“I thought the containment vessels were designed for this sort of event (meltdown).”
Nope. A containment vessel is meant to contain the situation if the pressure vessel is compromised, say, if the melted fuel rods puddle in the bottom of the pressure vessel and then melt on through that. Not good if the containment vessel has been breached, because the next thing after that is dirt.
The pressure vessels weren't compromised?