Nuclear weapons design means the physical, chemical, and engineering arrangements that cause the physics packageThe physics package is the nuclear explosive module inside the bomb casing, missile warhead, or artillery shell, etc., which delivers the weapon to its target. While photographs of weapon casings are common, photographs of the physics package are quite rare, even for the oldest and crudest nuclear weapons. For a photograph of a modern physics package see W80. of a nuclear weapon to detonate. There are three existing basic design types: Pure fission weapons are the simplest, least technically demanding, were the first nuclear weapons built, and so far the only type ever used in warfare, by the United States on Japan in World War II. Pure fission weapons have been the first type to be built by new nuclear powers, as a step toward building more advanced weapons. Boosted fission weapons are fission weapons that use nuclear fusion reactions to generate high-energy neutrons that accelerate the fission chain reaction and increase its efficiency. Boosting can more than double the weapon's fission energy yield. Staged thermonuclear weapons are arrangements of two or more "stages", most usually two, where the weapon derives a significant fraction of its energy from nuclear fusion (as well as, usually, nuclear fission). The first stage is typically a boosted fission weapon (except for the earliest thermonuclear weapons, which used a pure fission weapon). Its detonation causes it to shine intensely with X-rays, which illuminate and implode the second stage filled with fusion fuel. This initiates a sequence of events which results in a thermonuclear, or fusion, burn. High-energy neutrons from the fusion reactions can be (and usually are) used to produce an additional large number of fission reactions, which can double the already-large yield of the weapon. This process affords potential yields hundreds or thousands of times greater than those of fission weapons, and to develop weapons that are relatively compact for their yield.. Article on the Soviet Tsar Bomba test. Because explosions are spherical in shape and targets are spread out on the relatively flat surface of the earth, numerous smaller weapons cause more destruction. From page 35: "... five five-megaton weapons would demolish a greater area than a single 50-megatonner." Around 80% of the nuclear weapons deployed in the world today are thermonuclear weapons, as their ratio of size and weight to their energy release allows them great flexibility in deployment on bombers and in missile warheads.The deployed nuclear arsenal of the United States is believed to be comprised almost exclusively of two-stage warheads (B61, W76, W78, W80, W87, and W88), with a small number having the fusion secondary stage removed, and some (like the B61) having the option to prevent the secondary stage from firing (see dial-a-yield). At least 65% of Russia's deployed nuclear arsenal is believed to be composed of multi-stage strategic weapons, with the rest being non-strategic weapons, some of which may be fission-only. The nuclear weapons deployed by France, China and the United Kingdom are all of yields and types that signal that they are multi-stage weapons. Together these make up around 80% of all deployed global nuclear weapons as of 2025.
The United States pioneered most major nuclear weapon design innovations, but many were either subsequently developed independently by other nuclear states, or were obtained through espionage, gift, or sale.
In early news accounts, pure fission weapons were called atomic bombs or A-bombs and weapons involving fusion were called hydrogen bombs or H-bombs. Practitioners of nuclear policy, however, have favored the terms nuclear and thermonuclear, respectively.
Nuclear reactions Nuclear fission separates or splits heavier atoms to form lighter atoms. Nuclear fusion combines 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.The main source for this section is Samuel Glasstone and Philip Dolan, The Effects of Nuclear Weapons, Third Edition, 1977, U.S. Dept of Defense and U.S. Dept of Energy (see links in General References, below), with the same information in more detail in Samuel Glasstone, Sourcebook on Atomic Energy, Third Edition, 1979, U.S. Atomic Energy Commission, Krieger Publishing.
Fission
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 (for 235U three about as often as two; an average of just under 2.5 per fission). The fission chain reaction in a supercritical mass of fuel can be self-sustaining because it produces enough surplus neutrons to offset losses of neutrons escaping the supercritical assembly. Most of these have the speed (kinetic energy) required to cause new fissions in neighboring uranium nuclei.
The uranium-235 nucleus can split in many ways, provided the atomic numbers add up to 92 and the mass numbers add up to 236 (uranium-235 plus the neutron that caused the split). The following equation shows one possible split, namely into strontium-95 (95Sr), xenon-139 (139Xe), and two neutrons (n), plus energy:Glasstone and Dolan, Effects, p. 12.
The immediate energy release per atom is about 180 million electron volts (MeV); i.e., 74 TJ/kg. Only 7% of this is gamma radiation and kinetic energy of fission neutrons. The remaining 93% is kinetic energy (or energy of motion) of the charged fission fragments, flying away from each other mutually repelled by the positive charge of their protons (38 for strontium, 54 for xenon). This initial kinetic energy is 67 TJ/kg, imparting an initial speed of about 12,000 kilometers per second (i.e. 1.2 cm per nanosecond). The charged fragments' high electric charge causes many inelastic coulomb collisions with nearby nuclei, and these fragments remain trapped inside the bomb's fissile pit and tamper until their kinetic energy is converted into heat. Given the speed of the fragments and the mean free path between nuclei in the compressed fuel assembly (for the implosion design), this takes about a millionth of a second (a microsecond), by which time the core and tamper of the bomb have expanded to a ball of plasma several meters in diameter with a temperature of tens of millions of degrees Celsius.
This is hot enough to emit black-body radiation in the X-ray spectrum. These X-rays are absorbed by the surrounding air, producing the fireball and blast of a nuclear explosion.
Most fission products have too many neutrons to be stable so they are radioactive by beta decay, converting neutrons into protons by throwing off beta particles (electrons), neutrinos and gamma rays. Their half-lives range from milliseconds to about 200,000 years. Many decay into isotopes that are themselves radioactive, so from 1 to 6 (average 3) decays may be required to reach stability.Glasstone, Sourcebook, p. 503. In reactors, the radioactive products are the nuclear waste in spent fuel. In bombs, they become radioactive fallout, both local and global.
Meanwhile, inside the exploding bomb, the free neutrons released by fission carry away about 3% of the initial fission energy. Neutron kinetic energy adds to the blast energy of a bomb, but not as effectively as the energy from charged fragments, since neutrons do not give up their kinetic energy as quickly in collisions with charged nuclei or electrons. The dominant contribution of fission neutrons to the bomb's power is the initiation of subsequent fissions. Over half of the neutrons escape the bomb core, but the rest strike 235U nuclei causing them to fission in an exponentially growing chain reaction (1, 2, 4, 8, 16, etc.). Starting from one atom, the number of fissions can theoretically double a hundred times in a microsecond, which could consume all uranium or plutonium up to hundreds of tons by the hundredth link in the chain. Typically in a modern weapon, the weapon's pit contains of plutonium and at detonation produces approximately yield, representing the fissioning of approximately of plutonium.
Materials which can sustain a chain reaction are called fissile. The two fissile materials used in nuclear weapons are: 235U, also known as highly enriched uranium (HEU), "oralloy" meaning "Oak Ridge alloy", or "25" (a combination of the last digit of the atomic number of uranium-235, which is 92, and the last digit of its mass number, which is 235); and 239Pu, also known as plutonium-239, or "49" (from "94" and "239").
Uranium's most common isotope, 238U, is fissionable but not fissile, meaning that it cannot sustain a chain reaction because its daughter fission neutrons are not (on average) energetic enough to cause follow-on 238U fissions. However, the neutrons released by fusion of the heavy hydrogen isotopes deuterium and tritium will fission 238U. This 238U fission reaction in the outer jacket of the secondary assembly of a two-stage thermonuclear bomb produces by far the greatest fraction of the bomb's energy yield, as well as most of its radioactive debris.
For national powers engaged in a nuclear arms race, this fact of 238U's ability to fast-fission from thermonuclear neutron bombardment is of central importance. The plenitude and cheapness of both bulk dry fusion fuel (lithium deuteride) and 238U (a byproduct of uranium enrichment) permit the economical production of very large nuclear arsenals, in comparison to pure fission weapons requiring the expensive 235U or 239Pu fuels.
Fusion
Fusion produces neutrons which dissipate energy from the reaction."neutrons carry off most of the reaction energy", Glasstone and Dolan, Effects, p. 21. In weapons, the most important fusion reaction is called the D-T reaction. Using the heat and pressure of fission, hydrogen-2, or deuterium (2D), fuses with hydrogen-3, or tritium (3T), to form helium-4 (4He) plus one neutron (n) and energy:Glasstone and Dolan, Effects, p. 21.
The total energy output, 17.6 MeV, is one tenth of that with fission, but the ingredients are only one-fiftieth as massive, so the energy output per unit mass is approximately five times as great. In this fusion reaction, 14 of the 17.6 MeV (80% of the energy released in the reaction) shows up as the kinetic energy of the neutron, which, having no electric charge and being almost as massive as the hydrogen nuclei that created it, can escape the scene without leaving its energy behind to help sustain the reaction – or to generate x-rays for blast and fire.
The only practical way to capture most of the fusion energy is to trap the neutrons inside a massive bottle of heavy material such as lead, uranium, or plutonium. If the 14 MeV neutron is captured by uranium (of either isotope; 14 MeV is high enough to fission both 235U and 238U) or plutonium, the result is fission and the release of 180 MeV of fission energy, multiplying the energy output tenfold.
For weapon use, fission is necessary to start fusion, helps to sustain fusion, and captures and multiplies the energy carried by the fusion neutrons. In the case of a neutron bomb (see below), the last-mentioned factor does not apply, since the objective is to facilitate the escape of neutrons, rather than to use them to increase the weapon's raw power.
Tritium production