Difference Between Hydrogen and Uranium Bomb

Main Difference – Hydrogen vs Uranium Bomb

The theory of special relativity completely changed the classical ideas of mass, energy, time and more. The famous Einstein’s equation E= mc2 revealed a top secret between mass and energy, known as the mass- energy equivalence. According to this equation, we should be able to convert energy into mass and vice versa. 

When neutrons and protons combine or fuse into a nucleus, an incredible amount of energy is released. So, the mass of the resultant nucleus is less than that of the total mass of its parent particles. This reduction of the mass is given by the Einstein’s equation. Physicists realized that a vast amount of energy could be generated by either fusing small nuclei into heavy nuclei or braking heavy nuclei into light nuclei. Also, they realized that this energy could be used to generate electricity and to make mass destructive bombs as well.

The best fuels for fission bombs are Uranium and Plutonium whereas the best fuel to design fusion bombs is Hydrogen. As the names suggest, Uranium bombs use Uranium as their fissile fuel while Hydrogen bombs use Hydrogen as their fuel. In Uranium bombs, energy is released when Uranium nuclei break into light nuclei. But in Hydrogen bombs, energy is released when small nuclei fuse into He nuclei. The main difference between Hydrogen and Uranium bomb is that Uranium bombs are nuclear fission bombs whereas Hydrogen bombs are fusion bombs. This article focuses on the differences between Hydrogen and Uranium bomb.

What is Hydrogen Bomb

When light nuclei combine into a heavy nucleus, the mass of the resulting nucleus is less than the total mass of its parent nuclei. When they fuse, the loss of the mass is converted into energy according to the Einstein equation. This energy can be used to generate electricity. Unfortunately, the same idea can be used to make a fusion bomb because a vast amount of energy is released in fusion.

The best element as a fusion fuel is Hydrogen. Hydrogen has three isotopes namely Protium, Deuterium and Tritium. But, Hydrogen is naturally a gaseous element. For the fusion reaction, a very high temperature and a very high fuel density must be achieved. If Hydrogen is used as liquid Hydrogen, a cooling mechanism must be coupled with the bomb which adds an extra weight and volume to the bomb. So, Hydrogen is used in the form of LiD (Lithium Deuteride) which is solid and thus eliminate the need for a cooling mechanism.

Deuterium and tritium are the best isotopes for the fusion reaction. By using Deuterium as LiD, a very high fuel density can be achieved. The other advantage of Deuterium is being a stable isotope. The abundance of Deuterium in naturally occurred Hydrogen is around 0.015%. So, water is a good source of Deuterium.

In a fusion bomb, a very high temperature is needed (about 108 K) for the fusion reaction. So, a fission bomb is used in fusion bombs in order to achieve such temperatures. Once the fission bomb is detonated, the required temperature is achieved. In other words, a fission bomb is used in fusion bombs to ignite the fusion bomb. After the fission bomb is detonated, the fusion reaction initiates. First, a Li nucleus absorbs a neutron and fissions into a Helium nucleus, a tritium nucleus plus energy. Next, a Deuterium nucleus combines with a Tritium nucleus to produce a Helium nucleus, a neutron plus energy. So, the overall reaction can be shortened into the following equation.

D+ Li→ 2He+ energy

In the above fusion reaction, no radioactive nuclei are produced. The energy released per nucleon in the above fusion reaction is much higher than that of in the fission reaction of Uranium.

Difference Between Hydrogen and Uranium Bomb

What is a Uranium Bomb

Uranium has several isotopes such as Uranium-238, Uranium-235, and Uranium-239. Nevertheless, Uranium-238 accounts for 99.7% of naturally occurring Uranium. Uranium-239 is very unstable, so its half-life is very short. So it decays into Plutonium very soon. Uranium-238 is the most stable Uranium isotope. Uranium-235  is unstable and its natural abundance is around 0.72%.

When a Uranium atom absorbs a neutron, it breaks into two fission fragments (two smaller atoms) plus several neutrons. In this fission reaction, a huge amount of energy is released as kinetic energy of fission fragments and EM waves. If the resulting neutrons were absorbed by other Uranium atoms, the process becomes a chain reaction braking more and more Uranium-235 nuclei. However, some of the neutrons produced in the process escape from the Uranium sample. So those escaping neutrons do not participate in the nuclear fission. The fraction of the neutrons that escape from the sample depends on the mass of the sample. For a chain reaction, there is a threshold mass for Uranium called critical mass. Critical mass is the minimum mass of a fissile fuel that must be present in order to sustain the chain reaction once it is initiated. In addition, if the Uranium sample is a natural unenriched one, most of the neutrons would be absorbed by Uranium-238 atoms (because its abundance is around 99.7 %) which then produce Uranium-239. So it is a waste. In order to minimize the number of neutrons that are absorbed byUranium-238, the percentage of Uranium-235 must be improved. This process is called Uranium enrichment.

A nuclear bomb should be able to release a vast amount of nuclear energy in an instant. So, both escape of neutrons and number of neutrons absorbed by Uranium-238 must be reduced as much as possible. These requirements are achieved by using highly enriched Uranium (HEU) samples having a larger mass than critical mass. In Uranium bombs, Uranium is enriched nearly up to 90% of Uranium-235.

In modern day nuclear weapons, a high voltage vacuum tube coupled with a small particle accelerator is used as the neutron generator which is the initiator of the chain reaction. The following figure depicts the basic structure of a Uranium bomb.

Main Difference - Hydrogen vs Uranium Bomb

Prior to the detonation, the Uranium sample is kept as two parts separately each having a mass less than the critical mass. The total mass of these two samples exceeds the critical mass. This separation allows us to keep the bomb in the subcritical state until it is detonated. In other words, the bomb can’t sustain a chain reaction until the two parts join together since the mass of each sample is less than the critical mass.

First, the conventional explosive (TNT) is detonated which causes the Uranium bullet to rush and combine with the Uranium target. After they are combined into a single sample of Uranium, its mass exceeds the critical mass leading to a chain reaction and thus a nuclear explosion. This explosion releases a vast amount of energy in the form of kinetic energy of fission fragments and radiation burning the victims. The resulting fission fragments are also almost radioactive. So, there are many medical issues associated with the radioactive fallout caused by a nuclear explosion.

Difference Between Hydrogen and Uranium Bomb

Fuel:

Uranium bomb: Uranium bomb is fueled by Uranium-235.

Hydrogen bomb: Hydrogen bomb is fueled by LiD (Lithium Deuteride).

Initiation:

Uranium bomb: A neutron source is used as an initiator.

Hydrogen bomb: Hydrogen bombs are ignited by fission bombs.

Nuclear Reactions:

Uranium bomb: There are several fission paths for. For an example,Difference Between Hydrogen and Uranium Bomb -1

Hydrogen Bomb:Difference Between Hydrogen and Uranium Bomb - 2

By combining the first and second steps, we get the overall fusion reaction, 

Difference Between Hydrogen and Uranium Bomb - 3

Energy released per nucleon:

Uranium bomb: Energy depends on the fission path of Uranium-235. For the above fission path of , energy released per nucleon is ~ 0.70MeV

Hydrogen bomb: Energy released per nucleon is ~ 2.8MeV (For LD)

Important Requirements:

Uranium bomb: Critical mass and a neutron source are the most important requirements.

Hydrogen bomb: Very high temperature around 108 K and high fuel density are required.

Radioactive Fallout:

Uranium bomb: The radioactive fallout is high.

Hydrogen bomb: The radioactive fallout is less.Difference Between Hydrogen and Uranium Bomb - infographic

Image Courtesy:

“Ivy Mike” by The Official CTBTO Photostream – “Ivy Mike” atmospheric nuclear test – November 1952  via  

About the Author: Kuma


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