TEPCO thinks graphite melts? It vaporizes. At what temperature? Over 7,0000F degrees. Unless they know something else was going on inside Unit 3's reactor vessel (higher pressures, etc) they are still guessing to what happened.
Mock setups for meltdowns show fuel melts and collects in a blob to heat up the bottom of the reactor vessel then blasts out the bottom all at once with the help of overpressure from off-gassing. The iron bottom of the reactor vessel is going to glow red and give way long before any graphite does.
Unit 2 shows a hole in the grating from a hot blob passing thru. I suppose outer fringes of the reactor bottom might survive with some control rods and drive equipment intact below.
Simplyinfo has a pic of Unit 5's pedestal area in pristine condition to compare against.
From what I read, after Unit 4 blew, then Unit 3 pressures dropped. Possibly showing they might have been linked via tunnels and piping.
Doubt Unit 4's pool was ever hot enough to meltdown so melted fuel fallout releases were less possible. Might have been a lot of off-gassing (rod tube pressure failure) and some zirconium reactions there but not a lot of irradiation like at Unit 3 (neutron bombardment).
Still would like to know where all the scattered fuel pellets came from.
Unit 3's spent fuel condition will be hidden with the new cover being put in place.