What many people don’t realize is that when a ship is built its entire service life is planned out. For military vessels this includes possible upgrades and major maintenance.

Here is an image for the USS Enterprise at the Newport News Shipyard as it was being prepared for decommissioning.

Here is a closer view of the important part.

Here is a image of the USS George Washington currently undergoing a Refueling and Complex Overhaul.

Note the square hatches along the center-line of the flight deck. Those are the reactor access points. They are not permanent hatches, but are areas designed to be removed and replaced in a shipyard. They are cut open for maintenance then welded closed when the work is done.

The important information to note is that the path between the flight deck and reactor compartment has no significant structure in it. The ship is designed for the plates for each deck to be removed to open the access shaft.

It is very common for ships to have portions temporarily cut out during heavy maintenance.

The Enterprise was built with eight reactors because they were essentially used to replace the boilers in the existing conventional carrier design.

This added enough to the cost that the Nimitz and later designs only use two larger reactors. This complexity has added significantly to the cost of disposal of the Enterprise.