The sarcophagus would have to include a giant heat sink, and would still need active cooling. Otherwise the pressure and temperature would continue to build.

Above a couple of thousand K concrete loses its integrity. If the main assembly melted at very high temperatures it could eat its down through the core catcher and into the ground water.

That would be a major fail.

To make the idea practical you'd have to use an industrial ceramic - which might not be the easiest thing to build at short notice.