The aging Chalk River nuclear reactor is back up and running after an unplanned shut down.

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The control room at the Atomic Energy Canada Limited (AECL) Chalk River nuclear facility is seen during a media tour in Chalk River, Ont., on Dec. 19, 2007. ((Chris Wattie/Reuters) )

Atomic Energy of Canada Limited says repairs were made to the fuel rod flask — which fuels and de-fuels the reactor.

The Chalk River facility in Ontario is one of the world's largest producers of medical isotopes.

The shutdown took place at a time when two of the world's three other major producers were also out of operation, leading to worries of a possible major shortage of medical isotopes.

AECL said the nuclear facility has begun processing again and will resume supplying medical isotopes over the weekend.

It also said there were no nuclear safety issues around the shut down and the repairs.