A couple of comments. I worked at Watts Bar for 6 years - from just before they restarted construction until 2013. I now work out at one of the new reactors under construction at VC Summer.
First off, WBN2 and WBN1 share structures. Actually, all the structures except for the reactor building itself is shared. The units are what is considered an "opposite hand" configuration, which means that essentially a piece of equipment, piping, or valve on the far west side of the plant for U1 would be on the far east side, at the same northing, for U2 with everything matching up in the middle. The units also share many systems, and in order for them to start up U1, they had to have those systems (and many of the U2 pumps, valves and other support equipment) in service. The units also share a control room, spent fuel pool, diesel generators, and more. The only completely independent structure is the reactor building, which was structurally complete when they halted construction. Most everything inside was complete (major equipment set, piped in, etc). Most of what was lacking were control systems, instrumentation, and some valves. Also, all of this equipment was under temperature and humidity controls during the layup period.
One other thing - all of these structures are reinforced concrete. The unique thing about concrete is they get stronger with age unless you have something like saltwater causing problems. They're also *very thick* and *heavily reinforced* concrete - as in, the age isn't a handicap at all.