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Unit shut down

Learn how Ontario Power Generation and our project partners are safely shutting down each of the four CANDU® reactors at our Darlington Nuclear Generating Station as part of the station’s refurbishment.


Disconnecting one of our Darlington Nuclear Generating Station reactors from Ontario’s power grid, through a process also known as ‘breaker open’, marks the beginning of a reactor refurbishment. But shutting down a reactor is not just a matter of pulling the plug.

Workers must first remove fuel and heavy water from the reactor to ensure it’s separated from the rest of the station.


The first major step in this phase involves using remote-controlled tooling to remove nuclear fuel from the reactor.

Fuel is removed using ‘flow defuelling,’ a process that uses heat transport coolant moving over the nuclear fuel bundles to help push them into the fuelling machine for safe removal.

It takes extraordinary planning and preparation to ensure the defuelling process is completed safely while we continue to operate Darlington’s three other units at full power.

After the reactor is completely defuelled, workers drain the heavy water from the reactor and heat transport system, transferring it to safe storage.


With the fuel and heavy water removed, the unit undergoing refurbishment must be separated from the operating plant through a system of controls and physical barriers – a process known as Islanding.

This allows the unit to be refurbished safely and efficiently while minimizing impact on the operating station.

Islanding includes the installation of 16 bulkhead panels, each weighing in at about 5,443 kg or 12,000 lbs.

Separating the refurbished unit from the rest of the station allows OPG to open both airlock doors safely, enabling workers and parts to pass through them easily and efficiently.

Refurbishment in action

See how OPG is refurbishing the Darlington Station.

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