If you have 20,000 welds that each need to last 30 years without being fixed later, that's an order of magnitude difference in quantity than the corresponding 2,000 for a coal boiler, in addition to a few orders of magnitude difference in the necessity of lasting a long time without repair.

It's a hell of a lot more than a steam boiler, even if ultimately that's the goal. The nuclear island is a hell of a beast of complexity, size, and quality.