Refueling involves taking the expended core out of the reactor and putting in a new core with fresh nuclear fuel. They also replace the coolant water which is also contaminated. The have to cut through the hull to the deepest most protected part of the ship. And they are doing this into two different areas of the ship, one for each reactor.

Because the Gerald R Ford class is such a new design they will be building two more Nimitz class before they build the next Ford class. This gives them time to see what works and what needs to be improved. They did this with the first nuclear carrier, U.S.S. Enterprise CVN 65, which had 8 nuclear reactors compared to the Nimitz's two.