Ontario Power Generation (OPG) said on 8 November that refurbishment of its Darlington NPP had achieved another milestone with completion of calandria tube installation at unit 2. This prepares the way for bellows replacement followed by fuel channel installation. Work to replace two bellows assemblies began 29 October. Bellows assemblies, which are made of nickel-chromium alloy strips configured with 14 convolutions, provide flexibility between the fuel channel and the reactor end shields to reduce movement due to thermal changes as well as seal the space between the calandria tube and pressure tube, which houses the nuclear fuel bundles.
Refurbishment of Darlington 2 formally began in October 2016 after six years of planning and is expected to take 40 months. Removal and replacement of the reactor's 480 calandria tubes, 480 pressure tubes and 960 feeder pipes are the most significant project tasks.
All four Candu units at Darlington are being refurbished as part of a phased CAD12.8bn ($9bn) project scheduled for completion by 2026. The calandria tube installation at unit 2 involved teams of boilermakers and millwrights, and was the first series where much of the work was completed manually on the face of the reactor, with procedural direction supplied by the Re-tube Control Centre. The installation process involved installing the calandria tube into a lattice site, verifying correct positioning on both reactor faces, then rolling the calandria tube, finishing with a leak test.
“This was a crucial work evolution, leaving us with a pipe-tight reactor on the moderator side, which now allows us to remove the moderator dry systems, opening up a lot of important new work, as we march forward on the road toward filling the moderator in January,” said Mike Allen, OPG Senior Vice President, Nuclear Refurbishment. Getting to the point where workers were no longer required to wear plastic suits was one key to success of the series, said CanAtom superintendent Brian Flynn. To accomplish this, the vault underwent a deep clean by CanAtom labourers, with support from OPG. When post-clean surveys indicated it was safe, workers were able to proceed without plastic suits, increased productivity and reduced potential radiation exposure.
Darlington 3 will be the second unit to undergo refurbishment. The plans require refurbishment of Darlington 2 to be completed before work starts on Darlington 3 to benefit from “lessons learned”. Work on Darlington 3 is scheduled to begin in February 2020, on Darlington 1 in July 2021 and Darlington 4 in January 2023. The project will enable the 3512MWe plant to continue operating until 2055. Each unit is being taken out of service for three years to allow the work to be carried out. Between 2016 and 2033, ten Candu units will be refurbished – four at Darlington and six at Bruce NPP. The Pickering nuclear plant will continue to operate until 2024 to provide baseload electricity during the Darlington and Bruce refurbishments.