The head of Atomic Energy of Canada Limited (AECL) quit Friday, one day after the forced restart of a nuclear reactor responsible for producing more than half of the world's medical isotopes.

Prime Minister Stephen Harper announced the resignation of Michael C. Burns late Friday afternoon. Burns was chair of the Board of AECL, which operates the National Research Universal (NRU) reactor in Chalk River, Ont.

The reactor was closed on Nov. 18 for repairs. That's when federal nuclear inspectors discovered that the facility had been operating without emergency power backup for its cooling pumps and insisted on upgrades before it could be reopened.

The extended closure sparked an international shortage of radioisotopes used in diagnostic tests for cancer, heart problems, and bone ailments.

On Thursday, emergency legislation was enacted to override the inspectors' decision and force production of medical isotopes to resume.

Burns, a Tory fundraiser, was appointed chair of AECL by the Harper government one year ago.

Harper announced Friday that Burns will be replaced on Dec. 31 by Glenna Carr, a former Ontario bureaucrat. Carr has served as the deputy minister of three Ontario ministries.

"I would like to express my appreciation to the former Chair of the Board, Michael C. Burns for his service to AECL," Harper said in a press release.

Harper also appointed business executive Hugh MacDiarmid the new chief executive officer.

Emergency legislation was passed by the House of Commons on Wednesday, with support from all parties. It approved the restart of the reactor in order to alleviate the shortage of isotopes.

The legislation bypassed the Canadian Nuclear Safety Commission (CNSC) and allows AECL to restart operation of the reactor for 120 days, after which it will have to close again to complete safety upgrades.

Harper had warned he would be looking into the closure of the nuclear reactor, owned and operated by AECL.

"I can certainly assure the House that when this is all behind us the government will carefully examine the role of all actors in this incident and make sure that accountability is appropriately restored," he said in the House of Commons on Tuesday.

Harper had previously laid the blame for the isotope crisis on the CNSC.

He said the "Liberal-appointed" commission put lives at risk by insisting the reactor remain offline until safety measures were installed.

Harper said past Liberal appointments to the CNCS, which oversees the AECL, were partly to blame for the crisis.

"Look who's incompetent and partisan now,'' Liberal MP Omar Alghabra told The Canadian Press after hearing about the resignation of the Conservative-appointed Burns.

Alghabra said Burns' removal proved the Liberals were right in thinking that AECL was primarily responsible for the closure.

The publicly owned reactor, almost 50 years old, supplies medical isotopes, which are used to test for cancer, heart problems, and bone ailments.

The reactor was supposed to have been decommissioned in 2005. But a plant designed to replace the aging facility is six years behind schedule and will not produce radioisotopes until late 2008.