Bruce A Unit 2 restart activities resuming: minor moderator system repair complete
March 20, 2012
Late Monday afternoon, March 19, Bruce Power began to restart Unit 2 at the Bruce A Nuclear Generating Station following minor repair. Unit 2 had been returned to a guaranteed shut down state on Saturday, March 17, after the detection of a leak due to a defective gasket in the moderator system. The leak was entirely contained and the gasket replaced. There was no release to the environment and no plant worker received any dose as a result of the event. The other units in the station were not affected.
CNSC on-site staff monitored the investigation carried out by Bruce Power, reviewed and accepted its conclusions, and are satisfied with the actions taken to prevent recurrence. As planned, CNSC staff were in the Bruce Power main control room to monitor the resumption of restart activities. This minor incident does not affect last week’s decision by the CNSC to authorize the removal of shutdown guarantees and allow Bruce Power to proceed with the restart of the recently refurbished Unit 2.
About the moderator system
In CANDU stations, a cylindrical tank called calandria contains the moderator (heavy water) and the pressure tubes in which the nuclear fuel is inserted. The heavy water in the calandria is used to slow down neutrons to sustain nuclear fission. Heavy water is also used in the primary heat transport system of CANDU stations, bringing heat produced by the fuel to the steam generators. The moderator and primary heat transport systems are separate and remain at different temperatures when the reactor is operating.