Kathleen M. Sullivan, arguing for the plant's New Orleans-based operator, Entergy Nuclear Operations Inc., said the judge had relied on a "cherry orchard" of comments by legislators showing that safety was the primary concern. She said the state was overreaching.
"It's given itself the power to interfere with the federal scheme," she said.
Judge Christopher F. Droney questioned what happens to the plant if the state is found to have mixed motives rather than just safety concerns.
Sullivan answered that the lower-court judge had fully analyzed all concerns in a 101-page ruling.
Frederick said states have the final say in whether nuclear plants can operate in their boundaries, and taxpayers can face huge bills related to a nuclear plant.
The plant, which began operating in 1972, provided as much as a third of the state's electrical supply before last year, when its output for Vermont residents decreased dramatically after March.
The court did not indicate when it will rule.