Hiroshima needs billions of criticity masses to happen. Hypercriticity is achieved through violent compression and addition of hydrogen. Getting exactly at critical mass means the fission of a single atom, it produces a ridiculously low amount of energy (3,2 * 10^-11 joule). Critical mass is inversely proportional to density. Uranium and all actinids are highly ductile. The closer the U235/Pu239 atoms are, the less likely it is that a neutron escapes without causing fission. Compression brings the effective mass of the highly enriched uranium (HEU) bullet to millions of kilograms at detonation time. Hydrogen (tritium as well) slows down neutrons so that they spend more time inside fissile atoms, reducing as well critical mass (that's "moderation" for specialists). 6kgs of Pu239, for instance, is the COLD critical mass, without any compression and hydrogenation. So very small HEU bullets are systematically used with all "conventional" missiles and shells. The addition of depleted uranium is a crime against HUMANITY, ecocide and genocide.