What will happen WHEN the nuke plant in Nebraska is flooded by the Mississippi River?

Are you referring to the Cooper plant? Probably nothing. They plan to shut the reactor down when the river reaches a certain level. In Japan, they had no warning of the earthquake and less than an hour before the tsunami hit. In Nebraska, there is no earthquake and they'll have hours, if not days, to prepare. And the current prediction is that it will crest below the level where they would need to shut the plant down.