Candu is owned by Montreal-based engineering giant SNC-Lavalin (TSX:SNC), and its reactors supply nearly half of Ontario's electricity and 16 per cent of Canada's overall electricity requirements.
With operations in Ontario, Quebec and New Brunswick, Candu also supplies more than 22,000 megawatts of power at sites around the world. The company hasn't said yet how the job action might affect operations.
The Society of Professional Engineers and Associates, which represents the workers, said power plants will not shut down but there will a noticeable effect on operations.
"There's not enough people to replace us, so work will definitely be impacted," spokeswoman Michelle Duncan said.
The union has said the main sticking points in the labour dispute involved wages and seniority.
SPEA president Peter White said a key issue is what he calls the company's desire to move away from nuclear industry standards and compensate its employees differently from other workers in the field.
He said a full strike threatens the future of Canada's nuclear industry as it will almost certainly guarantee the loss of technological talent.
Senior engineers with years of expertise are choosing to leave the company, which could cause the design and service capabilities at Candu to decline, he said.
Duncan said the union would like to return to the bargaining table, but will only do so if SNC-Lavalin presents ``a fair and competitive deal.''
"We need to be competitive if we're going to maintain the expertise," she said.
"Our members can work anywhere in the world, they are employable and they are making their decisions by walking with their feet."