All four of the Candu units that make up the Bruce A generating station in Canada have been taken offline for periodic inspection and maintenance work on the plant's vacuum building in a once-in-12-years outage that has been years in the planning.

Bruce A's vacuum building (Image: Bruce Power)

Vacuum buildings are unique to multi-unit CANDU generating stations and are part of their robust safety systems. The structure is specifically designed to quickly and safely lower pressure inside the reactor building in the case of an accident, by releasing steam and hot gases from the reactor building into the vacuum building. This provides an additional protective barrier to prevent the release of radioactivity to the environment.

According to the Canadian Nuclear Safety Commission (CNSC), the system works without powerand is tested periodically under the supervision of CNSC inspectors.

A vacuum building outage - or VBO - takes place every 12 years and requires all four of the units to be taken offline. As all of the units need to be taken out of service for this work, VBOs are scheduled during periods of the year when demand is expected to be low, usually the spring or autumn. The timing of the outage means the units will be operational for the summer peak period.

"With four operating units out of service, this is not a typical outage campaign," Bruce A Vice President Frank Payne said. "Years of planning and preparation goes into our VBO outages and we are ready to safely carry out our maintenance and inspection work and successfully return these units to service."

Bruce Power plans to install an innovative new filtration system during the outage, which it said will provide an extra layer of safety and protection. The new system is expected to be fully commissioned and available by the end of 2022.

The four units that make up Bruce A - Bruce units 1-4 - were placed in long-term shutdown in the 1990s, returning to commercial operation in 2012. Units 1 and 2 were refurbished prior to their return to operation; units 3 and 4 are scheduled to undergo refurbishment as part of Bruce's ongoing Major Component Replacement, or MCR project, with work scheduled to begin on unit 3 in 2023 and unit 4 in 2025. The MCR will add about 30 to 35 years of operational life to each reactor, and is part of Bruce Power's Life-Extension Program which began in 2016.

Researched and written by World Nuclear News