The reactor makes more than two-thirds of global supply of the radioisotopes used in cancer tests. Shortages were quickly triggered last month when the reactor was shut down.
The reactor was supposed to be closed from Nov.
18 to Nov. 23 for planned maintenance, but the Canadian Nuclear Safety Commission refused to let it be restarted without an upgrade on the cooling pumps that prevent the reactor core from melting down.
The commission had discovered that AECL had been operating the reactor for 17 months without a battery-operated starter on the pumps, which the commission said was a condition of renewing the 50-year-old reactor's licence.
AECL spokesperson Dale Coffin said the Crown corporation voluntarily extended the maintenance shutdown to address the safety commission's concerns.
"We're still operating very safely and the reactor with the new upgrade in place already is even safer," said David Torgerson, the president of Atomic Energy of Canada's research and technology division.
Parliament pushed through legislation recently to bypass the commission's objections and get the reactor back online.
Meanwhile, the Conservative government says it's just a coincidence the AECL chairman quit last week following a furor over the shutdown of the reactor.
Health Minister Tony Clement said Michael Burns thought the chairman's post was a part-time one when he took it a year ago.
"As it turns out, there's a lot of work to be done," Clement said yesterday on CTV's Question Period. "So I think there were some indications this (resignation) might be coming up."
Prime Minister Stephen Harper announced the departure of Burns, a former Tory party fundraiser, in a terse news release.
The Tories appointed former Ontario bureaucrat Glenna Carr as the new chairman and named business executive Hugh MacDiarmid as chief executive officer.