Complex sentence: Little information about "Narwhal" 's career is available, but it was not uneventful and included a very heavy deployment rate interrupted only by three overhauls (two involving reactor refueling).
Split into simpler sentences: Little information about "Narwhal" 's career is available. It was not an uneventful career; it included a very heavy deployment rate interrupted only by three overhauls (two involving reactor refueling).