The Physics Behind Nuclear Weapons

Introduction

I must inform you that the heading that this section is given may be slightly misleading. Certainly I shall discuss certain aspects about how the atoms interact; however, for the most part I will be explaining the simple steps required to construct a weapon of such caliber. Contrary to what it may seem, I am not a young teenager bent on world destruction or the death of others. This information is being provided for the simple reason of interest. The data provided below is not intended in any way to be used in the serious construction of a nuclear weapon. Not only that, but anyone wishing to construct such a weapon would most likely end up killing himself either through nuclear detonation or, more likely, from radiation exposure. The simple reason this page exists is for people such as myself who find nuclear weapons interesting in both their method of construction as well as detonation.

Types of Nuclear Weapons

This section will go over the two basic categories of nuclear weapons. These two categories are nuclear weapons and thermonuclear weapons. Although both categories possess an unparalleled amount of destructive power, the designs are somewhat different for each.

A nuclear weapon (often refered to as an atomic bomb) uses a process called fission to instigate an explosion ranging from 1 kiloton (about 1,000 sticks of TNT) to 50 megatons (about 50,000 sticks). Essentially what happens during the fission process is that a nucleus is split, releasing neutrons. As these neutrons travel away from the initial atom, they colide with other nuclei, causing them to split as well until a chain reaction is occurring. Such a great amount of energy is released during each split that this is the reason for such a large explosion. In order for this reaction to occur, there must first be a supercritical mass of Uranium or Plutonium available.

The second category is thermonuclear weapons. Thermonuclear weapons, unlike nuclear weapons, use fusion to create their explosion. However, a fission detonating head is always used to create the correct conditions for fusion to take place. Once the fission detonating head is ignited, it superheats a mixture of Tritium and Deuterium and ignites a U-235 "Spark Plug" located in the core. This begins a fusion reactions, where the tritium and deuterium are combined together into one enormous mass. The amount of energy released in a fusion bomb is much greater than that from a fission bomb. This design technique also theoretically has no limit on the size of the bomb.

How a Nuclear Weapon Works

Nuclear Weapons rely on fission. Fission splits the nucleus of a U-235 or Plutonium atom, releasing neutrons. These neutrons in turn colide with other nuclei, splitting them and releasing more neutrons. This process continues until a chain reaction is sufficiently sustained. Fusion bombs rely on this method to start the reaction. However, once it begins, a fusion reaction is also sustained using Tritium and Deuterium. This significantly increases the power of the weapon.

Construction of a Simple Nuclear Device

To begin with, we must first discuss the types of materials that can be used in a nuclear device. There are essentially two types of materials: "fissile" and "fissionable". Fissile materials can be induced to fission by fast or slow neutrons. There are three practical materials available that are fissile: U-233, U-235, and Pu-239. Fissionable means that the material can be induced to fission by neutrons moving at a sufficiently high energy. U-238 is a good example. For this entire section, only fissile materials are applicable. Only in the thermonuclear section will fissionable materials become useful.

The most simple method to construct a nuclear device is to essentially place two sub-critical masses of U-235 in a containment area. Launching both masses at each other using some type of gun assembly will achieve supercriticality. Once a supercritical state is achieved, neutrons must be introduced in order to initiate a chain reaction. Therefore some type of a neutron producer needs to be used. The most reliable method is probably a "modulated" neutron intiator. A simple beryllium/plutonium 210 initiator (like the one used in the Manhattan Project) will suffice. This entire design is considered a subsonic assembly, and it is important to note that this method almost always results in pre-detonation, which means the reaction starts before both pieces are completely fused together. This in turn reduces the total yield of the bomb. However, it is sufficient to create a workeable design. It is also important to note that Plutonium cannot be used in this design, for it requires a more rapid method to induce supercriticality.

A second, and more precise, method of construction is to engineer a compression device. This will allow for a smaller mass of Uranium or Plutonium to be compressed to a supercritical state. Termed an "implosion assembly," this technique uses shock waves produced by military grade explosives. There are many methods to create an implosion assembly. Howewver, the simplist and probably easiest method of design is linear implosion. This allows for a low density, non-spherical mass to be compressed into a supercritical configuration without using symmetric implosion designs. This assembly is accomplished by embedding an elliptical shaped mass in a cylinder of explosive. The explosive is detonated on both ends, and an inert wave shaping device is required in front of the detonation points. Extensive experimentation is needed to create a workable form, but this design enables the use of Plutonium as well as Uranium.

Construction of a Complex Thermonuclear Device

A thermonuclear device builds on the above concept. After constructing a fission-based weapon, that is placed in front of a fusion bomb. The fusion bomb consists of a chamber filled with tritium and deuterium. In the center of this mixture is another chunk of U-235. After the fission detonator explodes, it superheats everything, including the U-235 spark plug. This in turn ignites the sparkplug, which begins the fusion process. Another component often found in a thermonuclear device is a U-238 reflector. Although not fissionable under normal conditions, temperatures produced through a fusion reaction are sufficient to cause U-238 to begin a fission reaction. Essentially in a thermonuclear weapon, then, are three reactions: the initial fission reaction, the fusion reaction, and a final fission reaction.

The initial requirement for a thermonuclear weapon then is the detonating head. Once that is obtained one may begin work on the other parts of the weapon. The chamber containing the tritium and deuterium has a half sphere of U-238 placed in front of to act as a shield. The chamber is surrounded by a polystyrene filled radiation channel which directs the X-rays released from the fission detonation. This powerful flux of energy is channeled and used to superheat a mixture of tritium and deuterium (or in many cases lithium-6 deuteride). It also ignites a hollow plutonium or uranium "spark plug" which aids in initiating the fusion reaction. Another addition to the weapon which is not necessarily required is a U-238 shell coating the fusion bomb. Since U-238 is not a fissile material, it requires extremely high energy levels to accomplish fission. These conditions are created in a fusion reaction. Thus adding U-238 around the exterior of the bomb would increase the final yield.

Conclusion

Although somewhat brief, this page has discussed the fundamental ideas behind nuclear weapons. Realistically one could not use this information to construct a practical weapon for the simple reason that it does not go into enough detail. Again, I would like to emphasize that it this is here merely because of interest, not because of any intent to harm another person or group of persons. Hopefully sometime in the near future this page will be expanded to include diagrams, more specific information, and pictures. However, I am extremely busy and will probably not be able to work on this page again for quite some time.


The Atomic Age: A Brief Overview
This specific page was last updated on: 01/04/2000 at 10:50pm EST
2000 Jeff Turkstra. All Rights Reserved.