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.
An oldie...