This may be a naive question, but after the Fukushima Daiichi partial meltdown and studying the aftermath of Chernobyl it seems they could be helped by this idea.
In Chernobyl, the liquidators that cleaned up the disaster tunneled concrete under the reactor core and covered the whole complex in a big containment unit. Why could they not have a 30 meter pit below a reactor filled with water with a trap door holding the reactor up? This pit could be very thick reinforced concrete similar to a missile silo If there were a meltdown or imminent meltdown and there were no other options the reactor core could be dropped into this pit by triggering the trap door and then perhaps have lead shot dumped on it or sealed in concrete.
This seems very simple but there must be some reason why this is not practical. I know that the core would still be hot and it would still be a problem but would this not be better than it being exposed to air?
Best Answer
If you read about the Molten-Salt Reactor, you will find that it has something rather similar to this. In the image below, item number 13 is a freeze valve. If the reactor overheats, the plug melts and the molten core flows down to the tanks 10 and 11. This "escape route" would probably be passively cooled by something like atmospheric convection. Of course, this was just one design and we can't speak for all systems that might be used.
In the generation of reactors being built today, AREVA has implemented something called a "core catcher". It is basically a passively cooled sheet of metal at the bottom of the containment building that a molten core would fall on, and then be safely contained and cooled.
My latter example, however, is only a worst-case scenario because if at all possible, we would prefer to not harm the physical integrity of the fuel. The rods of present-generation nuclear fuel hold in radioactive gases at a pressure significantly above sea-level atmosphere pressure. If many of the rods break, then that is still a core failure, and will release lots of radioactivity. Because of our reliance on the physical integrity of a small thickness of heavily irradiated metal cladding, we really can't move it except under controlled conditions and very slowly.