Complex sentence: The concept core would use a Super Scale Transport rocket to put a Mars transit vehicle into orbit, this would use NERVA type nuclear rockets to allow a fast low radiation trajectory to Mars and allow an equally fast return.
Split into simpler sentences: The concept core would use a Super Scale Transport rocket to put a Mars transit vehicle into orbit. The MTV would use NERVA type nuclear rockets to allow a fast and thus low radiation trajectory to Mars, and allow an equally fast return.