I've noticed that quite a few commercial nuclear reactors have an extended outage (they close down for several months) some time late in their third decade, and the maintenance works during the closure involve significant investment. This cost has been high enough in some cases that the reactor has permanently shut down rather than undergo the works (e.g. the Vermont Yankee, Dec 2014) because the business case just didn't stack up.
What are the works involved, in terms of scope and cost; how much do these vary depending on the type of reactor (PWR, BWR, ...)?