By Scott Anderson

TORONTO (Reuters) - Canada's MDS Nordion started shipping medical isotopes on Wednesday after a prolonged shutdown of a nuclear reactor at Chalk River, Ontario, disrupted the supply of isotopes to health facilities around the world.

Nordion, which supplies about 50 percent of the world's medical isotopes, has been hard hit by the shutdown of the nuclear plant where they are made. The Chalk River plant had been off line since November and resumed shipments to MDS Nordion on Tuesday.

Nordion spokeswoman Shelley Maclean said the company shipped the isotopes, used in cancer tests and other medical procedures, to customers on Wednesday and was "able to meet 100 percent of the orders."

Maclean said, however, that it was uncertain how the backlog in orders for the isotopes would be settled.

"Our customers are still trying to sort through how they are going to manage that, so we are just taking our lead from them, based on what they are going to need," she told Reuters.

The Chalk River plant is operated by Atomic Energy of Canada Ltd, a government-owned nuclear technology firm. It was returned to service on Sunday after the federal government pushed through legislation allowing it to restart despite concerns expressed by Canada's nuclear safety regulator.

The reactor makes more than two-thirds of global supply of the medical radioisotopes. Its shutdown caused shortages of isotopes around the world.

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.

(Reporting by Scott Anderson; Editing by Peter Galloway)