I remain cautiously optimistic that you may be right here.
Cladding damage? Yes, probably. Fuel pellet drop? Maybe some. Fuel pellet melt? Very unlikely. Vaporization? No way.
What happens when you have an entire fuel load sitting in an unconfined, open container, such as the fuel storage pools. If you get enough melting and subsequent reactions going on in these storage areas, couldn't this grow until vaporization begins to occur?
The geometry doesn't favor it. The worst-case geometry is that everything slumps and congeals in a mass with minimal surface area, which would be a sphere. A perfect sphere is unlikely to be formed. But it you use a realistic term for heat source, a perfect sphere isolated in ambient air, the radiative heat loss is such that you don't approach the melting point of uranium oxide, much less vaporization.
If you don't have optimum geometry (non-sphere, or a congealed irregular mass, or puddle), then the surface area is larger and you get greater heat transfer.
You can take further credit for conductive heat transfer since the material is in contact with other things, and also convective heat transfer, which is always operative except in a vacuum. It could be relatively poor, as in a stagnant atmosphere, but it will not be zero. That further reduces the mass temperature.