The Unit 2 nuclear reactor at Bruce Power was shut down over the weekend because of a leak as it was being restarted after a refurbishment.
The Canadian Nuclear Safety Commission says there was no release to the environment because of a moderate leak Saturday, and no plant worker received any dose of radiation.
The unit will remain shut down while the leak is investigated and steps are taken to prevent a recurrence. Other units in the Bruce A station were not affected.
Bruce Power announced Friday that it had been given the go-ahead to complete final safety checks and prepare to put the reactor into operation.
The reactor at the Bruce Power site on Lake Huron has been out of service since 1995.
Work on Unit 1 is still underway, and operations on that reactor should restart during the third quarter of 2012.