The Canadian energy group said plans to replace the reactor in Chalk River, Ont., wouldn't move forward due to those plans being over budget and significantly behind schedule, the Canwest News Service said Friday.

AECL Chief Executive Officer Hugh MacDiarmid said in a statement that "our board of directors and senior management have concluded that it is no longer feasible to complete the commissioning and start-up of the reactors."

The National Research Universal reactor in Chalk River went into operation in 1957 and was the focus of a temporary shutdown last year by federal nuclear regulators.

While the aged nuclear site is again producing medical isotopes, it has been the focus of replacement talks when its current license expires on Oct. 31, 2011, Canwest said.