Application of the leak before break concept to CANDU feeder piping with service induced cracking

Abstract of the technical paper presented at:
20th International Conference Structural Mechanics in Reactor Technology
(SMiRT 20- Division 2, Paper 1854)
Espoo, Finland
August 9–14, 2009

Prepared by:
John C. Jin and Raoul Awad
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

It has been claimed that leak before break (LBB) is the failure mode for CANDU feeder piping experiencing service induced intergranular cracking at Point Lepreau Nuclear Generating Station (PLGS) and Gentilly-2 (G-2) operating in Canada. The feeders at PLGS will be replaced in the latter part of 2009 as part of the refurbishment program for plant life extension and G-2 is also scheduled to undergo refurbishment in 2011.  No such cracking has been discovered in the feeders at other CANDU stations. However, due to the limited understanding of the cracking mechanism, particularly the root causes, it is currently not possible to preclude the susceptibility to the same cracking mechanism of the feeders at other CANDU stations which have similar manufacturing history and operating conditions to those of PLGS and G-2. Although the root cause is not yet confirmed, it is generally accepted that tensile residual stress plays a critical role in initiating and propagating the service induced cracks. Accordingly, cracking should be considered credible at the feeders possessing sufficient levels of residual tensile stresses.  Once service induced cracking becomes an active degradation mechanism and there are uncertainties in the assurance of prevention of through wall cracking during operation based on crack growth rate estimation and detection limits of the in-service inspection technique, then LBB should be demonstrated as one of the elements supporting the fitness for continued service of the entire feeder population. Through the LBB assessment, it should be confirmed that a feeder rupture is an unlikely event even when cracks are not identified during an expanded inspection program scope and those remaining cracks would grow through wall during the next operating interval.  

It is the regulatory view in Canada that the procedures for demonstrating LBB to support the continued operation of degraded components should be developed in a different manner from those used for conventional LBB which is used for the elimination of pipe whip restraints.  It should incorporate actual characteristics of the degradation mechanisms, such as crack morphology, aspect ratio, material properties and so on. Particularly, demonstrating that there is a procedure to ensure actions are taken in a timely and safe manner would be the most important element. The safety margins adopted in the leak rate assessment and margin to crack stability should be subjected to regulatory review and acceptance.

A detailed regulatory perspective on the application of LBB to CANDU feeder pipes with susceptible service induced cracking is discussed in this paper.

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