It's an interesting approach, freezing the ground has been used for quite a long time in large projects(Léon Chagnaud did in the early 1900's to build tunnels through the water logged banks of the Seine (if you want an interesting story about large scale civil engineering that one has it all, the stations where built on the surface then "sunk" to position and effectively where designed to be water tight while the tunnels where built by sinking caissons in and then connecting them, incredible work))
So what would the consequences be of letting the core just melt and eat its way downwards? It's not as if the environment around the plant and seawaters hadn't had a hefty portion of radioactive waste already and the structural damage hadn't happened. A quick look at Wikipedia tells me that in most nuclear accidents the worst had already happened before the actual meltdown.