Hiroshima needs billions of critical 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 (compressible). The closer the U235/Pu239 atoms are, the less likely it is that a neutron escapes without causing fission. Hydrogen (tritium as well) slows down neutrons so that they spend more time inside fissile atoms, reducing as well critical mass (for specialists : "moderation"). 6kgs of Pu239, for instance, is the COLD critical mass, with neither compression nor moderation. So very small HEU/Pu bullets are systematically used with all "conventional" missiles and shells. The addition of depleted uranium is a crime against HUMANITY, ecocide and genocide.