Canada's nuclear safety watchdog warned last month that it was concerned about an "erosion of safety margins" at the Pickering B nuclear station near Toronto.

A letter from the Canadian Nuclear Safety Commission surfaced Tuesday on the eve of a hearing in which Ontario Power Generation will make the case for renewing the station's operating licence until 2014.

The commission's assessment of the station was sent to Patrick McNeil, a senior executive at the provincially owned Crown corporation, on April 7.

"CNSC staff remain concerned with the trend in erosion of safety margins ... and expect that OPG will address the identified outstanding issues, and, in particular, will develop and implement engineered solutions to re-establish robust safety margins," says the letter.

Greenpeace obtained a copy of the document and cited it as proof the nuclear station poses an unnecessary risk to the Greater Toronto Area.

"Nuclear technology is an unforgiving technology," said Shawn Patrick Stensil, a Greenpeace researcher on energy issues.

"When your safety margins are declining, you're increasing your risk of accidents, and accidents from nuclear stations can cause irreversible harm."

Safety margins set the boundaries of acceptable risks at a nuclear facility. That risk level determines what safety systems must be in place in case of accidents.

The Pickering nuclear facility is located roughly 40 kilometres northeast of downtown Toronto. Statistics Canada's 2006 census data shows about five million people live in the Toronto census metropolitan area.

There are eight reactors at the Pickering nuclear facility. Four are at the older Pickering A station, which began commercial operation in the early 1970s, and four are at the Pickering B station.

The commission document deals only with the Pickering B station. Its reactors were added on between 1983 and 1985.

McNeil told the Canadian Press that it's important to put the document in context. He said it is one of many written exchanges in the nuclear station's environmental assessment and safety review.

The bottom line, he said, is that the Pickering B nuclear station is safe, and Ontario Power Generation wouldn't operate it if it weren't.

"We constantly assess how much wiggle room [there is].... We would not operate a facility that was unsafe and did not have adequate safety margins to meet the needs of the plant.

"The safety margin is completely adequate to meet the needs of the plant…. We're looking at a probability of less than one in a million for an incident."

He declined further comment on the reference to eroding safety margins.

McNeil also noted that commission staff have recommended the watchdog's tribunal grant a five-year licence extension to the Pickering B station.

The tribunal will hear from the public and special interest groups during Wednesday's CNSC hearing in Ajax, Ont.

The Ontario government is expected to decide this year whether to refurbish the Pickering B reactors or build new ones instead.

The nuclear safety commission did not return e-mails and calls for comment Tuesday.