It's not the cube that counts
It's all the effort you have to go through to create that cube from tons of very nasty fuel elements.
I'm not opposed to nukes, I just think it's essential to do this first, and, you're right, we need to get started immediately. As it is, if we start building both steam plants and fuel processing facilities right now, it will probably take us a century before the waste fuel elements start to decrease in number.
Jimmy Carter is probably one of the few men in government who really understood all this at an engineering level, I've always wondered what he could have possibly been thinking. That Plutonium scare was a smokescreen for something else.