaw is volumetric in nature, and is comprised of two axially oriented features that were joined by a ligament.  This flaw was evaluated for delayed hydride cracking initiation in accordance with the Canadian Standards Association Standard N285.8.  Two-dimensional elastic finite element stress analysis of the flaw was performed.  An adjustment factor on the applied stress that took into account the finite axial length of the flaw was used to produce a lower and more representative peak flaw-tip stress as compared with an assumed “infinite” length of the flaw in the two-dimensional finite element stress analysis.  The proximity of the two features of the flaw generated a stress shielding effect which was also accounted for in the evaluation for delayed hydride cracking initiation. The flaw met the acceptance criteria for prevention of delayed hydride cracking initiation, and the pressure tube was returned to service.  This paper provides a summary of the delayed hydride cracking initiation evaluation of this flaw, and highlights the methods adopted to reduce conservatism in the evaluation.

Presenting Author: Douglas Scarth Kinectrics Inc.

Presenting Author Biography: Doug Scarth is a Ph.D. in Materials Science from the University of Manchester (2002). Currently employed with Kinectrics Inc. in Toronto, Canada as a Technical Director for Fracture Programs. Performed structural integrity evaluations of nuclear and fossil plant components containing flaws.  Involved in development of improved methodologies for evaluating flaws.  Participated in development of engineering codes and standards for fitness for service assessments of plant components, including activities of Canadian Standards Association N285B Technical Committee, and Section XI of the ASME Boiler and Pressure Vessel Code. Involved in ASME Pressure Vessels and Piping Division. Doug Scarth is a ASME Fellow.

Authors:

Douglas Scarth Kinectrics Inc.
Steven Xu Kinectrics Inc.
Preeti Doddihal Kinectrics
Dennis Kawa Kedward Kawa & Associates Ltd.
Monique Ip Bruce Power