Federal regulators have added several things to the checklist of more than 450 items to be addressed before the troubled Fort Calhoun nuclear power plant can restart.

The Nuclear Regulatory Commission said Tuesday it has added to the to-do list for the Omaha Public Power District.

Regulators and utility officials say the plant about 20 miles north of Omaha won’t restart unless it’s safe. Fort Calhoun has been shut down since April 2011.

The new items on the checklist shouldn’t surprise OPPD officials because they have been discussed at recent meetings with regulators.

OPPD must explain the causes of several safety concerns identified at Fort Calhoun. And OPPD must address concerns about whether structures inside the containment building could withstand extreme circumstances, such as an earthquake or high radiation levels.