gnu-

It would be a combination of a hydrogen explosion and burning of the radioactive fuel. The fuel rod cladding burns first but I believe the fuel itself will burn if it reaches a high enough temperature.

Here's what CNN says about Unit 3:

"That reactor is of particular concern, experts have said, because it is the only one at the plant to use a combination of uranium and plutonium fuel, called MOX, that is considered to be more dangerous than the pure uranium fuel used in other reactors."

Fortunately, unlike Chernobyl, there is no graphite (moderators) to further fuel an ensuing fire.

Charley Noble