I think its power is very overestimated, nukes cannot destroy taht much and be used in that number. Now they are more powerfull than hiroshima and nagasaki times, but search for some documentaries of people that survived both, some of them were like 2-3km away and nothing happened to them, imedialitely or long time because of radiation. States always try to figure it as total and complete destruction and that after it there's killer radiation everywhere.