My understanding is that on commercial plants, they don’t remove the “whole top” rather that there is a mechanized process that removes the old fuel and then inserts new ones, usually from the top down, I think.
According to a former nuc sub cow-irker, they install the reactor through an opening that is later welded shut.
He doesn’t recall any boats that have been refueled. The boat wears out before the initial fuel is used up.
Pretty amazing.