I'm not an expert or even barely knowledgeable about how nuclear power plants are designed. While I protested against Watts Bar (which is near enough to me where if it had a meltdown I'd have to evacuate), most of what I'm learning about modern plants is based on reporting on the mostly-new plant in Japan undergoing several critical failures. But I do know how these plants work at the most basic level.
As I understand it, the plant in Japan is wholly reliant upon some form of power being available to keep the core from overheating. Pumps, mechanisms and the like were supposed to be powered by diesel generators in case of catastrophe. Except, in case of REALLY BIG catastrophes, this is a tough gambit. Why not rely on the one thing that will never fail -- gravity.
By designing a plant where rods will *fall away* from each other should power be cut, those rods will not be close enough to overheat. Seems like a simple enough idea. I'm sure engineers will claim it's tough to do, but I have faith in human ingenuity. I've been lucky enough to witness the impossible many times in my short life.
UPDATE: The very smart Richard Gaywood (check out his awesome food blog here) noted that nuclear engineers already use gravity to drop boron rods into the core, stopping fission. As he explains: "the problem comes from residual heat and decay heat, neither of which depends on the fuel rods being close to each other."
So... back to solar, you damn hippies!
one man's journey into creating gibblybits