A temporary shutdown at the Chalk River, Ont. nuclear reactor is causing a shortage of medical isotopes, forcing Canadian doctors to scramble to cancel and rearrange appointments with their patients.

The isotope shortage is expected to last until the middle of next week, CTV News has learned.

The shortage is expected to affect Ontario, Quebec, parts of the Maritimes, the northern United States and perhaps even Mexico.

Atomic Energy of Canada Ltd., responsible for the Chalk River nuclear facility, told CTV News that the shutdown was "normal" on Thursday night, but on Friday said the shutdown was "longer than expected."

The reactor was shut down Thursday of last week, and was expected to be back online with three to four days. However, it was down for an entire week, only going back online late Thursday night.

In a statement released Friday evening, the AECL explained that the reactor was shut down Thursday, Dec. 4 to carry out a "required configuration change to conduct research, unrelated to medical isotope production."

The outage was extended to address "unanticipated technical challenges." The AECL pointed out that before any maintenance outages it produces additional isotopes within the reactor and that this allows it to maintain delivery of isotopes throughout the outage period.

"Any possible impact from the shutdown is expected to be short-lived as the NRU (nuclear reactor) has come back on schedule," the AECL said in the release.

But last Saturday, AECL sent out a warning to government agencies that they would be a shortage of medical isotopes but never made the warning public.

Health Canada said in an email to CTV News that they followed the proper protocols to inform the medical community of the problem.

"Health Canada on December 8 alerted provincial and territorial officials and the nuclear medicine community of the potential short-term shortage this situation may cause," spokesperson Paul Duchesne said in the email.

CTV parliamentary correspondent Graham Richardson said Friday he has been hearing from the medical community all across Ontario that they are scrambling to move around appointments to make due during the shortage.

Health Canada would only say that "demand and supply vary across the country" and said the shortage is expected to be "short-lived."

The normal backup for medical isotopes in Canada is unavailable as a Dutch nuclear facility is down and not expected to be back online until well into 2009.

About half of the world's medical isotopes are produced at the Chalk River plant. The Chalk River reactor, known as the NRU, was shut down for more than a month in November 2007, sparking a crisis in Ottawa and a worldwide medical shortage.

Chalk River produces about 80 per cent of Canada's medical isotopes. Medical isotopes are used for a variety of tests and treatments, including for cancer, heart problems, and bone ailments.

The NRU is the world's oldest nuclear reactor, as it went online in 1958. It has an operating licence valid until October 2011, and the AECL has said that it will work on keeping that reactor open beyond that date.

With files from CTV parliamentary correspondent Graham Richardson