Some reliability engineer calculated the chances of failure of something important which is almost everything on board , probably vs time and what risk is acceptable . Metal fatigue is a bitch . everything is fine one minute and a world of pain the next , and we cannot predict when that minute will happen . Dallas could serve another 50 years and be fine , fissile uranium in the core notwithstanding or could blow ten important pipes on her next deployment . The odds say it ' s her time , according to some Excel spreadsheet somewhere , and that time comes for all things . No ship lasts forever . Except the Constitution . We ' ve still got plenty of oak trees . <| sep |> Thank you it ' s been a long struggle to get here and I am grateful for everything