Zirconium is what surrounds the fuel.  It is used because it is strong at operating temperatures (the fuel, though formed into a ceramic, is not strong) and has low neutron absorption and therefore does not effect or interfere with the fission reactions.  The fuel rods are hollow tubes of zirconium alloy with pellets of fuel inside.  

Zirconium oxidizes spontaneously.  Normally this is good, as the oxide sticks to the zirconium as a tough protecting layer, much as aluminum instantly oxidizes to form the tough layer which is actually what you are looking at when you look at aluminum.  

But.  At high temperature the oxide of zirconium flakes away and the zirconium keeps burning.  Also, the affinity of zirconium for oxygen is so fierce at high temperatures that zirconium will pull the oxygen out of steam (vaporized water) leaving hydrogen which will burn as soon as it meets more oxygen--which is presumably what fueled the several large explosions of reactor buildings.  

At the "low" temperatures of normal operation (above the boiling point of water but well below the melting point of zirconium) ziconium is benign and stable, but at the high temperatures of a meltdown it is malign and burns uncontrollably.  

Both zirconium and hydrogen release large amounts of heat when they burn.  

The Fates are kind.