The Society of Professional Engineers and Associates says negotiations with the company, which is a subsidiary of SNC-Lavalin (TSX:SNC), have failed to find a compromise and a deal is unlikely before employees are in a legal strike position as of 7 a.m. Monday.
The union says key sticking points are seniority and compensation.
Candu reactors supply about half of Ontario's electricity and 16 per cent of Canada's overall electricity requirements. They also supply more than 22,000 megawatts of power at sites around the world.
The company could not immediately be reached for comment.
SPEA president Peter White says 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 says a full strike threatens the future of Canada's nuclear industry as it will almost certainly guarantee the loss of technological talent.
He says senior engineers with years of expertise are choosing to leave the company, which could cause the design and service capabilities at Candu to decline.
The union says if a strike goes ahead, picket lines will be set up at various nuclear sites in Ontario, Quebec and New Brunswick.