In the event of a thermo nuclear war, the extreme heat anywhere near the epicenter of a blast would simply vaporize these things.
They will either need to be kept far, far away from a blast….or buried underground or water to avoid those blast furnace temps.
That's why the goal is a thousand or so widely scattered archives, some will be in front of city libraries, some in Appalachian hollers, some on the back 40, some in deep woods, some in disused salt mines, some....