Roger,

At times I can be away from my Internet connection for a day or longer. So, responses to notes such as this one can be delayed until I finally see them.

This is not my area of expertise by any means. But,

What happens when a molten core burns its way out of the power plant likely depends on the composition of the molten material.

If it is largely composed of a liquid blob of something like molten radioactive uranium then it might keep going deeper and deeper into the crust until it is finally diluted to the point where it is not hot enough to melt anything.

Or, if it hits a water pocket then the water might keep it solidified enough that it stays in one place.

Other materials could react differently.

Some of the materials in the core could actually burn and produce radioactive gases that head off into the atmosphere.

I believe that the zirconium covers on the uranium pellets in a core can react with water and produce explosive hydrogen. That is what cause the Japan nuclear power plants to explode.

Other types of radioactive gases such as a radioactive version of iodine and perhaps even a radioactive gaseous compound of cesium might also be released into the atmosphere.

Ground water would undoubtedly be contaminated with radioactive materials such as radioactive cesium. And that water would then begin traveling away form the nuclear power plant.

These are personal opinions.