How to Build a Thermonuclear (Hydrogen) Bomb

By the end of WWII, it was possible to build atomic bombs using fission (the splitting of atoms) that could create explosions equal to several hundred tons of TNT. Scientists suspected, however, that even more powerful weapons could be built by forcing atoms together, an effect called fusion. The United States tested the first of these mega-weapons in 1952 with a blast equal to 10.4 million tons of TNT. How are these thermonuclear super bombs built?

Almost all the energy we come into contact with on Earth is the result of chemical reactions. As a tree grows, it takes energy from the sun and uses it to separate the carbon atoms from the oxygen atoms in the carbon-dioxide in our air. The carbon it uses to make its trunk and branches and it sets the oxygen free. When we burn those branches, we reverse the process and let the oxygen in the air combine with the carbon, returning the energy from the sun in the form of light and heat from the fire. These are examples of chemical reactions.

Fission vs. Fusion

Nuclear weapons, however, produce energy from the splitting apart (fission) and combining of atoms (fusion). Nuclear reactions can be at least a million times more powerful than chemical reactions. The easier of these two nuclear reactions to use is fission . This was used to make the first atomic bombs (For more information on fission and atomic bombs see our article on "How to Build an Atomic Bomb").

Forcing atoms together as a way of releasing energy is called fusion. It's the source of power for the stars, including our sun. Stars are mostly balls of hydrogen gas, the simplest of all elements (the atoms are composed of just one proton and one electron and sometimes some neutrons). Normally atoms do not like to fuse, but under the enormous pressures and temperatures inside a star, it happens. Typically the nuclei (the centers) of six hydrogen atoms will go through a series of reactions and emerge as two helium atoms, two hydrogen atoms, a couple of positrons (electrons with a positive charge) and, most significantly, energy in the form of gamma rays.

When scientists first thought about releasing energy by fusing hydrogen atoms they had a real problem: There was no way on Earth to generate the heat and pressures necessary to cause the hydrogen to fuse. With the invention of the atomic bomb, however, they realized that they now had a device that could produce those energies. For this reason, every fusion bomb uses a fission bomb as its trigger.

Almost all thermonuclear bombs are multi-stage weapons. This means that the energy from the earlier stages are used to trigger the later stages. The first stage in such a bomb is a conventional fission device: essentially a ball of uranium-235 or plutonium-239 surrounded by high explosives. Upon detonation, the explosives compress the uranium or plutonium until a supercritical reaction (the splitting of atoms that releases high energy neutrons that split even more atoms) takes place, liberating energy in the form of gamma rays along with a flood of high energy neutrons.

The energy from this first stage would then be used to squeeze and heat the hydrogen in the second stage until the point where the atoms will fuse together to form helium and release energy. Exactly how to direct the energy from the first stage to the second stage was probably the most difficult problem in designing this type of bomb, however.

Teller-Ulam Design

In 1951 physicists Edward Teller and Stanislaw Ulam came up with a scheme that is now used in almost all multi-stage nuclear weapons. In their design the second stage, shaped like a cylinder (in some later designs the second stage was shaped like a sphere, but does the same job), sits next to the first stage. The cylinder consists of a pusher/tamper on the outside (often made of some heavy metal such as lead or depleted uranium), the hydrogen fuel on the inside and a "sparkplug" at the very center. The rest of the interior of the bomb was filled with a secret material that was a plastic similar to Styrofoam. As the gamma rays from the first stage hit the plastic, it turns into a very hot plasma which, in turn, starts to heats the out layers of the pusher/tamper in the second stage. In addition, x-rays from the primary stage reflect off the outer case of the bomb and are absorbed by the tamper/pusher, further increasing its temperature. As the outer layers of the tamper/pusher heat up, they vaporize, turn to hot gas and the gas expands rapidly, putting huge amounts of pressure on the remaining parts of the pusher/tamper. As the pusher/tamper is compressed, it puts pressure on the hydrogen fuel (which is why it's called the "pusher").

At the same time the first stage fission is radiating huge amounts of high energy neutrons. These are focused by a part of the bomb called the "Neutron Focus Lens" onto the second stage. The "spark plug" at the center of the second stage is made of fissionable material (either uranium-235 or plutonium-239) and as these high energy neutrons enter it, atoms hit by those neutrons start to split in the "spark plug," releasing energy. The sparkplug is, at the same time, being compressed by the hydrogen fuel. The compression also aids in the fission of the sparkplug which explodes and starts to push outward.

The hydrogen fuel is then caught between the pusher, pressing in on it and the spark plug pressing out. This puts the fuel under enough pressure and heat for the hydrogen atoms to start to fuse, release energy and the second stage explodes.

One of the functions of the "pusher/tamper" is not just to push inward on the hydrogen fuel, but to resist its outward motion as it starts to explode (which is why it is also called a "tamper"). The longer the hydrogen is kept compressed, the more fusion takes place, making the bomb more powerful.

Deuterium-Tritium Fuel

One of the problems that beset the designers of the bomb was the fusion fuel. While stars use the fusion reaction described above, scientists realized it would be more practical in a bomb to use a form of a fusion reaction that would fuse deuterium (a hydrogen atom with one neutron) with tritium (a hydrogen atom with two neutrons). This reaction, in addition to producing a helium atom and some gamma rays, also produced a free, high energy neutron.

The problem with this idea was that tritium is a radioactive form of hydrogen that decays with time. Constantly replacing the tritium in the second stage of the bomb would be difficult if you had a large arsenal of weapons. What designers came to realize, however, is that if they used lithium-deuteride as the fuel, it would be turned into tritium and deuterium when it was bombarded by free neutrons coming from the 1st stage of the weapon. This method of creating the fuel as the bomb was actually detonating greatly reduced the maintenance considerations of long-term storage.

While the above describes the basic functions of a thermo-nuclear device, almost all of these bombs also include certain additional features that increase the yield of the weapon.

Boosting a Bomb

One of these features is a mechanism that "boosts" the power of the bomb by injecting a small amount of deuterium/tritium gas into the center of the 1st stage of the bomb shortly before detonation. As the first stage explodes, the gas is compressed until some of the hydrogen atoms fuse. This fusion adds to the power of the bomb directly, but more importantly, it also releases high energy neutrons which then radiate out to split additional atoms in the first stage's uranium-235 or plutonium-239 primary fuel. Boosting can double the power of the first stage without adding additional, rare fissile material. Single stage atomic bombs can also use this mechanism.

Output can also be increased in the second stage by replacing the lead in the pusher/tamper with uranium-238. Uranium-238 is not usually considered "fissile" material because it does not produce the high energy neutrons needed to split atoms. However, when uranium-238 is put next to the fusion fuel, its atoms can be split by the high energy neutrons coming from the fusion reaction. In fact, this mechanism is so effective it can double the power of the bomb without adding any more rare fissile material.

Neutron Bomb

Because they are so efficient at increasing the power of the bomb, first stage boosting and using uranium for the pusher/tamper are standard for almost all thermonuclear devices. The one major exception is the so called neutron bomb. In the 80's and 90's the American planners were concerned about the large number of tanks in the Warsaw Pact nations' military that might be used to invade NATO countries. Soldiers in those tanks were relatively well protected from nuclear blasts, so designers sought to minimize the blast and long term fallout from the bomb while increasing the high-energy free neutrons. Since these neutrons can readily penetrate tank armor, such a bomb could incapacitate the tank crew while doing fairly little damage to the defending nation's land and infrastructure. One of the keys to making the neutron bomb work was to use a lead pusher/tamper which reduced the blast effect without reducing the output of high energy neutrons.

Three Stage Bombs

Though in theory it is possible to build weapons with an almost unlimited number of stages (and therefore with unlimited power), the largest bombs ever built only had three stages (an example is the so-called "Tsar Bombe," a Russian device that could produce a 100-megaton blast, but was only ever tested at 50 megatons). In reality, there is little military necessity to build gigantic bombs with more than two stages. In general, it is more effective to use a number of smaller bombs to complete an objective, rather than one big one.

Fortunately, thermonuclear weapons are difficult to make and only a few nations have mastered this skill. It isn't that the design is so complicated, but the uranium-235 or plutonium-239 needed to build the first stage is rare, difficult to produce and very closely guarded. It is also why nuclear proliferation of these materials is such a concern throughout the world.

Copyright Lee Krystek 2016. All Rights Reserved.