The heavy forged sections of a pressure vessel are really thick and there is a limit to how thick a single weld can be using conventional techniques. So you have to do many passes. Each one of those requires bringing the temperature of the massive forged sections that are being welded up to temperature, cooling it, and then x-raying the partial weld before proceeding with the next cycle. The actual time spent doing the weld is cumulatively not more than a week.