Nuclear weapon design

Nuclear weapon design involves the physical, chemical, and engineering arrangements required to detonate a physics package. The physics package is the core explosive module located inside a bomb casing, missile warhead, or artillery shell. While the outer casings are often photographed, the internal physics packages are rarely seen in images. These designs focus on managing nuclear reactions to release energy. These reactions are far more powerful than chemical ones. In fact, nuclear reactions can be one million times more powerful than comparable chemical reactions.

There are three primary design types used in nuclear weapons. The first is the pure fission weapon. These are the simplest designs and require the least technical demand. They were the first nuclear weapons ever built. The United States used this type during World War II. The second type is the boosted fission weapon. These use nuclear fusion reactions to create high-energy neutrons. These neutrons accelerate the fission chain reaction and increase its efficiency. This process can more than double the energy yield of the fission reaction.

The third and most complex type is the staged thermonuclear weapon. These arrangements use two or more stages to release energy. Most commonly, these weapons use two distinct stages. The first stage is typically a boosted fission weapon. When the first stage detonates, it emits intense X-rays. These X-rays illuminate and implode a second stage filled with fusion fuel. This triggers a thermonuclear burn. This sequence allows for energy yields that are hundreds or even thousands of times greater than pure fission weapons.

To understand these designs, one must understand nuclear fission. Fission occurs when a free neutron strikes the nucleus of a fissile atom. For example, hitting a uranium-235 nucleus causes it to split. This split creates two smaller fission fragments and releases more neutrons. In a supercritical mass, these neutrons strike neighboring nuclei. This creates a self-sustaining fission chain reaction. The number of fissions can theoretically double a hundred times in a single microsecond.

Fission releases massive amounts of energy through different mechanisms. When an atom splits, it releases immediate energy in the form of gamma radiation and kinetic energy. About 93% of this energy is the kinetic energy of the charged fission fragments. These fragments fly apart at speeds of about 12,000 kilometers per second. Within a microsecond, this energy converts into heat. This creates a ball of plasma several meters wide with temperatures in the tens of millions of degrees Celsius. This heat produces X-rays that create the final explosion.

Nuclear fusion is the complementary process to fission. While fission splits heavy atoms, fusion combines light atoms to form heavier ones. The most important reaction in weapons is the D-T reaction. This involves fusing deuterium (hydrogen-2) with tritium (hydrogen-3). This reaction produces helium-4, one neutron, and a large amount of energy. Although the energy per reaction is lower than fission, the energy output per unit mass is about five times greater. To maximize this, weapons often use a heavy material like uranium to capture the resulting neutrons. This can multiply the energy output tenfold.

Different materials are required to sustain these reactions. Fissile materials like uranium-235 or plutonium-239 are used for fission. Uranium-235 is often called "oralloy" or "25." Plutonium-239 is sometimes called "49." For fusion, scientists use hydrogen isotopes. Tritium is often produced by bombarding lithium-6 with neutrons in a nuclear reactor. In two-stage weapons, neutrons from the first stage can also produce tritium inside the lithium deuteride fuel. This allows for the creation of very large nuclear arsenals.

Weapon design also considers how to best distribute energy. Because explosions are spherical, spreading out many smaller weapons can cause more destruction than one massive weapon. For instance, five five-megaton weapons can demolish a larger area than a single 50-megaton weapon. Most modern nuclear powers utilize two-stage thermonuclear weapons. These are considered the most compact and cost-effective option for large arsenals. This is possible because they utilize both fission and fusion to achieve high yields.