On Tuesday, federal regulators added additional requirements to the checklist that Nebraska's Fort Calhoun nuclear plant must complete before it can restart.

The Omaha Public Power District has spent the better part of two years addressing a flood in 2011, a fire in a switchgear room and other issues that have kept the single-unit plant offline. In a release, the NRC announced the plant must also "identify the causes and resolve two existing conditions inside containment. Plant workers discovered that structures supporting equipment do not meet all design requirements." Additionally, the NRC said the plant must address insulation on containment building electrical conduits that might degrade if exposed to high levels of radiation during an accident.

A confirmatory action letter issued the same day also required that the plant conduct a third-party safety culture assessment, develop an integrated organizational effectiveness assessment and submit flood recovery plan updates.

Based on work required by an earlier version of the checklist, the OPPD had placed its target restart date in the first quarter of this year. The district previously hired Exelon to take over day-to-day management of the plant.