A medical isotope shortage is growing on both sides of the U.S.-Canada border, as the aging nuclear reactor in Chalk River, Ont., remains offline for repairs.

The NRU reactor previously produced about one-third of the world's medical isotopes, which are used for detecting cancer and heart ailments. It was shut down in May after a radioactive water leak was discovered.

On Wednesday, Atomic Energy of Canada Limited announced that the 52-year-old reactor will not be operational until the first quarter of next year, leaving parts of the U.S. and Canada scrambling for isotope supplies for even longer than expected.

And as recently as Friday morning, Natural Resources Minister Lisa Raitt said the government cannot say for sure when exactly it will be returning to service.

"We don't have a precise sense as to when the NRU will be back up and running and producing medical isotopes," she said from Toronto during a phone interview with CTV's Canada AM.

Kevin Tracey, the vice-president of the Ontario Association of Nuclear Medicine, said the supply problem will become more dire as time goes on.

The upcoming shutdown of a major European reactor in February will leave Canada with even fewer isotope suppliers than it has today, he said.

Tracey calls it a "looming perfect storm," which he said should be prompting the federal government to acting with a sense of urgency.

"There are a lot of words, but we're not seeing the action that the professionals in this business, the medical leadership, would like to see for their patients," he told CTV's Canada AM during an interview from Windsor, Ont., on Friday morning.

Jeffrey Norenberg, the head of the National Association of Nuclear Pharmacies in the U.S., also said the dual shutdown creates a dangerous situation.

"With both of them offline, it's very perilous," he said.

In the U.S., the supply shortage is already an issue in a country where some 40,000 patients undergo isotopes-related tests each day.

A June survey by the Society of Nuclear Medicine said that 91 per cent of participating hospitals, pharmacies and commercial imaging groups reported being affected by the shortage.

In some cases, patients are being treated with formerly-used isotopes.

Michael Graham, the president of the Society of Nuclear Medicine, says such treatments give a radiation dose that "is somewhat higher" than if other isotopes were available, they take longer to do and produce an image quality that "may not be quite as good."

Doctors in Canada have said that the isotope shortage has left them depending on imported isotopes, which appears to have pushed up their cost of acquisition considerably. In some cases, isotope shipments are costing up to $30,000 more than they used to.

In the U.S., Graham says he expects the per-isotope cost to double in cases where new contracts suddenly come into effect.

Raitt said Canada has been relying on its international partners to obtain the isotopes it needs as the shortfall continues.

But she said some of the isotope cost increases were known before the reactor was shut down three months ago.

"It was indicated by some distributors, even before Chalk River had their unplanned, unexpected shutdown in May, that there would be increases to the cost associated with medical isotopes," Raitt said. "It's trying to determine exactly how much extra is it costing because of Chalk River or if this is something that was just coming with the market."

When AECL announced that the reactor would not be back online until next year, a number of senior members of the Conservative government -- including Raitt, Health Minister Leona Aglukkaq and Prime Minister Stephen Harper -- said they were disappointed with the news.

With files from The Associated Press and The Canadian Press