My (albeit short and 30+ years ago) experience in the nuclear industry as an apprentice would lead me to believe that it's about quality control and inspection as well as it probably being a long-winded task if the pressure vessel is 8 inches thick. I'd think it would need to be entirely welded at that depth, with each weld pass being X-ray inspected.
Everything I did when briefly working in a reactor (3 months of my 4 years) took a LOT of time because of safety checks and routine.