Atomic Energy of Canada Ltd. said Friday it won't know until next week how long the Chalk River nuclear reactor that supplies 30 per cent of the world's medical isotopes will be offline.

The 52-year-old National Research Universal (NRU) reactor at Chalk River Laboratories in Ontario was shut down last week because of a water leak.

"AECL anticipates that the NRU reactor will remain out of service for more than one month," the agency said in a statement.

"However, the visual inspection process, once completed, will provide a more accurate return to service timeframe."

Scientists are trying to figure out why the reactor is leaking heavy water. Remote cameras have been lowered inside the reactor, and no abnormalities were visible.

On the weekend, scientists will use ultrasound to probe the interior wall of the reactor.

Maximizing supply

Canadian nuclear medicine specialists are also expecting an update from European providers on the state of their reactors, which could help meet some of the demand usually filled by the Chalk River reactor.

"Effective this Monday, May the 25th, we will see reductions in the total amounts of radioactivity available for patient imaging," said Alan Thibeau of the nuclear medicine division at Ottawa Hospital.

Thibeau is trying to make the existing supply go further by injecting a little less into each patient and running tests on Sunday to use up the isotopes before they decay.

In Saskatoon, dozens of patients set to have nuclear medicine procedures next week are having to reschedule.

Likewise, in New Brunswick, Health Minister Michael Murphy said he's concerned the looming shortage of medical isotopes will at some point lead to rationing of tests, with istopes reserved for urgent cases.