The casks are too thick to be penetrated by civilian firearms. They could be further protected from man-portable weapons (e.g. anti-tank guided missiles) by loc

The casks are too thick to be penetrated by civilian firearms. They could be further protected from man-portable weapons (e.g. anti-tank guided missiles) by locating them shallowly underground, where monitoring/maintenance is still easy. Military attack on the storage site with heavy weapons? That's approximately equal to the case of a military attack on a live power reactor, with the same dire consequences and responses. (If a shooting war has escalated to war crimes against nuclear powers, I expect that MAD is just around the corner.)