Thousands of patients in Canada and around the world are having their medical tests postponed because an Ontario nuclear reactor that produces radioisotopes has been temporarily shut down.

Radioisotopes are used in nuclear medicine in a number of ways. They can be injected into patients to allow nuclear imaging equipment to produce scans of the body to help diagnose a number of diseases, such as cancer, cardiovascular diseases and bone ailments.

The government-run Chalk River NRU reactor supplies more than two-thirds of the world's demand for medical radioisotopes. Canada also supplies about 80 per cent of the radioisotopes used in the United States.

The reactor was closed for scheduled repairs two weeks ago, but inspectors found more problems to repair than expected, extending the shutdown.

The longer-than-expected shutdown has led to shortages across the country. About 150 seriously ill Nova Scotians had their medical tests cancelled this week in Halifax and Sydney. And hospitals in Toronto are reporting to CTV News that they are triaging their patients, allowing only patients of the highest priority to undergo diagnostic scans and asking others to wait.

Dr. Christopher O'Brien of the Ontario Association of Nuclear Medicine calls the shutdown "devastating."

"This is just really bad planning and management," he told CTV Newsnet. "I think at least -- and our association thinks this as well -- if we knew we were having a planned shutdown why do it now? Why not when we have slower activity, such as over the Christmas holidays?

"The other component is: where was the contingency plan to ensure if they found something, that we could ensure that we would have access to the isotopes that we need to help our cancer patients to help our children, the elderly, grandparents sitting in hospital beds waiting to find out if they have fractures?

"We are devastated by this and exceedingly frustrated by the poor planning."

MDS Nordion, which creates medical isotopes and radiopharmaceuticals from the reactor, says they have been told by Atomic Energy of Canada Ltd. (AECL), which owns the reactor, that they are targeting a return to full production in early to mid-January.

MDS says it is working with its back-up supply network "to mitigate the impact."

Some hospitals that need to use radioisotopes are setting priorities, giving tests to their most urgent patients first.

"This one gap in service has provided a nationwide, if not global, shortage of an essential medical diagnostic product. We shouldn't be in this position," Dr. Sandor Demeter of the Canadian Nuclear Medical Society told CTV News.

The radioisotopes could not have been stockpiled ahead of time because they have a short shelf life. There are also few other sources, with only a few suppliers in the world.

AECL released a statement Tuesday afternoon that reads: "AECL recognizes the important role NRU plays in the supply and delivery of medical isotopes to patients around the world... We understand that patients will be impacted by this development and we are focused on completing the work precedures as quickly as possible."

AECL has been working on two replacement reactors, but they are nearly 10 years behind schedule and over budget because of flaws that potentially affect safety.

The NCU reactor at Chalk River has been in operation for 50 years. It was supposed to have been decommissioned in 2005, but the Canadian Nuclear Safety Commission extended the reactor's operating license until the problems with the new reactors can be sorted out.

With a report from CTV Ottawa