The Russian and American nuclear weapons which are currently active, are much more powerful than the ones which were dropped over Hiroshima and Nagasaki. The Hiroshima bomb killed people within a 10 sq. km area (around 2,500 acres). But an average Russian / American nuclear weapon (current) can wipe off life from at least 100 sq. km of area. If some one detonates 10,000 nukes, then that will be capable of wiping out life from the entire continental USA + Alaska.