A nuclear reactor in Canada that’s been out of service for almost two decades shut after a leak as it was being restarted, said the operator of the world’s second- largest plant.

The 750-megawatt Unit 2 at the Bruce Power complex near Kincardine on Lake Huron, about 250 kilometers northwest of Toronto, closed March 17 as it was being returned to service following refurbishment, said John Peevers, a Bruce spokesman at the plant. The leak was heavy water, which is used to slow nuclear reactions, he said.

“Staff safely and quickly isolated the leak,”Peevers said in an e-mailed response to questions. “Repairs are in progress and will take a few days to complete.”

There was no release to the environment and no plant worker received any contamination, the Canadian Nuclear Safety Commission said in a statement posted on its website.

The unit, one of eight at the plant which has a total capacity of 6,300 megawatts, has been shut since 1995 for renovations to its Candu reactor design, Peevers said. Unit 1 is scheduled to be restarted after similar refurbishment in the third quarter, Peevers said.

Bruce Power, based in Tiverton, Ontario, is a partnership of Cameco Corp.; TransCanada Corp.; BPC Generation Infrastructure Trust, established by the Ontario Municipal Employees Retirement System; the Power Workers’ Union; and The Society of Energy Professionals.

The Kashiwazaki-Kariwa plant in Japan has the world's largest capacity with 7,965 megawatts, according to the International Atomic Energy Agency. The plant is run by the Tokyo Electric Power Company.