This caught my eye: In some non-nuclear tests the weapon's exterior casing has melted from the friction. In tests to date, weapons have penetrated only a few dozen feet.
Seems like something like kevlar encasing the bomb's outer stucture with a super sprayed on Teflon material might work. Teflon coated bullets basically run out of energy instead of being stopped by a barrier when they strike a target.
When,we retrofitted our Church a few years ago for the new earthquake codes, the new outer shell may be more important to saving the structure in an earthquake than the new steel supports we added.