This looks like a more hardware focused job listing, so if you have a typical CS background you probably lack some of the expected skills for it. Note the mentions of RTOS, embedded, hardware schematics/bring up, and ARM core knowledge.

I wonder if largest meant “most people in a building”?

The trick with the economics of nuclear is that building it is the main cost, once the plant is operable the cost is much lower than other sources. So by the time you have a working plant, you might as well use it to get value out of it since you have already paid for it.

It is not, these control rooms are rather central to the plant. If I am not mistaken this is the reactor 4 panel, and immediately to the right of this image is the original concrete sarcophagus. The plant still has electrical power for some of the monitoring stuff, and the lights are still on (at least last I knew about) for maintenance and tour groups.

Here is another angle with the lighting and concrete visible. As you can tell, the concrete on the left is not the same type as the ceiling.

The control rooms for reactors 1-3 are still in pretty decent shape, as they were operating much after the meltdown. There are also schematics available online if you want to see how these rooms are positioned within the plant.

Java/C# would have types before the variables:

double sum = 0d;
for (int i = 0; i < 4; i++)
	sum += 3 * i;