Regulatory Perspective on CANDU Feeder Lifecycle Management

An abstract of the technical paper presented at:
PHWR Safety 2014 / CANSAS-2014
Ottawa, Ontario
June 23–26, 2014

Prepared by:
S. Liu, S. Eom and J. Jin
Canadian Nuclear Safety Commission
Ottawa, Ontario, Canada

Abstract

This paper provides an overview of the operating experience for CANDU feeder piping, including major forms of degradation and the strategies to manage them. The regulatory perspective to ensure feeder integrity is discussed, based on operating experience.

CANDU feeders are made of carbon steel piping, which is an integral part of the primary heat transport system. A major form of degradation is outlet feeder wall thinning, induced by flow-accelerated corrosion (FAC), which affects all CANDU reactors. Strategies to manage feeder thinning include primary water chemistry control, inspections, dispositions (in according to applicable codes and guidelines), and feeder replacement. Feeder thinning due to FAC can be predicted by monitoring feeder-thinning rates during inspections. 

Another form of degradation is feeder pipe cracking, which has happened in some CANDU reactors. Cracking was observed on a small number of tight-radius bends and one repaired weld on outlet feeders. Several mechanisms were proposed but none have been clearly confirmed. However, the investigation identified several key factors, such as cold work, high residual stress and high operating temperature, which increase cracking susceptibility. The strategies to manage outlet feeder cracking include inspections at high risk areas and enhancement in leak monitoring. In the Canadian regulator’s view, the feeder section with confirmed service-induced cracks must be replaced.

The potential risk of primary water stress corrosion cracking of dissimilar metal welds (DMWs) between carbon steel feeder pipes and Alloy 600 flow devices is an emerging issue for some CANDU reactors with the DMWs located at outlet feeders, because of the high operating temperatures at that location. Licensees have found that the volumetric inspection on the DMWs in accordance with the requirements of CSA N285.4 is difficult to perform because of high dose rates and access limitations. Instead of performing the inspections, licensees have proposed to perform leak-before-break (LBB) assessments as an alternative approach. If LBB is to be credited to exempt inspections as required by CSA N285.4, the LBB assessment shall incorporate the characteristics of the degradation mechanisms, such as crack morphology, aspect ratio, material properties, etc. Safety margins on the leak rate and crack size shall be sufficient. The LBB assessment is subject to the Canadian regulator’s review and acceptance.

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