I should stay out of this discussion but there are a few things that I will say. The disaster in Japan has torn at everyones heart in the last few days and I am no exception. The force of the tidal wave (yes,I know it has a different name now)was beyond belief! God only knows what the final count of those lost will be but it is going to be in the tens of thousands from direct effect of the quake alone. As for the power plants in question if there is an emission from radioactive debris like cesium many more lives could be lost spanning decades to come.
I am no scientist but I do not believe that it is possible to create any atomic blast from fuel grade enriched uranium melted down or otherwise. I say this because this grade of refinement of uranium is not pure enough to create an uncontrolled chain reaction like a bomb. In order for a reactor to attain fission many fine tolerances must be met and in order to create fission of uranium of less than about 85% U-235 neutron bullets must be slowed down in order to gain a collision with its neighbour atom. These requirements do not happen by chance but require intentional input.
That being said any chemical based explosion releasing debris into the air is no trivial concern! I do however believe that to run around like Chicken Little proclaiming an impossible situation is irresponsible.
I am at a bit of a loss to comprehend why Japan would ever allow a light water enriched grade reactor on its soil. The plutonium by-product has attraction for nations wanting that element for weapons but Japan has no nuclear weapon use or manufacturing ability.
The CANDU system mentioned earlier has less volatility because it uses natural uranium which requires no enriching centrifuge and can be quickly shut down by inserting dampening rods into the reactor core as well as by replacing the deuterium bath with light water. I believe that the system is fail-safed to do this automatically in an emergency. It is also my understanding that a large pool of light water sits above the reactor core ready to spill in in an emergency situation as well requiring no powered pumps.
Many things in nuclear physics are beyond my feeble comprehension but I do understand panic!
               Sandy