In the U.S. when they build submarines, they build sections of hull (pipe Shapes) and populate them with all the internal equipment, then join the pipe shapes and connect everything together, welding the hull into one piece. When they retrofit a sub, and refurb the reactor, they don't separate and break it apart like they did in the video of the ship being extended that is hyperlinked above. Instead, they cut the internal equipment pieces up and fish them out through the normal access holes, and then bring in the replacement parts and weld them in. Due to the cramped spaces involved, they employ very small people and dwarves to get into the tight places to reweld the new equipment, sometimes having to hold mirrors so they can see around a corner to do the welding around the corner.