Meltdown = casing around the fuel rods melts, fuel concentrates in the bottom of the core. It is in close proximity then and gets MUCH hotter.
From there it depends. Worst case scenario:
Starts boiling off the water causing a steam buildup and explosion in the reactor vessel throwing a plume of radioactive debris into the sky like chernobyl. Burns through the bottom of the core, melts into the bedrock till it hits groundwater.
Bigger steam explosion. More radioactives into the air...