The company’s decision comes in response to a planned strike by the United Steelworkers Local 8914 starting 30 August.
Cameco said in a statement: "In response to the strike notice, Cameco issued a lockout notice effective 12:01 a.m. on August 30, 2014 to assure a safe and orderly shutdown of its facilities.
"Cameco is continuing to meet with the union during the 72-hour notice period."
The proposed closure will affect around 535 workers at the two operations.
Contract talks began in November 2013 before the previous four-year contract expired on 31 December 2013. Since then workers at the operations have been without a contract.
In July this year, Cameco and the union jointly applied for conciliation under the Canada Labour Code.
The company said that the labor disruption will not have an impact on its 2014 uranium delivery commitments to customers.
"Cameco may draw on a variety of supply sources including primary production, and existing purchase commitments and inventories."
