Complex sentence: The 2011 Missouri River floods surrounded the plant with flood water, an unprecedented event for a U.S. nuclear power plant due to both the level of flooding and the anticipated period of time the flood is expected to persist.
Split into simpler sentences: The 2011 Missouri River floods surrounded the plant with flood water. The flood is an unprecedented event for a U.S. nuclear power plant not only due to the level of flooding but for the anticipated period of time the flood is expected to persist.