TORONTO - It appears the critical shortage of medical isotopes isn't likely to be resolved before mid-summer.

Atomic Energy of Canada Limited announced late Thursday that the Chalk River nuclear reactor should be ready to resume production of isotopes by the end of July.

In a statement, AECL said 56 per cent of the repair work that began last May has been completed. A review of the schedule for the remaining work suggests the end of July target is feasible, the agency said.

"The new schedule has built in prudent contingency reflecting the difficulty inherent in these final repairs," it said in the statement, noting the review had been done with advice from outside experts.

The president of the Canadian Association of Nuclear Medicine said the latest estimate of when the reactor will be back online is a good news, bad news story.

"Let me put it this way: at this point in time, it's better to know that they are optimistic about the fact that the repair can be accomplished," Dr. Jean-Luc Urbain said in an interview.

"What we are very concerned about is that ... the repair might not be successful. And that would be definitely tragic. But at the same time, we're struggling to take care of our patients."

The extended period of shortage of supply has been difficult for hospitals, clinics and their patients. The isotopes are used in key diagnostic tests for some cardiovascular conditions as well as some forms of cancer.

The 52-year-old reactor went off-line last May after a heavy water leak was detected following a power outage in the area. Initially it was thought the repairs would take a month to complete, but it quickly became apparent the work might take considerably longer.

The shutdown of the Ontario reactor, which has coincided with a shutdown of a Dutch reactor, has led to a worldwide isotopes shortage. Chalk River and the Dutch reactor normally produce the lion's-share of the world's medical isotopes.

The AECL statement said eight of 10 weld sites have been completed. But the location of the two remaining sites to be repaired make the final steps extremely challenging, it said.

"It's very delicate," Urbain said of the needed repairs.

"I think the repair needs to be done properly. I think if they're not done properly, we might not have a reactor again."