While entombment may well be the eventual solution there are several good reasons why it is premature.
1. The piles are still hot and will continue to heat giving off radiation. The concrete does not entomb the reactor underneath just the top.
2. Without first stabilizing and cooling the piles the risk of melt down is not mitigated.
3. The concrete would not stop radiation from leaching into the water table and the ocean resulting in an ongoing source of contamination to the ocean and the surrounding countryside.
4. Without handling the cores and the stored spent fuel rods the concrete will quickly deteriorate. Already the core at Chernobyl is degrading the concrete to the point to where a new cover needs to be put on.
I suggested pumping mud into the thing and then spraying it with corexit. Eventually the only solution will be to bomb the place and let the chips fall where they may.