“But we do know that the corium was and may still be eating its way through the concrete core catcher.”
But (unless I’m misunderstanding) the article admits (I say ‘admits’ because it’s dramatized in a way that tortures the truth, i.e., “China Syndrome”) that the rods are still contained in the steel structure, ie, they are not in contact with the concrete superstructure.
In #1 the rods and the RPV internals have almost completely melted and have dripped out of the RPV. Actually under pressure it can actually partially jet out the bottom of the RPV. That molten mass is now eating the drywell floor and has yet to get to the last steel wall which is called the dry wall. There is concrete on the dry well floor above the dry steel wall. That is the concrete that Tepco admits the corium (melted rods and stuff) is eating through.
Now that Tepco has admitted a significant amount of concrete floor erosion, there is another problem that must also be occurring. Gets much more technical and has to do with the drywell sumps and suppression chambers. Basically the corium mass could have flowed across the drywell floor and down the downcomers into the suppression pools. That is where the wet wells are located. That can cause a steam explosion and may have been what happened in Unit #2.