Linda Keen, shown in January, has resigned from the board of the Canadian Nuclear Safety Commission. (Fred Chartrand/Canadian Press)

The former head of the Canadian Nuclear Safety Commission has resigned from the nuclear watchdog's board of directors.

Linda Keen sent a letter to Prime Minister Stephen Harper on Tuesday stating she no longer wants to stay on the board and risk being second-guessed by government officials.

"Given the circumstances of my demotion and the continuing court challenge, applicants, stakeholders and even the government will be in a position to challenge my decisions on the basis of perceived bias," she wrote. "I will be constantly second-guessed as to my motivations for reaching decisions."

Keen, the former commission president, was dismissed by federal Natural Resources Minister Gary Lunn in January after refusing to authorize the restarting of the aging Atomic Energy of Canada Ltd. reactor in Chalk River, Ont., because of safety concerns.

The reactor was closed for scheduled maintenance in November 2007. The shutdown was prolonged because the regulators found more problems, including two water pumps that didn't have earthquake-resistant emergency backup power.

However, the reactor produces two-thirds of the world's medical radioisotopes and its shutdown caused a worldwide shortage. In December, the government brought in emergency legislation to override Keen's decisions, and Parliament ordered the facility to reopen.

Keen said in January that she was acting according to the law when she refused to approve the restart of the reactor, pointing out that the mandate of the commission was to ensure public safety.

Keen said the risk of restarting the Chalk River reactor was 1,000 times higher than accepted international standards.

In the case of a nuclear fuel failure, the international standard for acceptable risk is one in a million, Keen said.

Keen's letter resigning from the board of directors said it would be inappropriate if she continued to sit in on hearings as a demoted member.

She is also suing the federal government over her dismissal.

With files from the Canadian Press