Here's an excerpt from the Union of Concerned Scientists daily briefing (I urge everyone interested to review each transcript) which emphasizes their continuing concerns:

"The IEA and others have reported that Units 1, 2, and 3 have had their cores at least or approximately half uncovered, and this has probably been the case since shortly after the initiation of the accident. So, it's been at least several days that at least 50 percent or the upper half of these cores have been uncovered. Now, that's a longer period of time for the situation than the core at Three Mile Island experienced, and the upper parts of the cores may have experienced significant damage. What may have happened is a swelling of the fuel rods that could cause a potential -- reduce the ability of coolant to actually get between those rods if the core is reflooded. So, the situation may not be -- these cores may not be as easily cooled as they would have been if they were undamaged.

Another issue that may have come up is the embrittlement of the zirconium cladding. When the zirconium becomes uncovered, after it expands and balloons, it can become oxidized and eventually becomes embrittled. If the core is then reflooded with water, that could cause cracking of the zirconium and release of fuel particles in what's called the debris bed. This was seen at Three Mile Island. The debris could then sink to the bottom of the reactor vessel, where it might then begin an attack on the steel of the reactor vessel. Also, it makes the configuration of the core, again, harder to cool.

One should also note that if part of the core fuses into a single mass, that it is, again, harder to cool the entire mass, and even if you get coolant on the outside of this mass, the central line may continue to heat up and eventually liquefy at the center and then drip down, again, to the bottom of the vessel. So, the fact that the cores have experienced some damage raises questions about the efficacy, even if the cooling is restored and they're reflooded."

They haven't posted the transcript of today's briefing but it should be available later today.

Charley Noble