I have made no secret that the graphics suck. I know they aren't great. I'm not an artist and it took me at least 40 hours to get the buildings to look the way they do now, and another 20 hours to make all the icon art. It was the worst part of making this mod. I say this in the sense of "I'm so bad at it that it took me forever to get anything done." As for how to make art, the way that the devs make the art is in Blender. They create a 3D structure, set the camera angle/offest, and then render the .png at the end. The artwork is all PNGs. If you want to have an animation, you need to make a giant PNG where each frame of the animation is in a grid in a single PNG. You can look in the factorio install directory (example: C:\Program Files (x86)\Steam\steamapps\common\Factorio\data\base\graphics\entity\assembling-machine-3) to see what I mean.
There are supposedly blender scripts/addons that will render the frames of the animation into a single PNG for you, but I didn't try to make anything animated. The smoke you see in-game is a separate effect. Also, if you intend for things to have a different color (IE: color coded buildings) make a mask for the parts that are going to be colored (see how barrels are done) to make it easy to apply a tint just to the mask layer and change color that way.
The one thing I would ask is to make it look like a nuclear reactor. For a LWR the primary containment vessel is always pill-shaped (the bottom half is underground) due to the pressures it has to withstand. For LMR / MSR reactors, secondary containment buildings normally have squared corners for the same reasons all industrial buildings are squared: it's cheaper. I don't actually care that the buildings have square corners. I'm just explaining why mine did. :p
The turbine hall is always a separate building from primary containment. That's largely because the turbines are as large/larger than the reactor core itself in most cases. And the rats nest of pipework needed to move the steam/working fluid to/from them, cool it down, heat-exchange it, etc.
The cooling tower shape is also fairly specific because it's designed to work passively. That iconic cooling tower shape is a physics choice, built at the scale they are, it naturally draws air in at the bottom and exhausts it out the top without any need for fans internally. You MUST have a heatsink for a nuclear reactor (the cooling tower is a heat sink). It doesn't have the be the hyperboloid shape we associate with nuclear power, but it does have to be pretty massive. If you don't have a hyperboloid cooling tower, you generally are using a giant body of water to cool your reactor.
So, I guess I'm trying to say, there are 3 buildings for every site. The reactor building, the turbine hall, and the cooling solution. The fence I have around all the buildings is to help place the entities, so you can see where the borders are. I have a different solution for that now, so if your models don't include a perimeter fence, I can easily add a border to them.
