The leak of an estimated 7,000 litres occurred during routine maintenance on the plant's Unit 2 heavy water transfer system, according to OPG spokesman Neal Kelly. The heavy water was contained and recovered.
"As a precautionary measure, staff near the incident area assembled and were accounted for and the unit was safely shut down," Kelly said in an email statement.
"There was no impact to the health and safety of employees or the public and there was no impact on the environment."
Kelly says OPG also informed the nuclear regulator about the leak Tuesday, as well as local mayors and fire and police departments around Darlington, which is on Lake Ontario, about 70 kilometres east of Toronto.
OPG expects to restart the unit on Thursday.