[lumost]

Like any engineering activity it'll depend on how much knowledge of the design the engineering team has. If it's not the same engineering and regulatory teams - then there will be a lot of risk in certifying the plant, little reward, and I'd expect every design decision to be expensively questioned.

Not to mention the hundreds of small decisions related to "X part supplier went out of business. Y part is similar with a slightly different alloy and mechanical properties, is it a suitable replacement? how can we verify this?"

The original design teams would have auxiliary artifacts that were vetted, and tribal knowledge to help quickly answer these questions. Subsequent design teams will not. The timescales between plant construction exceeds most engineer's memory at 5-10 years.