US nuclear Hiroshima

From Asia Times online: (but I'm not sure he reaches that 500,000 Hiroshimas number)
In addition, to the active stockpile of 5,113 warheads, there are somewhere around 8,000 to 9,000 "obsoleted" nuclear weapons - weapons that will not be kept in good repair, but will be dismantled eventually and their 30 tons or so of weapons grade metal recovered; and another 38 tons of military plutonium that has never been fabricated into a weapon.

That's perhaps 10,000 megatons' worth of nuclear metal.

For comparison purposes, Little Boy - the uranium bomb dropped on Hiroshima - was calculated to have a destructive power of less than 18 kilotons.

In other words, the United States has enough weapons-grade metal for 25,000 warheads - or 500,000 Hiroshimas.