• Employees gather to celebrate the return of Darlington Unit 3 to service following successful refurbishment.

    Darlington Refurbishment performance update Q3 2023

  • The Unit 3 generator at the Darlington Nuclear Generating Station.

    OPG celebrates the early completion of Darlington Unit 3

  • A worker trains on OPG's full-scale reactor mock-up at Darlington Nuclear.

    OPG achieves two major Darlington Refurbishment milestones

  • A view of the Unit 1 turbine hall at Darlington Nuclear GS. Unit 1 has set a new world record for continuous operation by a nuclear power reactor.

    Project is well into final half of 10-year execution phase

  • Performance update Q3 2023
  • OPG celebrates the early completion of Unit 3
  • OPG achieves two major milestones
  • Project well into final half

Securing decades more clean power for Ontario

In 2016, after years of detailed planning and preparation, Ontario Power Generation's (OPG's) team of project partners, industry experts, energy professionals, and skilled tradespeople successfully shut down the first of four Darlington reactors scheduled for refurbishment over the next 10 years.

Explore how this ambitious initiative took place, and how the Darlington Refurbishment Project continues to progress on time and on budget.

Refurbishment timeline

Project Status: UnderwayStart Date: February 2022Expected Completion: Q2 2025
Project Status: CompleteStart Date: October 2016Expected Completion: June 2020
Project Status: CompleteStart Date: September 2020Expected Completion: July 2023
Project Status: UnderwayStart Date: July 2023Expected Completion: Q4 2026
Shut down
First segment
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.
Refurbishment timeline
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.

Following years of detailed planning, approvals and preparations, the Darlington Refurbishment Project team carries out the first major step in project execution: Shutting down the reactor and disconnecting it from the power grid. This is done through a process also known as ‘breaker open,” which involves cooling the reactor and stopping nuclear fission in accordance with Canadian Nuclear Safety Commission regulations and operational procedures.

Workers at a GE plant in Poland prepare a massive generator stator for shipment to Darlington Nuclear.
Workers at a GE plant in Poland prepare a massive generator stator for shipment to Darlington Nuclear.
Defuelling
Removing fuel and heavy water from the reactor.
Refurbishment timeline
Defuelling
Removing fuel and heavy water from the reactor.

Now that the reactor has been safely shut down and the reactor disconnected, workers use remote-controlled tooling to remove 6,240 fuel bundles from the unit and place them in water-filled fuel bays for up to ten years of safe storage. Once the fuel has been removed, workers drain heavy water from the reactor and the heat transport system, then store, clean and purify it before pumping it back in, after unit reassembly.

Islanding
Safely separating the shutdown reactor from the operating plant.
Refurbishment timeline
Islanding
Safely separating the shutdown reactor from the operating plant.

With the fuel and heavy water removed, the unit undergoing refurbishment must be separated from the operating plant. This is done through a process known as Islanding, whereby workers disconnect equipment and put physical barriers in place.

Islanding allows the unit to be refurbished safely and efficiently while limiting impact on the operating units and rest of the station.

Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.
Refurbishment timeline
Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.

Workers perform a containment pressure test at this stage to confirm that the disconnected unit has been safely and completely isolated from the rest of the operating plant.

This process involves raising the pressure in the unit to ensure that any contamination is contained.

Disassembly
Feeder removal
Removing all 960 feeder tubes from the reactor.
Refurbishment timeline
Feeder removal
Removing all 960 feeder tubes from the reactor.

After opening the airlocks to allow for the free movement of materials and equipment, workers remove 960 feeder tubes from the reactor.

Feeder tubes carry heated heavy water to boilers and return the water back to the reactor for reheating.

Removal of the feeders is the first step in the disassembly of
the unit, and sets the stage for replacement of parts.

Fuel channel removal
Removing all 480 fuel channels from the reactor.
Refurbishment timeline
Fuel channel removal
Removing all 480 fuel channels from the reactor.

Fuel channels are made of several components: a pressure tube, two stainless steel end-fittings and annulus spacers, which separate the pressure tube from the calandria tubes, preventing them from touching.

Once all end fittings have been taken out, workers remove all 480 pressure tubes through a process that involves these of engineered tooling mounted on a massive Re-tube Tooling Platform, inside the vault.

Calandria tube removal
Removing all 480 calandria tubes from the reactor.
Refurbishment timeline
Calandria tube removal
Removing all 480 calandria tubes from the reactor.

A calandria tube is a long cylindrical tube made of zirconium that surrounds the pressure tube and forms a leak tight seal of the calandria vessel – the heart of the reactor.

A Darlington CANDU reactor contains 480 calandria tubes, which are removed using specialized tooling, which workers operate remotely and transport in protective flasks to OPG’s Re-tube Waste Processing Building for storage preparation.

Reassembly
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.
Refurbishment timeline
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.

With all the reactor components removed, workers carefully clean and inspect the calandria vessel to ensure materials show no degradation.

Once this process is complete, workers install 480 new calandria tubes inside the reactor.

Workers complete much of this work manually and directly on the face of the reactor, with direction provided by operators in our Re-tube Control Centre.

After installation, inspection and testing, workers reattach the bellows assemblies, which allow for any movement of pressure tubes and calandria tubes, caused by temperature changes.

Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.
Refurbishment timeline
Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.

Fuel channels are made of several sub-components: a pressure tube, two stainless steel end-fittings and annulus spacers.

The reactor’s 480 fuel channels are critical parts of the heat transport system, which hold the reactor’s fuel bundles.

At this stage, workers pre-assemble the fuel channels in a climate- and foreign material-controlled clean room then prepare them for delivery to the vault for installation.

Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.
Refurbishment timeline
Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.

Each of the reactor’s 960 feeder tubes is unique in shape, size and length, and these new components are delivered in three sections: upper, middle and lower. Because of their unique composition, installing them inside the reactor is a process similar to putting together a puzzle.

Starting with upper section, workers install feeder tubes, making their way down to the lower section, where the feeder tubes are attached to end fittings.

Once this work is complete and regulatory approval received, the project team fills the moderator with clean heavy water and prepares to load the reactor with new fuel.

Power up
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.
Refurbishment timeline
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.

At this stage, workers refill the moderator with heavy water, load each of the 6,240 fuel bundles into the fuel channels, then fill and pressurize the heat transport system.

A fuel bundle contains natural uranium in the form of ceramic pellets.

Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.
Refurbishment timeline
Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.

Unit containment is restored once we’ve removed all tooling, including the Re-Tube Tooling Platform, needed to perform refurbishment; the airlocks have been closed; the physical barriers removed from inside the vault; and the 59 key systems have been returned to service.

Operators in the Re-Tube Control Centre can then begin the process of achieving first criticality, which means sustaining the chain reaction of splitting atoms and releasing heat.

Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.
Refurbishment timeline
Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.

Throughout refurbishment, the Canadian Nuclear Safety Commission carries out inspections and grants approvals for the project to progress from one milestone to the next. At this stage, the CNSC continues this oversight, confirming the newly refurbished unit can be operated safely and granting approval for the reactor to be gradually brought back to full power and reconnected to Ontario’s power grid.

Shut down
Before disassembling a CANDU® reactor, workers must disconnect it from the power grid. But shutting down the unit is not just a matter of pulling a plug. Workers must remove fuel and heavy water, and isolate the unit from the rest of the station.
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.
Refurbishment timeline
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.

Following years of detailed planning, approvals and preparations, the Darlington Refurbishment Project team carries out the first major step in project execution: Shutting down the reactor and disconnecting it from the power grid. This is done through a process also known as ‘breaker open,” which involves cooling the reactor and stopping nuclear fission in accordance with Canadian Nuclear Safety Commission regulations and operational procedures.

Defuelling
Removing fuel and heavy water from the reactor.
Refurbishment timeline
Defuelling
Removing fuel and heavy water from the reactor.

Now that the reactor has been safely shut down and the reactor disconnected, workers use remote-controlled tooling to remove 6,240 fuel bundles from the unit and place them in water-filled fuel bays for up to ten years of safe storage. Once the fuel has been removed, workers drain heavy water from the reactor and the heat transport system, then store, clean and purify it before pumping it back in, after unit reassembly.

Islanding
Safely separating the shutdown reactor from the operating plant.
Refurbishment timeline
Islanding
Safely separating the shutdown reactor from the operating plant.

With the fuel and heavy water removed, the unit undergoing refurbishment must be separated from the operating plant. This is done through a process known as Islanding, whereby workers disconnect equipment and put physical barriers in place.

Islanding allows the unit to be refurbished safely and efficiently while limiting impact on the operating units and rest of the station.

Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.
Refurbishment timeline
Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.

Workers perform a containment pressure test at this stage to confirm that the disconnected unit has been safely and completely isolated from the rest of the operating plant.

This process involves raising the pressure in the unit to ensure that any contamination is contained.

Disassembly
Taking a reactor apart is a complex job that includes removing, storing or replacing thousands of critical components. With our project partners, skilled tradespeople and nuclear professionals, we’re safely disassembling our reactors one at a time.
First segment
Feeder removal
Removing all 960 feeder tubes from the reactor.
Refurbishment timeline
Feeder removal
Removing all 960 feeder tubes from the reactor.

After opening the airlocks to allow for the free movement of materials and equipment, workers remove 960 feeder tubes from the reactor.

Feeder tubes carry heated heavy water to boilers and return the water back to the reactor for reheating.

Removal of the feeders is the first step in the disassembly of
the unit, and sets the stage for replacement of parts.

Fuel channel removal
Removing all 480 fuel channels from the reactor.
Refurbishment timeline
Fuel channel removal
Removing all 480 fuel channels from the reactor.

Fuel channels are made of several components: a pressure tube, two stainless steel end-fittings and annulus spacers, which separate the pressure tube from the calandria tubes, preventing them from touching.

Once all end fittings have been taken out, workers remove all 480 pressure tubes through a process that involves these of engineered tooling mounted on a massive Re-tube Tooling Platform, inside the vault.

First segment
Calandria tube removal
Removing all 480 calandria tubes from the reactor.
Refurbishment timeline
Calandria tube removal
Removing all 480 calandria tubes from the reactor.

A calandria tube is a long cylindrical tube made of zirconium that surrounds the pressure tube and forms a leak tight seal of the calandria vessel – the heart of the reactor.

A Darlington CANDU reactor contains 480 calandria tubes, which are removed using specialized tooling, which workers operate remotely and transport in protective flasks to OPG’s Re-tube Waste Processing Building for storage preparation.

Reassembly
Putting the reactor back together requires quality, precision workmanship on every task. Rebuilding involves inspecting, cleaning and installing thousands of components to ensure the unit’s safe and efficient operation for at least another 30 years.
First segment
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.
Refurbishment timeline
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.

With all the reactor components removed, workers carefully clean and inspect the calandria vessel to ensure materials show no degradation.

Once this process is complete, workers install 480 new calandria tubes inside the reactor.

Workers complete much of this work manually and directly on the face of the reactor, with direction provided by operators in our Re-tube Control Centre.

After installation, inspection and testing, workers reattach the bellows assemblies, which allow for any movement of pressure tubes and calandria tubes, caused by temperature changes.

First segment
Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.
Refurbishment timeline
Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.

Fuel channels are made of several sub-components: a pressure tube, two stainless steel end-fittings and annulus spacers.

The reactor’s 480 fuel channels are critical parts of the heat transport system, which hold the reactor’s fuel bundles.

At this stage, workers pre-assemble the fuel channels in a climate- and foreign material-controlled clean room then prepare them for delivery to the vault for installation.

First segment
Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.
Refurbishment timeline
Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.

Each of the reactor’s 960 feeder tubes is unique in shape, size and length, and these new components are delivered in three sections: upper, middle and lower. Because of their unique composition, installing them inside the reactor is a process similar to putting together a puzzle.

Starting with upper section, workers install feeder tubes, making their way down to the lower section, where the feeder tubes are attached to end fittings.

Once this work is complete and regulatory approval received, the project team fills the moderator with clean heavy water and prepares to load the reactor with new fuel.

Power up
Restarting a reactor is a process that involves a number of step-by-step approvals to confirm the quality of work and to ensure that equipment, systems, operating procedures and trained staff are ready to proceed with the start-up process.
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.
Refurbishment timeline
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.

At this stage, workers refill the moderator with heavy water, load each of the 6,240 fuel bundles into the fuel channels, then fill and pressurize the heat transport system.

A fuel bundle contains natural uranium in the form of ceramic pellets.

Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.
Refurbishment timeline
Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.

Unit containment is restored once we’ve removed all tooling, including the Re-Tube Tooling Platform, needed to perform refurbishment; the airlocks have been closed; the physical barriers removed from inside the vault; and the 59 key systems have been returned to service.

Operators in the Re-Tube Control Centre can then begin the process of achieving first criticality, which means sustaining the chain reaction of splitting atoms and releasing heat.

Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.
Refurbishment timeline
Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.

Throughout refurbishment, the Canadian Nuclear Safety Commission carries out inspections and grants approvals for the project to progress from one milestone to the next. At this stage, the CNSC continues this oversight, confirming the newly refurbished unit can be operated safely and granting approval for the reactor to be gradually brought back to full power and reconnected to Ontario’s power grid.

Shut down
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.
Refurbishment timeline
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.

Following years of detailed planning, approvals and preparations, the Darlington Refurbishment Project team carries out the first major step in project execution: Shutting down the reactor and disconnecting it from the power grid. This is done through a process also known as ‘breaker open,” which involves cooling the reactor and stopping nuclear fission in accordance with Canadian Nuclear Safety Commission regulations and operational procedures.

Defuelling
Removing fuel and heavy water from the reactor.
Refurbishment timeline
Defuelling
Removing fuel and heavy water from the reactor.

Now that the reactor has been safely shut down and the reactor disconnected, workers use remote-controlled tooling to remove 6,240 fuel bundles from the unit and place them in water-filled fuel bays for up to ten years of safe storage. Once the fuel has been removed, workers drain heavy water from the reactor and the heat transport system, then store, clean and purify it before pumping it back in, after unit reassembly.

Islanding
Safely separating the shutdown reactor from the operating plant.
Refurbishment timeline
Islanding
Safely separating the shutdown reactor from the operating plant.

With the fuel and heavy water removed, the unit undergoing refurbishment must be separated from the operating plant. This is done through a process known as Islanding, whereby workers disconnect equipment and put physical barriers in place.

Islanding allows the unit to be refurbished safely and efficiently while limiting impact on the operating units and rest of the station.

Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.
Refurbishment timeline
Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.

Workers perform a containment pressure test at this stage to confirm that the disconnected unit has been safely and completely isolated from the rest of the operating plant.

This process involves raising the pressure in the unit to ensure that any contamination is contained.

Disassembly
First segment
Feeder removal
Removing all 960 feeder tubes from the reactor.
Refurbishment timeline
Feeder removal
Removing all 960 feeder tubes from the reactor.

After opening the airlocks to allow for the free movement of materials and equipment, workers remove 960 feeder tubes from the reactor.

Feeder tubes carry heated heavy water to boilers and return the water back to the reactor for reheating.

Removal of the feeders is the first step in the disassembly of
the unit, and sets the stage for replacement of parts.

Fuel channel removal
Removing all 480 fuel channels from the reactor.
Refurbishment timeline
Fuel channel removal
Removing all 480 fuel channels from the reactor.

Fuel channels are made of several components: a pressure tube, two stainless steel end-fittings and annulus spacers, which separate the pressure tube from the calandria tubes, preventing them from touching.

Once all end fittings have been taken out, workers remove all 480 pressure tubes through a process that involves these of engineered tooling mounted on a massive Re-tube Tooling Platform, inside the vault.

First segment
Calandria tube removal
Removing all 480 calandria tubes from the reactor.
Refurbishment timeline
Calandria tube removal
Removing all 480 calandria tubes from the reactor.

A calandria tube is a long cylindrical tube made of zirconium that surrounds the pressure tube and forms a leak tight seal of the calandria vessel – the heart of the reactor.

A Darlington CANDU reactor contains 480 calandria tubes, which are removed using specialized tooling, which workers operate remotely and transport in protective flasks to OPG’s Re-tube Waste Processing Building for storage preparation.

Reassembly
First segment
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.
Refurbishment timeline
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.

With all the reactor components removed, workers carefully clean and inspect the calandria vessel to ensure materials show no degradation.

Once this process is complete, workers install 480 new calandria tubes inside the reactor.

Workers complete much of this work manually and directly on the face of the reactor, with direction provided by operators in our Re-tube Control Centre.

After installation, inspection and testing, workers reattach the bellows assemblies, which allow for any movement of pressure tubes and calandria tubes, caused by temperature changes.

First segment
Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.
Refurbishment timeline
Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.

Fuel channels are made of several sub-components: a pressure tube, two stainless steel end-fittings and annulus spacers.

The reactor’s 480 fuel channels are critical parts of the heat transport system, which hold the reactor’s fuel bundles.

At this stage, workers pre-assemble the fuel channels in a climate- and foreign material-controlled clean room then prepare them for delivery to the vault for installation.

First segment
Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.
Refurbishment timeline
Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.

Each of the reactor’s 960 feeder tubes is unique in shape, size and length, and these new components are delivered in three sections: upper, middle and lower. Because of their unique composition, installing them inside the reactor is a process similar to putting together a puzzle.

Starting with upper section, workers install feeder tubes, making their way down to the lower section, where the feeder tubes are attached to end fittings.

Once this work is complete and regulatory approval received, the project team fills the moderator with clean heavy water and prepares to load the reactor with new fuel.

Power Up
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.
Refurbishment timeline
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.

At this stage, workers refill the moderator with heavy water, load each of the 6,240 fuel bundles into the fuel channels, then fill and pressurize the heat transport system.

A fuel bundle contains natural uranium in the form of ceramic pellets.

Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.
Refurbishment timeline
Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.

Unit containment is restored once we’ve removed all tooling, including the Re-Tube Tooling Platform, needed to perform refurbishment; the airlocks have been closed; the physical barriers removed from inside the vault; and the 59 key systems have been returned to service.

Operators in the Re-Tube Control Centre can then begin the process of achieving first criticality, which means sustaining the chain reaction of splitting atoms and releasing heat.

Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.
Refurbishment timeline
Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.

Throughout refurbishment, the Canadian Nuclear Safety Commission carries out inspections and grants approvals for the project to progress from one milestone to the next. At this stage, the CNSC continues this oversight, confirming the newly refurbished unit can be operated safely and granting approval for the reactor to be gradually brought back to full power and reconnected to Ontario’s power grid.

Shut down
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.
Refurbishment timeline
Breaker open
Shutting down the reactor and disconnecting it from Ontario's power grid.

Following years of detailed planning, approvals and preparations, the Darlington Refurbishment Project team carries out the first major step in project execution: Shutting down the reactor and disconnecting it from the power grid. This is done through a process also known as ‘breaker open,” which involves cooling the reactor and stopping nuclear fission in accordance with Canadian Nuclear Safety Commission regulations and operational procedures.

Defuelling
Removing fuel and heavy water from the reactor.
Refurbishment timeline
Defuelling
Removing fuel and heavy water from the reactor.

Now that the reactor has been safely shut down and the reactor disconnected, workers use remote-controlled tooling to remove 6,240 fuel bundles from the unit and place them in water-filled fuel bays for up to ten years of safe storage. Once the fuel has been removed, workers drain heavy water from the reactor and the heat transport system, then store, clean and purify it before pumping it back in, after unit reassembly.

Islanding
Safely separating the shutdown reactor from the operating plant.
Refurbishment timeline
Islanding
Safely separating the shutdown reactor from the operating plant.

With the fuel and heavy water removed, the unit undergoing refurbishment must be separated from the operating plant. This is done through a process known as Islanding, whereby workers disconnect equipment and put physical barriers in place.

Islanding allows the unit to be refurbished safely and efficiently while limiting impact on the operating units and rest of the station.

Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.
Refurbishment timeline
Containment pressure testing
Confirming the safe separation of the shutdown unit from the operating plant.

Workers perform a containment pressure test at this stage to confirm that the disconnected unit has been safely and completely isolated from the rest of the operating plant.

This process involves raising the pressure in the unit to ensure that any contamination is contained.

Disassembly
Feeder removal
Removing all 960 feeder tubes from the reactor.
Refurbishment timeline
Feeder removal
Removing all 960 feeder tubes from the reactor.

After opening the airlocks to allow for the free movement of materials and equipment, workers remove 960 feeder tubes from the reactor.

Feeder tubes carry heated heavy water to boilers and return the water back to the reactor for reheating.

Removal of the feeders is the first step in the disassembly of
the unit, and sets the stage for replacement of parts.

Endfitting removal
Removing all 960 endfittings from the reactor fuel channels.
Refurbishment timeline
Endfitting removal
Removing all 960 endfittings from the reactor fuel channels.

Each of the 480 fuel channels are made of the following components: two stainless steel endfittings on each side of the reactor vessel, an 8-metre pressure tube running the full length inside the reactor, and several annulus spacers wrapped around each pressure, tube preventing contact with the surrounding calandria tube.

The 960 endfittings are carefully severed at the face of the reactor, transported to a special reactor component processing facility and stored in shielded containers at a secure onsite storage building.

Calandria tube (CT) and Pressure tube (PT) removal
Simultaneously remove all 480 CT-PT from the reactor; final step of fuel channel removal.
Refurbishment timeline
Calandria tube (CT) and Pressure tube (PT) removal
Simultaneously remove all 480 CT-PT from the reactor; final step of fuel channel removal.

A calandria tube is a long cylindrical tube made of zirconium that surrounds the pressure tube and forms a leak tight seal of the calandria vessel – the heart of the reactor.

Once all endfittings have been taken out, workers remove the 480 calandria tube-pressure tube pairs together through a process that involves unique engineered and remotely-operated tooling mounted on a massive Retube Tooling Platform inside the vault, and then placed into protective flasks and transport in protective flasks to OPG’s Re-tube Waste Processing Building for storage preparation.

Reassembly
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.
Refurbishment timeline
Calandria tube installation
Inspecting and cleaning the calandria vessel then installing all 480 calandria tubes inside the reactor.

With all the reactor components removed, workers carefully clean and inspect the calandria vessel to ensure materials show no degradation.

Once this process is complete, workers install 480 new calandria tubes inside the reactor.

Workers complete much of this work manually and directly on the face of the reactor, with direction provided by operators in our Re-tube Control Centre.

After installation, inspection and testing, workers reattach the bellows assemblies, which allow for any movement of pressure tubes and calandria tubes, caused by temperature changes.

Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.
Refurbishment timeline
Fuel channel installation
Assembling 480 fuel channels in clean rooms then installing them inside the reactor.

Fuel channels are made of several sub-components: a pressure tube, two stainless steel end-fittings and annulus spacers.

The reactor’s 480 fuel channels are critical parts of the heat transport system, which hold the reactor’s fuel bundles.

At this stage, workers pre-assemble the fuel channels in a climate- and foreign material-controlled clean room then prepare them for delivery to the vault for installation.

Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.
Refurbishment timeline
Feeder installation
Installing all 960 feeder tubes prior to filling the moderator system.

Each of the reactor’s 960 feeder tubes is unique in shape, size and length, and these new components are delivered in three sections: upper, middle and lower. Because of their unique composition, installing them inside the reactor is a process similar to putting together a puzzle.

Starting with upper section, workers install feeder tubes, making their way down to the lower section, where the feeder tubes are attached to end fittings.

Once this work is complete and regulatory approval received, the project team fills the moderator with clean heavy water and prepares to load the reactor with new fuel.

Power Up
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.
Refurbishment timeline
Fuel load
Loading 6,240 fuel bundles into the reactor’s fuel channels.

At this stage, workers refill the moderator with heavy water, load each of the 6,240 fuel bundles into the fuel channels, then fill and pressurize the heat transport system.

A fuel bundle contains natural uranium in the form of ceramic pellets.

Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.
Refurbishment timeline
Containment restored
Confirming the containment of the unit has been restored then removing tooling and physical barriers.

Unit containment is restored once we’ve removed all tooling, including the Re-Tube Tooling Platform, needed to perform refurbishment; the airlocks have been closed; the physical barriers removed from inside the vault; and the 59 key systems have been returned to service.

Operators in the Re-Tube Control Centre can then begin the process of achieving first criticality, which means sustaining the chain reaction of splitting atoms and releasing heat.

Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.
Refurbishment timeline
Breaker close
Reconnecting the reactor to Ontario’s power grid following rounds of inspections and approvals.

Throughout refurbishment, the Canadian Nuclear Safety Commission carries out inspections and grants approvals for the project to progress from one milestone to the next. At this stage, the CNSC continues this oversight, confirming the newly refurbished unit can be operated safely and granting approval for the reactor to be gradually brought back to full power and reconnected to Ontario’s power grid.

A smart investment

According to an independent report by the Conference Board of Canada, the Darlington Refurbishment Project and the subsequent 30 more years of station operation, are expected to generate a total of $89.9 billion in economic benefits for Ontario, create 14,200 jobs per year, and boost personal income by an average of $1.6 billion on an annual basis.

With 96% of project costs spent in the province and a heavy reliance on Ontario-based contractors, for every $1 spent on the project, Ontario’s GDP will increase by an average of $1.40.

In addition to playing a significant role in strengthening Ontario’s economy, an independent report prepared by Intrinsik Environmental Sciences, noted that the continued operation of Darlington Nuclear to 2055 will take the equivalent of two million cars off Ontario’s roads per year by avoiding significant greenhouse gas emissions. This is an important step in Ontario and Canada’s fight against climate change.

Frequently asked questions

Find answers to some of the most frequently asked questions about our Darlington Refurbishment project.

Refurbishment involves replacing core reactor components to enable the plant to operate safely for at least another 30 years. Each reactor is taken out of service for about three years to allow for:

  • Replacement of fuel channels, feeder pipes, calandria tubes and end fittings;
  • Rehabilitation of steam generators, turbine generators and fuel handling equipment; and
  • System improvements and plant upgrades to meet current regulatory requirements

The cost of refurbishment is $12.8 billion including interest and escalation.

Clean, reliable, low-cost energy for Ontario – the Darlington Refurbishment will generate $14.9 billion in economic benefits to Ontario, and a total of $89.9 billion when calculating in at least another 30 more years of station operations, according to “Continued Operation of the Darlington Nuclear Generating Station: An Impact Analysis on Ontario’s Economy” (2016) an independent report by the Conference Board of Canada.

The project will also generate thousands of jobs in Darlington and at many of  Ontario’s Nuclear Supply Chain companies supplying services, components and manpower for the work. The investment also preserves about 3,000 jobs at the station as it continues providing clean, reliable, base load power for another 30 years at a cost lower than other alternatives considered.

Darlington Refurbishment is one of the largest clean energy projects in Canada. In 2026, Intrinsik Environmental Sciences released “Greenhouse Gas Emissions Associated with Various Methods of Power Generation in Ontario” (October 2016) stating that the total reduction in greenhouse gas (GHG) emissions from 2024 to 2055 associated with the continued operation of Darlington following refurbishment is estimated to be 297 megatonnes of carbon dioxide, or the equivalent of taking two million cars off the road.

In addition to 30 more years of employment at Darlington to operate and maintain the station, the refurbishment project is expected to provide an additional 2,000 direct jobs annually (primarily contractors) and thousands of additional indirect and induced jobs over the duration of the project. Various skilled trades will be required for this work. In the initial stages of the project, the needed skills include:

  • Boilermakers
  • Construction millwrights and industrial mechanics
  • Crane operators
  • Electricians
  • Carpenters
  • Heavy equipment mechanics and operators
  • Insulators
  • Ironworkers
  • Steam, pipe and gas fitters
  • Trades helpers and labourers; and
  • Welders.

In the later stages of the project, skilled trades and occupations from all sectors will be needed, such as carpenters, construction managers, contractors and contract supervisors, electricians, plumbers and sheet metal workers.

All contracts issued for Darlington refurbishment also include a provision for apprenticeship opportunities. OPG has set targets to achieve 20% apprenticeships for skilled trades working on the project.

We expect the majority of these workers will be resourced from across Durham Region and the Greater Toronto Area, but we are relying on trades to from across Ontario to supplement the project's high demand for resources. In Ontario, we’re lucky to have skilled, experienced trade workers in the nuclear industry but as the project progresses, we anticipate that the demand for this talent will rise.

OPG will establish financing as part of its work program. Financing for the initiation and execution preparation phases has been funded from OPG’s general operations.

The province of Ontario is responsible for long-term energy plans and for establishing the appropriate electricity supply mix. The province has determined that a mix of options is the best approach for ensuring a safe, secure and reasonably-priced supply of electricity for Ontarians. Nuclear energy is a base load generation source, designed to operate continuously and provide the foundation for a stable, clean and secure supply mix.

In November 2017, the Financial Accountability Office released a report, “An Assessment of the Financial Risks of the Nuclear Refurbishment Plan,” which concludes, “There are no alternative scenarios that are comparable to refurbished nuclear generation in terms of both cost and emissions.”

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