why not just let the thing meltdown? It would essentially bury the fuel. After it drops down a 1000' or so, fill the hole in with cement. I wouldn't be too worried about volcanic eruptions, radiation is what keeps the earth core nice and soft.
The most important reason is that 'corium' isn't actually hot enough to burn through the earth like that, nor does it conduct heat all that well, even if any part of it became hot enough.
The integrity of the fuel rods is challenged at 2200F (zircaloy-water reaction, which released the hydrogen that caused the reactor building roofs to blow off on three of the Daichi units.)
Steel melts at about 2600F. Concrete breaks down at about 1800F.
In addition, the fuel is a uranium-oxide mix, a sort of ceramic. This class of material is generally known for poor thermal conductivity. That's why the pellets are the size of a pencil eraser, they need to be small and have a high surface area in order to conduct heat from the center of the pellet- which might be at 3000F in normal operations- to the fuel cladding and into the reactor coolant, which might be around 600F.
Anyway, from what I know about western reactors (it's my line of work, but i'm not a reactor engineer per se), I seriously doubt the fuel would 'burn' through steel or concrete. The fission products escape because of physical destruction to the facility caused by the Tsunami, or because of relief valves that limit reactor coolant system pressure, or primary containment structure pressure.
Chernobyl's release was due to a massive overpressure event that physically broke the reactor vessel. Nothing ever burned through concrete (check out the photos of the 'elephant's foot')
Three Mile Island's core was found in the bottom of the reactor vessel; a small amount of fission products was released by mis-operation of support systems. The integrity of the reactor vessel was never threatened, though the containment building (much larger than Daichi primary containment structures) withstood several hydrogen burns.