OMAHA, Neb. (AP) — Omaha Public Power District officials remain confident in their plan to improve the Fort Calhoun nuclear power plant, and they disagree with an environmental group’s petition to revoke the plant’s license. OPPD spokesman Jeff Hanson said Thursday that the utility believes the nuclear power plant 20 miles north of Omaha is safe. The utility also says the plant is an important part of its power-generating portfolio. The Sierra Club of Iowa filed a formal petition with the Nuclear Regulatory Commission Thursday asking regulators to revoke Fort Calhoun’s license. David Lochbaum, nuclear safety director at the advocacy group Union of Concerned Scientists, says the NRC probably receives a similar petition about once a month. Most of the petitions are denied, but he said they can force changes at nuclear plants.