WASHINGTON — States cannot shut down nuclear plants over safety worries, the United States Court of Appeals for the Second Circuit ruled on Wednesday, upholding a lower court’s decision that allowed the Vermont Yankee plant to keep running despite a seven-year effort by the Vermont Legislature to close it.

“The nuclear power industry has just been delivered a tremendous victory against the attempt by any state to shut down federally regulated nuclear power plants,” said Kathleen Sullivan, a lawyer for Entergy, which owns Vermont Yankee.

The court found that states are “pre-empted” from regulating safety by the Atomic Energy Act of 1946, which made safety a federal responsibility. The Legislature had sought to shut the plant by denying Entergy a “certificate of public good” that is required for all power plants. But the court said Vermont was unpersuasive when it said that the reasons for the denial were that the reactor was too costly and unreliable, and that closing it would encourage the development of renewable energy from wind or wood.

In hearings and floor debate, Vermont legislators referred often to the idea that they could not legislate over the safety of the plant, which is on the Connecticut River near the Massachusetts border, and would have to find other reasons to close it.