you'd have to use an industrial ceramic

And you would have to get that ceramic UNDER the "core catcher". The "core catcher" will only hold the core if it is dispersed far below criticality and can cool below the temperature that will degrade the concrete. Covering it all with a solution of boric acid will help greatly. I would feel better if the PR idiots at GE and TEPCO could figure out that it is pointless to refuse to talk about meltdown containment when meltdown is occurring and actually release some diagrams that would clearly confirm that they actually do have a working passive containment system.  

"It is not necessary to have hope in order to persevere."