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Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5 Nb Pressure Tube Material

[+] Author Affiliations
Jun Cui, Gordon K. Shek, Douglas A. Scarth

Kinectrics, Inc., Toronto, Ontario, Canada

William K. Lee

Ontario Power Generation, Pickering, Ontario, Canada

Paper No. PVP2004-2301, pp. 53-65; 13 pages
doi:10.1115/PVP2004-2301
From:
  • ASME/JSME 2004 Pressure Vessels and Piping Conference
  • Fracture Methodologies and Manufacturing Processes
  • San Diego, California, USA, July 25–29, 2004
  • Conference Sponsors: Pressure Vessels and Piping Division
  • ISBN: 0-7918-4669-5
  • Copyright © 2004 by ASME

abstract

Flaws in Zr-2.5 Nb alloy pressure tubes of CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism called Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. The presence of small surface irregularities, or secondary flaws, at the bottom of service-induced fretting flaws in pressure tubes requires an integrity assessment in terms of DHC initiation. Experimental data and analytical modeling are required to predict whether DHC initiation can occur from the secondary flaws. In the present work, an experimental program was carried out to examine the impact of small secondary flaws with sharp radii on DHC initiation from simulated fretting flaws. Groups of cantilever beam specimens containing blunt notches with and without secondary flaws were prepared from unirradiated pressure tube materials hydrided to a nominal concentration of 50 wt ppm hydrogen. The specimens were subjected to multiple thermal cycles to form hydrides at the flaw-tip at different applied stress levels, which straddled the threshold value for DHC initiation. The threshold conditions for DHC initiation were established for different simulated fretting and secondary flaws. The experimental results are compared with predictions from the engineering process-zone DHC initiation model.

Copyright © 2004 by ASME

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