The China Syndrome is a fanciful “theory” that postulates that when a nuclear reactor undergoes a meltdown, the hot core material melts through the pressure vessel and through the concrete containment flooring below into the ground. All the way to China.
Well, this really can”t happen because the core, hot as it is and dense as it is, could only go to whatever depth matches it’s density. I don’t think many people really understand what happens to core material when it breaches the containment and encounters the subsurface. Further, it’s hard to say if the core material will remain intact as a single unit long enough to retain a critical condition as it spills outside of the reactor vessel assembly.
The fuel elements are, I believe, ceramic in nature, making them refractory. Refractory materials have quite high melting points. A reaction mass that has some fluidity might well split off isolated blebs which could then take the whole mass away from a critical condition. This would tend to dampen the reaction rate and allow the reaction mass to cool below the melting point of the mass.
Quite apart from the dispersion of the core material is the loss of moderator around the reaction mass. This would occur as the primary coolant water flashes to steam as the pressure vessel is breached. Loss of moderator reduces the number of neutrons in the resonant range and the power should drop accordingly. The decay heat from the fission products should be fierce.
As a molten parcel of reactor core heats up the surface material below it, the molten flooring, soil, or bedrock must be fluid enough to allow the core to displace it downward. It could be that the blob gets elongated and increases the surface to volume ratio enough to allow the loss of neutron flux to cause the blob to cool below the melting point of the ground. How a self-heating blob of core material behaves under the pull of gravity in a variable and possibly refractory rocky matrix is not an easy problem.
Ground water would be problematic for the neighborhood because eruptions of contaminated steam would be expected to issue from the crater.
I hope these poor fellows are able to get their reactors under control before the area gets too hot. If the reactor spaces and control rooms get too hot it is going to complicate the remediation.
It is worth reading the updates from NISA.