Is it possible to know how long a nuclear reactor has been running in Civ 6?
Solution 1:
Yes, it is.
When you move your mouse over the "Recommission Nuclear Reactor" option, you'll see a tooltip, where the "Reactor Age" in turns can be seen.
Note that recommissioning a Nuclear Reactor will reset its age to 0.
In this case, the reactor's been running for 84 turns since it's been built or recommissioned.