And that's NOT a ram-jet. Part of the thrust in a ram jet comes from the liquid fuel suddenly growing much larger as it's vaporized.

Using air directly would mean heating it to thousands of degrees. Then you need a material that can handle such temperatures (which don't exist), and still contain the nuclear fuel. And at those temperatures your fuel is molten.

When you burn liquid fuel - it burns at a thousand degrees or so, but then expands rapidly (and cools), so you don't actually have to contain anything at that temperature.

But if you heat air you do - and that's not easy.

Oh, and the entire point of ram and scram jets, is to provide oxygen without blowing out the flame (which is why it's so hard to do). That doesn't apply here.

Edit: Read this http://en.wikipedia.org/wiki/Nuclear_thermal_rocket and you'll see how hard it is. And they had control of the working fluid, you are proposing plain air.