[Errant]

Certainly if heat energy is absorbed by the cladding to the point of cracking or melting, that will carry off a significant amount of thermal energy.

I was referring to the likelihood (if any) that melting cladding and other, would mix with the melting pellets and help to control the reaction. Is that a false assumption on my part?

[chimera]

If you got enough heat to melt the uranium oxide, then for sure the cladding material would melt and do its part in heat absorption and radiant cooling. As far as a quenching-type reaction goes, yes, certainly, there may be some of that. It is likely a complex process on the microscopic level. Zircalloy starts to slump at about 1850 deg. C., with complete melting by 2200 deg. C. Uranium oxide melting is up close to 5000 deg. C.

At this point, with the decay time that has elapsed, it is hard for me to imagine reaching the melting point temperature of uranium oxide, if there ever really was a chance of getting to that, even in the early stages of cooling loss. That is just an incredibly high temperature, and there are just too many mechanisms for heat transfer from the thermal mass. But embrittlement of the cladding, maybe with some cracking and slumping, could very well have occurred. Just like with TMI-2, we won't know until we can snake a camera in there and get a look.