Nuclear weapon design

Nuclear weapons design means the physical, chemical, and engineering arrangements that cause the physics package 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.
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. The first stage is typically a boosted 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. This process affords potential yields hundreds or thousands of times greater than those of fission weapons.

Pure fission weapons have been the first type to be built by new nuclear powers. 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 technical base and industrial infrastructure are built.
Most known innovations in nuclear weapon design originated in the United States, though some were later developed independently by other states.
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, favor 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.

Fission

When a free neutron hits the nucleus of a fissile atom like uranium-235, the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons. 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 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. The following equation shows one possible split, namely into strontium-95, xenon-139, and two neutrons, plus energy:
The immediate energy release per atom is about 180 million electron volts ; i.e., 74 TJ/kg. Only 7% of this is gamma radiation and kinetic energy of fission neutrons. The remaining 93% is kinetic energy of the charged fission fragments, flying away from each other mutually repelled by the positive charge of their protons. This initial kinetic energy is 67 TJ/kg, imparting an initial speed of about 12,000 kilometers per second. 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, this takes about a millionth of a second, 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, 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 decays may be required to reach stability. 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. 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, "oralloy" meaning "Oak Ridge alloy", or "25" ; and ²³⁹Pu, also known as plutonium-239, or "49".
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 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 and ²³⁸U 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. In weapons, the most important fusion reaction is called the D-T reaction. Using the heat and pressure of fission, hydrogen-2, or deuterium, fuses with hydrogen-3, or tritium, to form helium-4 plus one neutron and energy:
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 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 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, 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

An essential nuclear reaction is the one that creates tritium, or hydrogen-3. Tritium is employed in two ways. First, pure tritium gas is produced for placement inside the cores of boosted fission devices in order to increase their energy yields. This is especially so for the fission primaries of thermonuclear weapons. The second way is indirect, and takes advantage of the fact that the neutrons emitted by a supercritical fission "spark plug" in the secondary assembly of a two-stage thermonuclear bomb will produce tritium in situ when these neutrons collide with the lithium nuclei in the bomb's lithium deuteride fuel supply.
Elemental gaseous tritium for fission primaries is also made by bombarding lithium-6 with neutrons, only in a nuclear reactor. This neutron bombardment will cause the lithium-6 nucleus to split, producing an alpha particle, or helium-4, plus a triton and energy:
But as was discovered in the first test of this type of device, Castle Bravo, when lithium-7 is present, one also has some amounts of the following two net reactions:
Most lithium is ⁷Li, and this gave Castle Bravo a yield 2.5 times larger than expected.
The neutrons are supplied by the nuclear reactor in a way similar to production of plutonium ²³⁹Pu from ²³⁸U feedstock: target rods of the ⁶Li feedstock are arranged around a uranium-fueled core, and are removed for processing once it has been calculated that most of the lithium nuclei have been transmuted to tritium.
Of the four basic types of nuclear weapon, the first, pure fission, uses the first of the three nuclear reactions above. The second, fusion-boosted fission, uses the first two. The third, two-stage thermonuclear, uses all three.