tornado missiles, tidal waves or floods, earthquakes, terrorism, deliberate airplane crash, foundation design and seismic response
thicker steel, more concrete, different sizes 5-Mw to 300 Mw, different manufacturers, etc.
If its in the same spot. Seabrook Station in NH was stopped after Unit-1 was completed, the Unit-2 foundation is there, look at the wiki in post #9, should be easy to put Unit-2 on, but I'm sure it will be difficult. Back to the bottom of post #9 and see that Indian Point was closed by governor Cuomo, see how easy it would be to turn it back on. The NRC does their reviews, it takes what it takes.
...and it takes time during construction, we don't want to cut corners, remember?