Candu Energy Inc. enacted a contingency plan Monday after about 800 nuclear scientists, engineers and technologists walked off the job when negotiators failed to reach a deal by the deadline.
But the Society of Professional Engineers and Associates (SPEA), which represents the workers, said there will be a noticeable effect on operations.
Candu is owned by Montreal-based engineering giant SNC-Lavalin, and its employees design, build and service nuclear reactors that supply nearly half of Ontario's electricity and 16 per cent of Canada's overall electricity requirements.
In Canada, the reactors Candu designed are operated by Ontario Power Generation, Bruce Power, Hydro Quebec and New Brunswick Power.
Reactors designed by Candu supply more than 22,000 megawatts of power at sites around the world.
Wages, seniority among key issues in dispute
"There's not enough people to replace us, so work will definitely be impacted," union spokeswoman Michelle Duncan said.
Duncan said Candu employees have stopped the ongoing safety analysis that's performed on reactors, and are not available to make any repairs to the reactors.
Duncan said the work stoppage may also have a trickle-down effect on companies that manufacture the components for the reactors, if fewer contracts are being carried out.
Candu spokeswoman Katherine Ward said the company does not operate any nuclear power plants, so the strike action should have no impact on the day-to-day operations at the plants.
The union has said the main sticking points in the contract 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 because 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."
The federal government agreed last year to sell the Candu reactor division of Atomic Energy of Canada Ltd. to SNC-Lavalin for $15 million plus future royalties.
Under the deal, SNC agreed to protect about 1,200 AECL jobs.
Industry Minister Christian Paradis said he's confident Candu will be able to resolve the situation through bargaining.
SNC-Lavalin is one of the world's largest engineering and construction firms.
Shares in the company were down 15 cents at $38.06 on the Toronto Stock Exchange Monday afternoon.