In my tests there was another BIG factor to watch out for. Magmafication. My testing showed that odds are at least one or two blocks of your bunker are going to get turned into magma in the final seconds, no mater how catastrophic your cooling failure. If on the other hand you are approaching a meltdown extremely slowly, you can easily magmafy a LOT of blocks. So, to avoid this you need to build your bunker outside the magmafication range (greatly increasing the number of needed blocks). My testing was done with a bit older version tho (the one in the tekkit pack for MP) so if they have prevented this in new versions kindly ignore me.