CONTRIBUTING FACTORS OF CANDU GRAYLOC FITTING LEAK

抄録

Grayloc connections are part of CANDU Primary Heat Transport System (PHTS) and connect the inlet and outlet feeders at the End Fittings (EF) to the Fuel Channels (FC). In each CANDU reactor, the number of Grayloc hubs is in the range of 760 to 960. Grayloc leak has been a lingering operational issue in past decades, causing unacceptable amount of forced or extended planned outages.

Following multiple Grayloc leaks at Pickering Nuclear Generation Station (PNGS) since 2019, extensive investigations [1, 2, 3] have been conducted to find root causes or contributing factors to the leaks. In this paper, major potential contributing factors are identified, reviewed and discussed.

In the end, no single/sole contributing factor is identified in causing recent PNGS Grayloc leaks. Several major factors or more precisely - their combined effects were recognized for the recent leaks: weak original Grayloc design; local Flow Accelerated Corrosion (FAC) near the hub/seal ring contact surface to initiate the potential breakthrough leak path via steam cutting; hubs with improper installed Grafoil tape; higher feeder load causing accelerated stress corrosion; the extent of local FAC effect increasing with service years. Impact ranking of potential contributing factors is also given in the paper.