"Within three to four days we start delivering (radioisotopes) and within seven days we're at full production," said AECL spokesman Dale Coffin.
The reactor might restart as soon as Wednesday if the Conservative government succeeds in pushing through emergency legislation allowing AECL to use a back-up method.
When injected into the body, the isotopes give off radiation that can be seen by a camera to diagnose cancer, heart disease and other medical conditions.
The Ontario Association of Nuclear Medicine says about 50,000 Canadians and 160,000 Americans would have their tests postponed for each month the reactor remains shut down.
Opposition politicians accused the government of incompetence and some medical professionals said they were surprised at how the affair had been handled.
"I think the community was gobsmacked. The fact that we didn't learn about it until late, the fact that it happened at all, is truly a great shock," said Dr. Alexander McEwan, a physician at the Cross Cancer Institute in Edmonton, Alberta.
"In the United States it's impacted as much as Canada. In Japan it's impacting them very, very, very, badly, they're getting 20 percent of their normal shipment moved into the country," McEwan said.
Chalk River produces medical isotopes for Canadian health care company MDS Inc and its MDS Nordion division, which is responsible for about 50 percent of world supply.
"From our understanding, the primary shortage is in North America, Latin America and South America, not as much in Europe," said MDS Nordion spokeswoman Tamra Benjamin.
AECL says it could restart the reactor now using an emergency pump but the Canadian Nuclear Safety Commission does not like this idea. The government is trying to push through legislation temporarily sidelining the watchdog and allowing the reactor to operate for 120 days.
While the Conservatives hold only a minority of seats in the House of Commons and would need the support of opposition parties for the legislation to pass, officials said they expect the House of Commons to pass the legislation and send it to the Senate on Tuesday night. Senate sources said the bill should pass on Wednesday at the latest.
Prime Minister Stephen Harper lashed out at Nuclear Safety Commission boss Linda Keen on Tuesday for refusing to allow the reactor to reopen immediately.
"There will be no nuclear accident but what there will be is a growing crisis in the medical system here in Canada and around the world," an infuriated Harper told Parliament.
Harper's comments were a clear sign he has lost confidence in Keen. A senior government official said her position was becoming increasingly difficult, but a spokesman for the commission said she intended to stay on.
McEwan -- also president of the Virginia-based Society of Nuclear Medicine -- said the Chalk River problem could lead to the construction of a reactor in the United States to produce medical isotopes.
"The question that is going to be asked is why does the United States not have it own medical reactor, why are they dependent on foreign sources," he told Reuters.
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