For a nuclear plant, the reactor building is mostly underground, so all you really need is a small building. Make it rectangular. Then, since it is the desert, and you need to cool that reaction down, you'll need the big, parabolic cooling towers most commonly associated with nuclear power.
Another thing you could do is to make a model of a coal fired power plant with a scrubber on the stack. The vapor coming out of the stack looks like pollution, but it is actually just water vapor. Matt's photos show a dirty stack. Believe it or not, there's a lot of sulfur in that shot. [ I take it back. It looks like there might be a scrubber on the stack all the way to the left. However, it is clear that the stack on the right is not scrubbed. See photo below for a recent stack with a scrubber.
This is the pleasant prarie generating station near Kenosha, WI. The stack with the 'smoke' coming out is actually scrubbed so that there is less than 3% sulfur content. The stack on the left is the old stack and I think it was out of service by this photo (Dec 06)