You have probably read in history books about the atomic bombs used in World War II. You may also have seen fictional movies where nuclear weapons were launched or detonated (Fail Safe, Dr. Strangelove, The Day After, Testament, Fat Man and Little Boy, The Peacemaker, just to name a few). In the news, while many countries have been negotiating to disarm their arsenals of nuclear weapons, other countries have been developing nuclear weapons programs.
Photo courtesy NARA
Atomic Cannon Test, 1953
We have seen that these devices have incredible destructive power, but how do they work? In this article, you will learn about the physics that makes a nuclear bomb so powerful, how nuclear bombs are designed and what happens after a nuclear explosion.
Nuclear bombs involve the forces, strong and weak, that hold the nucleus of an atom together, especially atoms with unstable nuclei (see How Nuclear Radiation Works for details). There are two basic ways that nuclear energy can be released from an atom:
• Nuclear fission - You can split the nucleus of an atom into two smaller fragments with a neutron. This method usually involves isotopes of uranium (uranium-235, uranium-233) or plutonium-239.
• Nuclear fusion -You can bring two smaller atoms, usually hydrogen or hydrogen isotopes (deuterium, tritium), together to form a larger one (helium or helium isotopes); this is how the sun produces energy.
In either process, fission or fusion, large amounts of heat energy and radiation are given off.
To build an atomic bomb, you need:
• A source of fissionable or fusionable fuel
• A triggering device
• A way to allow the majority of fuel to fission or fuse before the explosion occurs (otherwise the bomb will fizzle out)
The first nuclear bombs were fission devices, and the later fusion bombs required a fission-bomb trigger. We will discuss the designs of the following devices:
• Fission bombs (in general)
• Gun-triggered fission bomb (Little Boy), which was detonated over Hiroshima, Japan, in 1945
• Implosion-triggered fission bomb (Fat Man), which was detonated over Nagasaki, Japan, in 1945
• Fusion bombs (in general)
• Teller-Ulam design of a hydrogen fusion bomb, which was test-detonated on Elugelap Island in 1952
A fission bomb uses an element like uranium-235 to create a nuclear explosion. If you have read How Nuclear Radiation Works, then you understand the basic process behind radioactive decay and fission. Uranium-235 has an extra property that makes it useful for both nuclear-power production and nuclear-bomb production -- U-235 is one of the few materials that can undergo induced fission. If a free neutron runs into a U-235 nucleus, the nucleus will absorb the neutron without hesitation, become unstable and split immediately.
TAs soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom happens to split). The two new atoms then emit gamma radiation as they settle into their new states (see How Nuclear Radiation Works). There are three things about this induced fission process that make it interesting:
• The probability of a U-235 atom capturing a neutron as it passes by is fairly high. In a bomb that is working properly, more than one neutron ejected from each fission causes another fission to occur. This condition is known as supercriticality.
• The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (1*10E-12 seconds).
• An incredible amount of energy is released, in the form of heat and gamma radiation, when an atom splits. The energy released by a single fission is due to the fact that the fission products and the neutrons, together, weigh less than the original U-235 atom.
The difference in weight is converted to energy at a rate governed by the equation e = m * c^2. A pound of highly enriched uranium as used in a nuclear bomb is equal to something on the order of a million gallons of gasoline. When you consider that a pound of uranium is smaller than a baseball and a million gallons of gasoline would fill a cube that is 50 feet per side (50 feet is as tall as a five-story building), you can get an idea of the amount of energy available in just a little bit of U-235.
In order for these properties of U-235 to work, a sample of uranium must be enriched . Weapons-grade uranium is composed of at least 90-percent U-235.
In a fission bomb, the fuel must be kept in separate subcritical masses, which will not support fission, to prevent premature detonation. Critical mass is the minimum mass of fissionable material required to sustain a nuclear fission reaction. This separation brings about several problems in the design of a fission bomb that must be solved:
• The two or more subcritical masses must be brought together to form a supercritical mass, which will provide more than enough neutrons to sustain a fission reaction, at the time of detonation.
• Free neutrons must be introduced into the supercritical mass to start the fission.
• As much of the material as possible must be fissioned before the bomb explodes to prevent fizzle.
To bring the subcritical masses together into a supercritical mass, two techniques are used:
Neutrons are introduced by making a neutron generator. This generator is a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. In this generator:
1. The foil is broken when the subcritical masses come together and polonium spontaneously emits alpha particles.
2. These alpha particles then collide with beryllium-9 to produce beryllium-8 and free neutrons.
3. The neutrons then initiate fission.
Finally, the fission reaction is confined within a dense material called a tamper, which is usually made of uranium-238. The tamper gets heated and expanded by the fission core. This expansion of the tamper exerts pressure back on the fission core and slows the core's expansion. The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction.
Types of Bombs
Gun-triggered Fission Bomb
The simplest way to bring the subcritical masses together is to make a gun that fires one mass into the other. A sphere of U-235 is made around the neutron generator and a small bullet of U-235 is removed. The bullet is placed at the one end of a long tube with explosives behind it, while the sphere is placed at the other end. A barometric-pressure sensor determines the appropriate altitude for detonation and triggers the following sequence of events:
1. The explosives fire and propel the bullet down the barrel.
2. The bullet strikes the sphere and generator, initiating the fission reaction.
3. The fission reaction begins.
4. The bomb explodes.
Little Boy was this type of bomb and had a 14.5-kiloton yield (equal to 14,500 tons of TNT) with an efficiency of about 1.5 percent. That is, 1.5 percent of the material was fissioned before the explosion carried the material away.
Implosion-Triggered Fission Bomb
Early in the Manhattan Project, the secret U.S. program to develop the atomic bomb, scientists working on the project recognized that compressing the subcritical masses together into a sphere by implosion might be a good way to make a supercritical mass. There were several problems with this idea, particularly how to control and direct the shock wave uniformly across the sphere. But the Manhattan Project team solved the problems. The implosion device consisted of a sphere of uranium-235 (tamper) and a plutonium-239 core surrounded by high explosives. When the bomb was detonated, this is what happened:
• The explosives fired, creating a shock wave.
• The shock wave compressed the core.
• The fission reaction began.
• The bomb exploded.
Fat Man was this type of bomb and had a 23-kiloton yield with an efficiency of 17 percent. These bombs exploded in fractions of a second. The fission usually occurred in 560 billionths of a second.
Modern Implosion-Triggered Design
In a later modification of the implosion-triggered design, here is what happens:
• The explosives fire, creating a shock wave.
• The shock wave propels the plutonium pieces together into a sphere.
• The plutonium pieces strike a pellet of beryllium/polonium at the center.
• The fission reaction begins.
• The bomb explodes.
Fission bombs worked, but they weren't very efficient. Fusion bombs, also called thermonuclear bombs, have higher kiloton yields and greater efficiencies than fission bombs. To design a fusion bomb, some problems have to be solved:
• Deuterium and tritium, the fuel for fusion, are both gases, which are hard to store.
• Tritium is in short supply and has a short half-life, so the fuel in the bomb would have to be continuously replenished.
• Deuterium or tritium has to be highly compressed at high temperature to initiate the fusion reaction.
First, to store deuterium, the gas could be chemically combined with lithium to make a solid lithium-deuterate compound. To overcome the tritium problem, the bomb designers recognized that the neutrons from a fission reaction could produce tritium from lithium (lithium-6 plus a neutron yields tritium and helium-4; lithium-7 plus a neutron yields tritium, helium-4 and a neutron). That meant that tritium would not have to be stored in the bomb. Finally, Stanislaw Ulam recognized that the majority of radiation given off in a fission reaction was X-rays, and that these X-rays could provide the high temperatures and pressures necessary to initiate fusion. Therefore, by encasing a fission bomb within a fusion bomb, several problems could be solved.
Teller-Ulam Design of a Fusion Bomb
To understand this bomb design, imagine that within a bomb casing you have an implosion fission bomb and a cylinder casing of uranium-238 (tamper). Within the tamper is the lithium deuteride (fuel) and a hollow rod of plutonium-239 in the center of the cylinder. Separating the cylinder from the implosion bomb is a shield of uranium-238 and plastic foam that fills the remaining spaces in the bomb casing. Detonation of the bomb caused the following sequence of events:
1. The fission bomb imploded, giving off X-rays.
2. These X-rays heated the interior of the bomb and the tamper; the shield prevented premature detonation of the fuel.
3. The heat caused the tamper to expand and burn away, exerting pressure inward against the lithium deuterate.
4. The lithium deuterate was squeezed by about 30-fold.
5. The compression shock waves initiated fission in the plutonium rod.
6. The fissioning rod gave off radiation, heat and neutrons.
7. The neutrons went into the lithium deuterate, combined with the lithium and made tritium.
8. The combination of high temperature and pressure were sufficient for tritium-deuterium and deuterium-deuterium fusion reactions to occur, producing more heat, radiation and neutrons.
9. The neutrons from the fusion reactions induced fission in the uranium-238 pieces from the tamper and shield.
10. Fission of the tamper and shield pieces produced even more radiation and heat.
11. The bomb exploded.
All of these events happened in about 600 billionths of a second (550 billionths of a second for the fission bomb implosion, 50 billionths of a second for the fusion events). The result was an immense explosion that was more than 700 times greater than the Little Boy explosion: It had a 10,000-kiloton yield.
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