@tsarstepan,
15 megatons is unrealistically big for a modern nuke. Although China still has ten 5 megaton nukes aimed at the US.
As far as third degree burns go, if you are in a zone where people get third degree burns if they are in a direct line of sight to the fireball, and windows are all shattered and blow violently inward, Duck and Cover is a Very Good Idea.