OTTAWA (Reuters) - The Canadian government fired the country's top nuclear watchdog late on Tuesday, criticizing her for how she handled the closure of a key reactor which makes medical radioisotopes.

The sacking of Canadian Nuclear Safety Commission president Linda Keen will put more pressure on Conservative Prime Minister Stephen Harper, already under fire from opposition legislators and the media for his handling of the affair.

Keen lost her job for refusing to allow a 50-year-old reactor at the Chalk River facility in Ontario to reopen after regular maintenance in November. The reactor makes more than two-thirds of the global supply of medical isotopes.

Last month, as hospitals began complaining about widespread shortages, the Conservatives overruled Keen by forcing legislation through Parliament to order that the reactor be restarted for 120 days.

Harper publicly slammed Keen -- who said the move was too risky because some safety back-up systems were not working -- and said her judgment had been appalling.

"Having reviewed the actions of the president around the extended shutdown of the ... reactor, it is clear that the government is not satisfied that she demonstrated the leadership expected," Natural Resources Minister Gary Lunn said in a statement released at 11:46 pm (0346 GMT) Tuesday.

The isotopes, when injected into the body, give off radiation that can be seen by a camera to diagnose cancer, heart disease and other medical conditions.

"The extended shutdown of the reactor was threatening to cause a national and international health crisis. The president was aware of the importance of maintaining Canada's and the world's supply of medical isotopes," Lunn said.

Lunn and Keen are due to appear separately on Wednesday before a Parliamentary committee probing what happened to the reactor, which is operated by Atomic Energy Canada Ltd.

AECL's chief quit last month.

Last week, a report by Canada's auditor-general said AECL was falling short on plans to replace the aging reactor.

Keen, who made clear last week in an angry exchange of letters with Lunn that she would contest her firing, will be accompanied by a lawyer.

Opposition legislators accuse Lunn of incompetence, saying he had been aware of problems at Chalk River for two weeks before telling cabinet colleagues.

They are also unhappy that Harper focused on the fact that Keen had been appointed by the former Liberal government.

Chalk River produces medical isotopes for Canadian health care company MDS Inc and its MDS Nordion division, which is responsible for about half of world supply.

(Additional reporting by Scott Anderson in Toronto)

(Editing by Bernadette Baum)

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