A story in the Washington Post says there was a study showing that a full-speed impact by a commercial airline could penetrate a containment dome. The actual study is not available, for obvious reasons.

There are very simple additional defenses for this. It would be nearly impossible to "dive" a plane into the top of the building, and any successful attack would probably require a full speed run almost horizontally at low altitude. One possibility might be a closely-spaced ring of tall steel towers around the containment building which would wreck a plane and spill its fuel before it could hit the concrete containment unit. Even simpler might be the old "barrage balloon" defenses used during WWII.

This article describes these balloons, and discusses possible modern applications for protecting against low-level attacks.

This article describes barrage balloons with an offensive capability!