It would take not even a small nuke to blast away the hatch in the floor, let alone a 20 megaton hydrogen bomb. the bunker it covers would not be able to protect anybody inside from such an explosion even if the weapon missed it by miles, especially not if it was a surface detonation which would create a massive crater and shock waves like an earthquake.
None of the entire convoluted backstory explains why "Reddington" dragged Liz into his deal with the FBI.
The entire convoluted backstory fails to explain why "Reddington" did not simply reveal to Liz that the woman he shot in front of her was not actually Liz' mother. He would not even had to tell her who she actually was, or revealed much of the backstory. All it would have taken would have been to make Liz take a DNA sample from her for comparison with Liz' own DNA. It would have ended Liz' trauma about Reddington killing her mother and avoided sending Liz on her revenge-fueled rampage, at zero extra risk and next to no effort. Liz would have had further questions but "Reddington" has never been bothered by refusing to answer any of them.