I've been developing a number of questions as I watch the Fukushima news. Any reporters reading this post are free to use them without crediting me. These are what instantly spring to mind. I may have more later.
1. Does the fresh water now being used to cool the reactors and spent fuel ponds have boron added to it?
2. Are the coolant loops been operated in recirculate or open mode? By that I mean whether the water is being recirculated by pumps (new ones have been installed) or whether the water is taken in at one end and pumped out at the other. Or is water added and boiled off?
3. This is less a question than a general wondering about heat transfer issues. Let's say the core(s) were exposed and some damage occurred, perhaps even melting. That would have been at temperatures much above the boiling point of water. Pressure in the vessels makes water boil at somewhat higher temperatures, but adding water to the heated/ing fuel elements made really a lot of steam for a while. Now some of the cores are reported to be partially uncovered by water. But as long as they are partly covered by water, the ambient temperature inside the reactor vessel has to be lower than the boiling point of water at whatever the pressure is. So how much higher than that could the temperature of the exposed parts of the fuel elements go? They're going to be partially cooled by conduction into the parts covered by water.
And I've seen some stuff about temperature layering in the reactor vessel. How about convection? Have the laws of physics been suspended? Generally things have to be pretty stable or big to get temperature layering, as in lakes or the ocean. Temperature layering would be very unlikely if the water is being circulated, or if it's boiling off.