TORONTO -- About 20 litres of heavy water containing radioactive tritium spilled in a weekend mishap at the nuclear plant near Kincardine, Ont., Bruce Power said on Monday.

The spill on the floor of the idled Unit 6 at Bruce B occurred during replacement of the reactor's major components.

"Crews were flushing a system as part of layup activities on Unit 6, which is shut down," Bruce Power spokesman John Peevers said. "There have been no adverse impacts to our employees, the environment or the public."

The incident was, as required, reported to the Canadian Nuclear Safety Commission, which put out a brief public statement on Saturday saying it was monitoring the situation and would provide updates. The commission did not immediately respond to a request for comment on Monday.

Tritium is a naturally occurring radioactive isotope of hydrogen used for nuclear reactors. It can pose a health risk if it is ingested through drinking water or food, or if it is inhaled or absorbed through the skin.

James Scongack, vice-president of Bruce Power -- the country's only private sector nuclear generator that produces about 30 per cent of Ontario's power -- said anything to do with heavy water is carefully managed.

Peevers said staff moved promptly to stop the spill and clean it up. Employees were moved from the affected area during the cleanup and barriers were set up to restrict personnel. The cleanup, he said, was not difficult.

Tritium levels were being monitored, the company said.

Unit 6 was shut down Jan. 17, and its fuel had been removed. The refurbishment of the reactor is expected to last until the end of 2023.

The plant's life-extension program started on Jan. 1, 2016. It involves the gradual replacement of older systems in the eight reactor units during regularly scheduled maintenance outages. As part of that program, the company began the intensive replacement of key reactor parts in several of its units. The parts include steam generators and pressure tubes.

The COVID-19 pandemic, Peevers said, had no impact on the response to the situation.

This report by The Canadian Press was first published on March 30, 2020.