The vacuum building design is a unique safety feature of CANDU reactors. It is designed to prevent the release of radioactive material to the environment in the event of an accident.
The building itself is enclosed in nearly 1.2 metre-thick (four feet) concrete walls and maintained at negative pressure or vacuum. Each of the station’s reactors, which are also enclosed in their own concrete buildings, is connected to the vacuum building by a large duct.
In the event of a large leak in the reactor cooling system, radioactive steam and water would be sucked into the vacuum building and condensed. It would then be cooled by water sprays from the dousing tank located at the top of the vacuum building. The douse tank contains 12 million litres of water. High-efficiency filters and charcoal absorbers would remove 99.9% of the radiation.