OPG spokesman Neal Kelly says they don't know the exact amount but estimate about 7,000 litres of heavy water leaked during routine maintenance Tuesday afternoon, while the reactor was running.
Kelly says all the heavy water was contained and recovered, and the reactor was safely shut down.
OPG assembled all the staff to make sure everyone was OK, and the utility says there was no impact to their health and safety or to the environment.
Further investigations into the leak from the heavy water transfer system at Darlington are underway.
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.
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