Talks between Entergy and the union that represents security guards at Grand Gulf Nuclear Station had continued for months before workers were locked out Sunday over work shifts in excess of federal regulations, according to a statement issued Thursday by the union.

The one-page statement from the United Government Security Officers of America Local 36 said waivers allowed by the Nuclear Regulatory Commission “in certain circumstances” to ensure security at all times at the Claiborne County plant were used too often. NRC mandates plant operators document when a security officer works more hours than permitted by federal regulations.

“Unfortunately, the use of waivers has been abused at Grand Gulf,” read part of the statement.

Workers picketed on Thursday along the access road and near the plant entrance. UGSOA said information handed out near the picketers dealt with working conditions at the plant.

“Specifically, the officers are tired of the disparity in treatment and Entergy’s failure to acknowledge or correct the staffing issues that have resulted in excessive shifts and mandatory overtime without providing negotiated vacation time,” the union said.

The utility did not respond late Thursday to the union’s statement.

A contract with the Colorado-based union expired at midnight Sunday, Entergy officials said earlier this week. Non-union workers and staff from its other nuclear plants were being employed during the lockout, Entergy said.

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 utility’s Pilgrim nuclear plant in Plymouth, Mass., were locked out after a tentative contract was rejected. The union has filed a complaint with the National Labor Relations Board over the alternative staffing plan.