""No pilots may operate an aircraft in the areas covered by this NOTAM," referring to the "notice to airmen," effective immediately.
Since last week, the plant has been under a "notification of unusual event" classification, becausing of the rising Missouri River. That is the lowest level of emergency alert.
The OPPD claims the FAA closed airspace over the plant because of the Missouri River flooding. But the FAA ban specifically lists the Fort Calhoun Nuclear Power Plant as the location for the flight ban"
Read more: http://www.businessinsider.com/faa-closes-airspace-over-flooded-nebraska-nuclear-power-plant-2011-6#ixzz1Q8Zs5gJ4