Nuclear Fusion Weapons

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Fusion nuclear devices, also known as thermonuclear weapons or hydrogen bombs, were developed as more powerful and more efficient nuclear weapons after the invention of fission bombs. The Tsar Bomba ("king of the bombs"), detonated by the Soviet Union on August 30, 1961, was the largest hydrogen bomb ever exploded on Earth. It was estimated to have a destructive yield equivalent to 60 million tons of TNT. In comparison, the fission weapons deployed over Japan in WW II had a yield of 15 thousand tons of TNT.

A hydrogen bomb is thousands of times more destructive than an atomic bomb for two main reasons:

Atomic Mass of Hydrogen vs. Uranium/Plutonium

First, a given mass of deuterium or tritium contains many more atoms than the same mass of uranium or plutonium because the atomic mass of hydrogen is much less than that of uranium or plutonium. So, weight for weight, there are more deuterium or tritium nuclei available for fusion than there are uranium or plutonium nuclei. Theoretically, the complete fusion of a given mass of deuterium would yield three times as much energy as the complete fission of the same mass of uranium.

Size of a Fusion Bomb Is Not Constrained

In a fission weapon, the chain reaction can only continue as long as a critical mass is present. When the bomb explodes, much of the fissionable material is blown away unused and the chain reaction stops. In contrast, fusion can continue as long as fusionable material is available in a hydrogen bomb. It does not matter how small the amount, as long as the termperature is sufficiently high enough. Thus a larger proportion of fusionable material is used. Therefore, the size of a hydrogen bomb is limited only by the weight that a missile will be able to carry.

Nuclear fusion weapons have the capacity to destroy the world. To understand the mechanics behind a fusion weapon, one must understand the physics behind nuclear fusion first.

» View: Physics of Nuclear Fusion

Obstacles to a Fusion Weapon

Nuclear fusion's exponentially higher yield and efficiency makes it appealing both as an energy source and as a military weapon. Three obstacles have to be overcome when designing a nuclear fusion weapon:

1. Deuterium and tritium are hard-to-store gases.
2. Tritium has a short half-life, so it has to be continously resupplied.
3. Deuterium and tritium have to be highly compressed at high temperatures to start the fusion reaction.

Despite these obstacles, the United States was the first to develop fusion weapons in 1952.

Deuterium and tritium are hard-to-store gases

To overcome the storage problems, deuterium can be chemically combined with litium-6 to form a solid lithium-deuteride compound. Lithium-deuterium nuclei are packed closely together, making it an optimal compound for fusiont occur once heat is supplied.

How is tritium stored, then? Scientists developed an ingenious way to eliminate the necessity of storing tritium in the device at all:

Tritium has a short half-life, so it has to be continously resupplied

Neutrons emitted from a fission reaction can produce tritium from lithium.

Lithium-6 reacts with a neutron to form tritium and helium-4.

Lithium-7 reacts with a neutron to form tritium, helium-4, and a neutron.

 

Thus the lithium-deuteride, when bombarded with neutrons from a fission reaction, will contain both tritium and deuterium, the necessary reactants for a fusion reaction.

Deuterium and tritium have to be highly compressed at high temperatures to start the fusion reaction.

Fusion cannot occur spontaneously. Two nuclei will tend to repel each other because they both carry a positive charge. In order to fuse, the nuclei must be moving at very high velocities (high temperature) and must be very close to each other (high pressure).

The Polish mathematician Stanislaw Ulam solved the problem of how to initiate fusion in a hydrogen bomb. He recognized that most of the raditation emitted during a fission reaction was in the form of X-rays, and these X-rays could provide the high temperatures and pressures needed for the fusion reaction to start.

By incorporating a fission bomb within a fusion bomb, this problem could be solved. Not only does the fission bomb provide the heat necessary to initiate the fusion reaction, but it also bombards the lithium-deuteride with neutrons, causing tritium to form.

Fusion Bomb Design

Teller-Ulam Design of a Hydrogen Bomb - Click here to read about the components of a typical hydrogen bomb.