There was an estimated 50 tons of reactor fuel and 800 tons of graphite left in the reactor at Chernobyl after the explosion there.
Fukushima has an estimated 70 tons of fuel in unit 1 and 90 tons of fuel in units 2 and 3. It is not know how much was ejected in
the explosions at Fukushima. So the reactors at Fukushima may contain more fuel in the core, but Chernobyl has a large amount of
graphite mixed in.
Below is a rough drawing to show the concrete thicknesses, potential corium flow paths and structures at Fukushima. The illustrated
volume of corium in the drawing is not intended to be a visual estimate of volume, but is to aid seeing the potential areas corium could
flow. One structure of note is the small sand pit below the torus pipe where it meets the containment bulb. This creates a small dip in
the floor of containment at the edge where the pipe connects and shows a thin spot in containment to the pipe. There is an estimated
10.3 meters of concrete directly below the reactor. The reactor itself is housed in a tube of concrete, the control rod mechanisims are
inside so many penitration holes are likely. The concrete at the bottom of the supression chamber is about 2.7 meters and the side wall
of the supression chamber is about 1.5 meters thick. The sump hole in the bottom of the supression chamber leaves even less concrete
below it.
Image from inside the torus looking at one of the pipes that lead down into the torus from containment.
