Replacement security workers will guard Grand Gulf Nuclear Station until further notice after Entergy locked out unionized personnel when the two sides couldn’t agree on a new contract before the old one expired at midnight Sunday.

Talks between the New Orleans-based utility that runs the Claiborne County plant and the leadership of the United Government Security Officers of America Local 36, which represents Grand Gulf security officers, broke down, a news release issued Monday said.

The utility was using non-union workers and staff from its other nuclear plants to fill the gap today, the utility said.

“Since those employees would have been working without a contract, they would reserve the right to leave their posts and strike at any time — a situation that is unacceptable at an Entergy Nuclear plant,” said Mike Balduzzi, Entergy Nuclear’s senior vice president of nuclear technical services. “The health and safety of the public is foremost among our obligations, and this measure will ensure we meet the rigorous standards required for securing, operating and maintaining our nuclear power plant.”

The utility would not comment further and declined to disclose the pay scale of its security workers, spokesman Mike Bowling said.

A call to the union’s Westminster, Colo.-based president was not returned. Founded in 1992, the UGSOA International Executive Board has 12,000 members and has negotiated more than 75 contracts between May 2010 and May 2012, according to its website.

Entergy’s Wholesale Commodities segment owns or manages six nuclear plants in the northern U.S. Five others, including Grand Gulf and Louisiana plants Riverbend and Waterford 3, are owned by the utility or, in Grand Gulf’s case, owned and operated through a common stock arrangement.

In June, a six-month effort was completed to refuel the boiling-water reactor and upgrade its generating capacity by 13 percent.

Also in June, union workers at the Pilgrim nuclear power plant in Plymouth, Mass., rejected a tentative contract with Entergy. The dispute has become contentious with Entergy locking out union workers and the union filing complaints with the National Labor Relations Board and Entergy’s implementation of the alternative staffing plan.