[citation][nom]nexus9113[/nom]The biggest danger of a melt down is not the explosion of the containment chamber, but the possibility of the fuel becoming so hot that it burns through everything it comes into contact with, which will result in what is known as "China Syndrome", a slang term coined by US nuclear engineers meaning that the "rock" can get so hot when unchecked it could melt straight through the planet to China if it didn't hit any cooling material. The issue with that situation is that it is highly likely the "rock" would hit an underground source of water before it got to the other side, instantly vaporizing all the liquid into extremely high temperature steam, increasing the pressure in that pocket exponentially, and creating an explosion of contaminated steam that would jet into the air like an explosion, destroying the area around the meltdown, contaminating the atmosphere, and distributing fallout for an incredible distance (Chernobyl was literally felt around the world to some degree).[/citation]
As, CJL already stated, nothing of what you said has any reality to it.
Molten fuel rod assemblies, while extremely hot and completely unable to melt though the containment shield, simply takes far too much energy. The idea that it would then continue on through the rock surface below is just pure fantasy.
Chernobyl was an EXPLOSION. There was no containment shield and the reactor was opened to the environment. Steam and pressure vented a significant amount of radio-nucleotides into the air which was carried downwind. This is not a very likely scenario in Japan as their NPPs have containment shields.
To date, there has only been one real world test of a containment shield and that was 3 Mile Island, and it performed spectacularly well. A full scale meltdown and the molten fuel assemblies were unable to breach the containment shield.