The shutdown of Ontario's Chalk River nuclear reactor could have "dire" implications if it overlaps with the scheduled shutdown of a European reactor that also produces the vital medical isotopes, according to one expert.

The problem-plagued Ontario reactor has been shutdown over a heavy water leak that was discovered last week.

The closure is expected to last a month, and to create a major shortage in the global supply of the isotopes used for medical imaging and treatment.

A reactor in Petten, in the Netherlands, is scheduled for a 12-week closure over the summer, said Dr. Jean Luc Urbain of the Canadian Association of Nuclear Medicine.

"The implications of having both the Petten reactor and the Chalk River reactor down would be very dire as those two reactors pretty well cover 80 per cent of the world market," Urbain told CTV's Canada AM on Wednesday.

"So it would be very difficult to supply the whole world with the other reactors that supply only 20 per cent of the market."

An isotope shortage could begin as early as Sunday.

Power outages in eastern Ontario and western Quebec led to the shutdown of the Chalk River reactor last Thursday, according to Atomic Energy of Canada Ltd.

A heavy water leak was discovered the next day, which means the reactor may be out of commission for at least a month while officials investigate repair options.

The leaking water is being collected in drums, and "there is no threat to workers, the public, the environment or nuclear safety related to this event," the agency said in a news release issued on Monday.

The water is leaking at a rate of five kilograms per hour, the federally-owned AECL said. The leak is located at the base of the reactor vessel, where there is corrosion on the outside wall.

Urbain said isotopes are used in about 30,000 diagnostic tests each week in Canada.

When the shortage kicks in on the weekend, those procedures will likely be prioritized with only the most vital ones going ahead until the shortage ends.

Urbain said an Australian reactor is currently in the regulatory approval process, and will help ease the shortage once it gets the green light to produce isotopes.

In 2007, the Chalk River facility was shut down over safety concerns, leading to a worldwide isotope shortage until the federal government ordered the plant to resume production, despite the concerns.