Canada ordered a nuclear reactor to be re-opened even though the risk was far greater than international standards permit, a committee has heard.
Non-radioactive technology is also used in cancer detection
Linda Keen told a House of Commons committee that she was acting according to the law when she refused to approve restarting the reactor last year.
She was later sacked as head of the country's nuclear watchdog.
The plant produces two-thirds of the world's medical isotopes, and the shutdown created a worldwide shortage.
Ms Keen refused to re-open the Chalk River plant after it was shut for routine maintenance in November last year, because some safety back-up systems were not working.
Prime Minister Stephen Harper said in December that there was no risk of a meltdown, and his Conservative government pushed emergency legislation through parliament to get it working again.
Speaking on Tuesday, Ms Keen said the chance of a failure was one in 1,000, while the permitted international standard for nuclear fuel failure was one in a million.
"Ignoring safety requirements is simply not an option," she told the House of Commons committee that is reviewing the circumstances surrounding the shutdown.
Also on Tuesday, Auditor General Sheila Fraser told the committee that the government's decision to fire Ms Keen raised concerns about the independence of regulatory bodies.
However, Atomic Energy of Canada Limited, which run the Chalk River plant, said in a statement that Ms Keen's comments were misleading.
"There are no international standards related to one-in-one-million for fuel failures. All reactors experience fuel failures from time to time and there are no safety consequences to the public, employees or the reactor," it said.
Earlier this month, Natural Resources Minister Gary Lunn said the way Ms Keen had handled the closure did not meet "the very high standard of conduct the government and Canadians expect from public officeholders".
The isotopes produced at the plant are used for medical imaging and diagnostic scans for fractures, cancer and heart conditions.
When injected into the body they give off radiation that can be seen by a camera.