Thousands of patients in Canada, the U.S. and other countries are having their medical tests postponed because an Ontario nuclear reactor that produces radioisotopes for nuclear medicine was shut down for repairs.

The government-run Chalk River reactor supplies more than two-thirds of the world's demand for medical radioisotopes, which are used to diagnose cancer and cardiovascular diseases.

Once injected into patients, the radioisotopes allow nuclear imaging equipment to produce detailed scans of the body.

But the Chalk River reactor was closed for scheduled repairs two weeks ago, causing a cross-country shortage of radioisotopes. During the maintenance check, regulators found more problems to repair than expected.

Atomic Energy of Canada Ltd., which owns the reactor, said it won't be operating at full capacity for another 10 days.

About 150 seriously ill Nova Scotians had their medical tests cancelled this week in Halifax and Sydney. Halifax nuclear medicine specialist Dr. Andrew Ross said he has already cancelled tests for 100 patients and expects to cancel 100 more next week.

"It's an absolutely devastating turn of events for us," said Ross, of Halifax's Capital Health.

Without the radioactive injections, the department's scanning machines can't be used.

"This is the first full-scale interruption that I have experienced in my 20 years of practice," he said.

Nuclear medicine doctors say the biggest frustration is that there was no contingency plan in place before the reactor shut down. Radioisotopes can't be stockpiled because they have a short shelf life and there are only a few suppliers in the world.

Replacement reactors on hold

"Seeing as this shortage is going on for longer than expected, there's definitely going to be issues," said Dawn Marie King of Toronto's University Health Network.

"We are held hostage by it, to be frank."

A federal plan to build two replacement reactors has been on hold for 10 years.

"The plans are all done. It's just a matter of getting the regulatory approvals and the things built and once that happens, this kind of disaster should be far far less likely to happen to us," Ross said.