Not just the reactors themselves, but with a combination of other buildings. The immediate area around the reactors are not always as open as you might think, depends mostly on the plant and its location.
Assuming players wouldn't be able to get on top of the reactors/buildings and see everything, even something fairly simple as the following could suffice, although modifications to suit two teams would obviously be needed.
Using the above screenshot, maybe
something like this. Making the starred buildings the tallest (apart from the reactors) would arguably provide a fair amount of blockage.
Obviously, this is just an example, so don't take this specific layout so seriously.