Health Canada has warned doctors to expect a shortage of medical isotopes after an unscheduled shutdown this week at the Chalk River, Ont., nuclear plant that supplies them.

In an email sent to the Canadian Medical Association and doctors practising nuclear medicine, a Health Canada official encouraged doctors to "take steps to maximize the existing supply during this period."

Officials with Atomic Energy of Canada Ltd. (AECL) have confirmed they discovered a mechanical problem with the reactor on Sunday. A device used to extract the isotopes wasn't working properly, and in order to repair the device, the reactor had to be shut down, AECL officials said.

It is not expected to reopen until Tuesday.

"Obviously, we will experience some mechanical difficulties. It's a 51-year-old reactor," said Dale Coffin, spokesman for Atomic Energy of Canada Ltd. "It's not without its challenges, but it's a safe reactor, and safety is our number one priority."

Supplies could drop by 50 per cent

AECL says it's unable to predict what the precise impact of the shutdown will be on the supply of isotopes, which are used in sophisticated diagnostic medical tests such as bone scans.

However, one distributor of medical isotopes offered an estimate. In an email sent to Ontario hospital staff, GE Healthcare estimated supplies will be reduced by about 50 per cent until Saturday.

When contacted by CBC News, two doctors in nuclear medicine said emergency services will be maintained, and they don't expect any critical care issues to arise from the shortage. The doctors said some non-emergency procedures might have to be rescheduled.

"We've become very efficient at triaging and selecting those who can and can't be bumped [during shutdowns and shortages]," said one doctor in charge of nuclear medicine at several Ontario hospitals.

In December 2007, a shutdown for maintenance threatened to create a global shortage of isotopes. In December 2008, the reactor was shut down because of a leak involving radioactive water, temporarily reducing the isotope supply.

AECL officials said work is being done in order to have the reactor up and running by the end of Tuesday.

"The reactor is in the restart process now and on its way to high power as we speak", said Coffin.