Every modern nuclear weapon, from the smallest tactical device to a thermonuclear warhead hundreds of times the yield of Hiroshima, is still doing the same thing the 1945 bombs did: assembling a critical mass of fissile material suddenly enough to release a runaway chain reaction in about a millionth of a second. The basic designs are also unchanged. The Hiroshima bomb, “Little Boy,” used a gun-type barrel to fire a subcritical mass of uranium-235 into a target ring; the Nagasaki bomb, “Fat Man,” used precisely shaped explosive lenses to implode a plutonium core to more than twice its normal density. Modern weapons still use gun-type or implosion-type primaries — only the scale has grown.
The basic physics is nuclear binding energy. A heavy nucleus like U-235 is, on balance, less stable than the mid-mass fragments into which it splits. When a free neutron is absorbed, the nucleus deforms and breaks apart, releasing heat, two or three new neutrons, and gamma radiation. If the geometry is right and the mass is large enough, the second generation produces more neutrons than the first, chain runs away in roughly a microsecond. The smallest configuration that achieves runaway is the critical mass — about 52 kg for a bare sphere of U-235, 10 kg for Pu-239. See the Manhattan Project and the atomic bomb.
The Hiroshima bomb, “Little Boy,” used a gun-type assembly: a subcritical “bullet” of U-235 fired down a barrel into a subcritical target ring. About 64 kg of enriched uranium was in the weapon, but only about 1.4% underwent fission. The Nagasaki bomb, “Fat Man,” used implosion: 32 precisely shaped explosive lenses compressed 6.2 kg of plutonium to roughly 2.5 times normal density, achieving criticality in a much smaller mass. The Hiroshima yield was about 15 kilotons; Nagasaki about 21. Both bombs killed far more people from fire, blast, and acute radiation injury than from fission products released.
The physics has not changed since 1945. Fission bombs are still either gun-type or implosion-type. Thermonuclear weapons use a fission primary to fuse hydrogen isotopes, releasing energy in megatons. The destructive arithmetic is the same; only the scale has grown.