Comment by Animats

This is strange. There is no "containment vessel". That's the reactor pressure vessel. Usually, there's an outer containment vessel as well. Chernobyl didn't have one at all, which is why that was such a disaster. Fukishima's reactors had ones that were too small and didn't have enough expansion volume to diffuse the pressure after loss of coolant. There's a design assumption here that there will never be a meltdown.