OTTAWA - The shutdown of the Chalk River nuclear reactor could be a long one.

Atomic Energy of Canada Ltd. said Thursday that it can't rule out a prolonged closure of the reactor that produces half the global supply of radioactive isotopes used in medical imaging.

Executives from the Crown corporation said they won't be able to put a timetable on the restart until they gather more facts about a heavy-water leak.

"We have said at this point that it will be more than a month to bring the NRU back into service," said Bill Pilkington, AECL's vice-president in charge of the plant.

"As to what it might be beyond that, that really depends on what we find in our inspections.

"So your suggestion of it being in the order of eight months or more is certainly not crazy, but at the same time, we don't have facts yet to determine what that is."

The 52-year-old NRU reactor at Chalk River, Ont., was shut down May 14 after a power outage in parts of eastern Ontario and western Quebec.

A heavy water leak was detected within the facility the following day and officials said it would be out of service for more than a month while repair options are considered.

But some media reports say it could be down for much longer.

The agency said the heavy water is being contained and stored in drums, and there is no threat to workers, the public or the environment.

But officials also said they will run out of medical isotopes by Saturday.

Natural Resources Minister Lisa Raitt said other countries that produce medical isotopes have offered to help make up for the shortage.

"During this extended outage at Chalk River, other isotope-producing countries have the capacity to assist in minimizing production shortfall," she said in Mississauga, Ont.

"These countries all have different sets of constraints and capacity, and will need to work through those. But there is goodwill amongst all and a willingness to be helpful."

However, the world's four other medical isotope reactors can't fully match Chalk River's output.

The federal government is looking at other options for producing isotopes, such as McMaster University's 50-year-old reactor, which filled in for the NRU reactor in the 1970s when Chalk River was shut down.

Other possibilities include working with the TRIUMF laboratory and the University of British Columbia to produce alternative isotopes, Raitt added.

The federal and Ontario governments sent out advice Thursday to doctors struggling with a shortage of isotopes.

Guidelines obtained by The Canadian Press urge doctors to time procedures so patients who need higher doses of radiation go first, work evenings and weekends before the isotopes decay, use less isotopes but run tests longer, and prioritize scans so urgent tests are done first.