If a 40 megaton-equivalent hydrogen bomb were to explode, how far would the damage spread?

Writing a story here, and I'm trying to figure out how many nuclear bombs would need to be detonated to eradicate most of the US. So what I want to know is, uming a 40 megaton energy level, what's the radius in miles or km of a) total devastation, b) severe injury that will lead to death within days or weeks, and c) minor damage that isn't immediately fatal (meaning those in the area will survive the attack essentially intact, despite any health problems in years to come). Anybody know? Better yet, how many miles apart would the bombs have to be to essentially obliterate or condemn to destruction everything between them?