Nuclear Secret

It's called that because if word got out, the world will end in a fiery display of death and misery.

No it's not. This is what someone might consider their "deepest, darkest secret" that could emotionally destroy them if it ever got found out about. This could be the person's Dark Past, a Shameful Habit, or something more sinister like a Secret Plot. If the Nuclear Secret is ever made public, it could cause great humiliation or loss of trust. A Nuclear Secret is more serious than a Deep Secret as the discovery of the latter might not be as damaging to its owner's reputation. Also unlike a Deep Secret, someone may never be comfortable with the idea of others knowing about what they're hiding. However, if they are, they may tell only one person who promises to not tell anybody. For the sake of conflict in the plot, this person will go back on their oath and spill the cat out of the beans. They tell one of their friends who begs to be let in on the big secret. Because the two are friends, that entitles the other to knowledge of the Nuclear Secret.