Bruce Power has received permission from the Canadian nuclear regulator to restart unit 2 of the Bruce A nuclear power plant in Ontario, which has been idle for almost 20 years. However, the detection of a leak during start-up of the refurbished Candu reactor resulted in it being returned to a shut down state.
Candu reactors are designed to undergo refurbishment after approximately 25 years of operation, requiring a major outage but allowing reactor life to be extended by up to 30 years. At the Bruce A units, the retubing of unit 2 has been completed and regulatory approval was given in June 2011 to start reloading fuel into the reactor. Refurbishment work of unit 1 is also nearing completion.
|The four-unit Bruce A plant (Image; Bruce Power)
Following a review of submissions from Bruce Power and several inspections at the plant, the Canadian Nuclear Safety Commission (CNSC) concluded that the company had "met all of the conditions related to releasing reactor shutdown guarantees in unit 2."
Reactor shutdown guarantees consist of different measures to make criticality within the reactor impossible while the unit is out of service. In a Candu plant, a large amount of gadolinium is typically injected into the heavy water moderator contained within the calandria. The gadolinium acts as a 'poison', preventing fission occurring. The gadolinium can subsequently be removed by using the reactor's moderator purification ion exchange columns. Other measures include closing and locking the valves into the moderator purification system and controlling the moderator's chemistry.
Ramzi Jammal, executive vice-president and chief regulatory operations officer at the CNSC, said: "It's with a high degree of confidence today that the CNSC is authorizing the safe restart of unit 2 at the Bruce A nuclear generating station. Our assessments have provided us with the assurances that the operator has taken all necessary measures to protect workers, the public and the environment."
The announcement of the CNSC's decision on 16 March meant that Bruce Power could start removing those shutdown guarantees and progress with the remaining return-to-service activities, including final safety checks in preparation for synchronization to Ontario's electricity grid. Upon restart, the unit may increase power up to 50% of its capacity. Additional approval is required from the CNSC to raise reactor power beyond that level.
Bruce Power informed the CNSC on 17 March that a moderator leak had been discovered during restart operations at the unit. The regulator reported that "Bruce Power staff took the appropriate action and returned the unit to the shut down state while they investigated the cause." The leak was found to be inside confinement and has been isolated, it noted. There was no release to the environment and no worker received any dose, according to the CNSC.
Bruce Power resumed restart operations at unit 2 of the Bruce A plant in Ontario on 20 March after replacing a defective gasket in the moderator system which had caused a minor leak.
Bruce A unit 2 will now remain shut down while further investigations are conducted into the leak and steps are taken to prevent recurrence.
"A project of this magnitude has never been done before on a Candu reactor and that cannot be overlooked," said Bruce Power president and CEO Duncan Hawthorne. "We have learned many lessons from our work on unit 2 and have implemented them on unit 1, which is following very closely behind unit 2 and should achieve a similar milestone in a few short months."
TransCanada Corporation, which owns 49% of Bruce A and 32% of Bruce B, said that the restart authorization "effectively ends the construction and commissioning phases of the project." The company said that it expects Bruce A unit 2 to start commercial operations in the second quarter of 2012, while unit 1, which is also being refurbished, is expected to begin operations in the third quarter. It is not yet known how the leak will effect unit 2's restart schedule.
Units 1 and 2 at the four-unit Bruce A plant started up in 1977, but unit 2 was shut down in 1995 because a steam generator suffered corrosion after a lead shielding blanket used during maintenance was mistakenly left inside. In the late 1990s then-owner Ontario Hydro decided to lay up all four units at the plant to concentrate resources on other reactors in its fleet, and unit 1 was taken out of service in December 1997 with units 3 and 4 in following in 1998. The four units at sister power station Bruce B continued to operate. Bruce Power took over the operations of both Bruce plants from Ontario Hydro in 2001 and restarted units 3 and 4 by early 2004. Bruce A units 3 and 4 are likely to undergo a similar refurbishment once units 1 and 2 are back in operation.
The decision to refurbish the units followed a 2005 agreement by Bruce Power and the government of Ontario to refurbish the two 769 MWe Candu reactors as a faster option than building new ones in the face of impending power shortages.
Researched and written
by World Nuclear News