What does "nuclear-hardened" mean?
Nuclear hardened means able to withstand the nearby detonation of a nuclear device. I used to work* in such a building, part of the USAF SAGE program of the 1950s & 60s: https://en.wikipedia.org/wiki/Semi-Automatic_Ground_Environment The walls & floors were about 2-3 ft of reinforced concrete, intended to withstand a nuclear detonation on the airfield about 1/4 mile away.
*Not when it was in active military use, but some decades later, when it had been converted to an academic research facility.