Implosion-type nuclear weapon
Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three existing basic design types. In many existing designs, the explosive energy of deployed devices is derived primarily from nuclear fission, not fusion. (This includes fission of any U-238 tamper or casing due to fusion produced neutrons.)
Pure fission weapons historically have been the first type to be built by a nation state. Large industrial states with well-developed nuclear arsenals have two-stage thermonuclear weapons, which are the most compact, scalable, and cost effective option once the necessary industrial infrastructure is built.
Most known innovations in nuclear weapon design originated in the United States, although some were later developed independently by other states; the following descriptions feature U.S. designs.
In early news accounts, pure fission weapons were called atomic bombs or A-bombs, a misnomer since the energy comes only from the nucleus of the atom. Weapons involving fusion were called hydrogen bombs or H-bombs, also a misnomer since their energy comes mostly from fission. Practitioners favored the terms nuclear and thermonuclear, respectively.
The term thermonuclear refers to the high temperatures required to initiate fusion. It omits the equally important factor of radiation pressure, which was considered secret at the time the term became widespread. Many nuclear weapon terms similarly obfuscate because of their origin in a classified environment.
Nuclear fission separates or splits heavier atoms to form lighter atoms. Nuclear fusion combines together lighter atoms to form heavier atoms. Both reactions generate roughly a million times more energy than comparable chemical reactions, making nuclear bombs a million times more powerful than non-nuclear bombs, which a French patent claimed in May 1939.
In some ways, fission and fusion are opposite and complementary reactions, but the particulars are unique for each. To understand how nuclear weapons are designed, it is useful to know the important similarities and differences between fission and fusion. The following explanation uses rounded numbers and approximations.
When a free neutron hits the nucleus of a fissile atom like uranium-235 (235U), the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons. Fission can be self-sustaining because it produces more neutrons of the speed required to cause new fissions.
The U-235 nucleus can split in many ways, provided the atomic numbers add up to 92 and the atomic weights add to 236 (uranium plus the extra neutron). The following equation shows one possible split, namely into strontium-95 (95Sr), xenon-139 (139Xe), and two neutrons (n), plus energy:
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