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Unit reassembly

Putting a reactor back together again is an intricate process requiring precision and quality workmanship at every turn.

Explore how, over the course of several months, Ontario Power Generation’s Darlington Refurbishment team will rebuild 58 interconnected systems in sequence to restore a single nuclear reactor unit to full-power operation.


After inspecting the inside of the calandria, workers move on to ensure the holes (or bores) in the tube sheets, through which fuel channels are fed and secured, are free of any minor imperfections that would prevent a proper seal.

Unlike during unit disassembly, workers perform most of this work on the reactor face directly rather than by remote control.

Calandria tube installation

Workers next install the calandria tubes and at this time, they also replace any bellows assemblies that need to be changed. The bellows, made of nickel-chromium alloy strips, help reduce horizontal movement between the fuel channel and the reactor end shields caused by temperature changes, and seal the space between the pressure and calandria tubes.

Fuel channel assembly

Fuel channels have a critical role in the nuclear fission process and the heat transport system, by containing and supporting fuel bundles in the reactor.

A fuel channel is made of:

  • a zirconium pressure tube,
  • two stainless steel end fittings, and
  • four annulus spacers.

Annulus spacers play a crucial role in maintaining the necessary gap between the pressure tube and calandria tube.

Fuel channels are assembled in a climate and foreign material-controlled clean room, then serialized into kits specific to their lattice site location (reactor face bore) for delivery to the vault.

Feeder replacement

Workers then replace the feeders, which carry the primary coolant required to cool nuclear fuel and transfer the heat in large boilers to create high-pressure steam.

There are 960 upper and 960 lower feeders, which follow complex pathways to the header they’re welded to. The longest feeders require an additional middle section or they wouldn’t fit inside the vault.

With all of these components replaced, the final steps involve filling the moderator and loading new fuel into the reactor.

Refurbishment in action

Watch how OPG is refurbishing the Darlington station.