A large cylindrical structure, it is connected to the generating station by a pressure relief duct and kept at negative atmospheric pressure to any release of radioactive steam can be sucked into the vacuum building.

The Canadian Nuclear Safety Commission requires all CANDU operators to conduct a thorough examination of the structure every 12 years.

CANDU reactors are Canadian-designed pressurized heavy water power reactors that use heavy water (deuterium oxide) for moderator and coolant, and natural uranium for fuel.

Since the vacuum building is a shared safety system, the company will keep all four Bruce B units 5-8 offline during the inspection period. Unit 6, however, shut on Sept. 11 for its biannual inspection program.

Bruce Power expects Units 3 and 4 at the Bruce A to remain at high power throughout the Bruce B vacuum building inspection.

One MW powers about 1,000 homes, according to the North American average.

The 6,660 MW Bruce station is located in Tiverton, on the shores of Lake Huron, about 155 miles (249 km) northwest of Toronto. There are four 825 MW A units and 840 MW B units at Bruce. Units 1 and 2, built in the late 1970s, have not operated since the end of 1999 because they needed extensive upgrades.

The company is conducting a feasibility study to determine whether it makes sense to return units 1 and 2 to service, which could cost hundreds of millions of dollars. The decision to restore the units depends, in part, on changes the Ontario government may make to energy sector regulations.

Bruce Power, one of Ontario's largest power generators, is owned by Cameco Corp. (CCO.TO: Quote, Profile, Research) (31.6 percent), TransCanada Corp. (TRP.TO: Quote, Profile, Research) (31.6 percent), BPC Generation Infrastructure Trust, established by the Ontario Municipal Employees Retirement System (31.6 percent), the Power Workers' Union (4 percent) and the Society of Energy Professionals (1.2 percent).