The probability of a meltdown is "only" 1 / 2000. However beeing checked every turn and for evey city with a plant, it can add up...
If a meltdown occurs, exatly the same method (NuclearExplosion) is called, as when a city is hit by a Nuke. Some buildings are destroyed, the troops are dameged and some pop is lost.
A interesting side-effect might be, that Bomb Shelters do reduce the damage your city's population and troop will take from a meltdown.