Kathleen Duguay, a spokeswoman for NB Power, said the spill occurred in the reactor building.
She didn't know the exact amount, but said it was "minute."
"It was a small spill. It was all cleaned up immediately," she said.
Duguay said no one was hurt.
The spill occurred as the reactor's moderator system was being filled with heavy water as part of NB Power's plan to restart the generating station, which has been undergoing a lengthy and expensive refit.
Duguay said the spill was detected by radiation monitoring equipment and the reactor building was evacuated immediately.
She said workers in the area were wearing protective suits at the time and were not exposed.
The cause of the accident wasn't known.
Lepreau was taken offline in March 2008 for an overhaul that was supposed to take 18 months. But the project has been plagued by cost overruns and delays and is now not expected to return to service until the fall of 2012.
Atomic Energy of Canada Ltd. is overseeing the refurbishment.
The province has pegged the total cost overruns, including replacement power while the nuclear reactor is offline, at about $2.4 billion.