How Nuclear Bombs Work

You have pr obably 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). They're on TV, too -- Jack Bauer struggles to stop a nuclear bomb detonation on the TV show "24." In the news, while many countries have been negotiating to disarm their arsenals of nuclear weapons, other countries have been developing nuclear weapons programs.

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. 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)

Atomic Structure

Atoms are in your body, the chair you are sitting in, your desk and even in the air.

Before we talk about the physics of atomic bombs, it's a good idea to go over the basic properties of atoms.

Atoms are incredibly small -- the smallest is about 10^-8 cm in diameter. For an idea of how small this really is, think of abaseball. The diameter of a baseball is about 7 cm. If an atom were the size of a baseball, an actual baseball would be about 3044 miles high.

An atom is made up of three subatomic particles -- protons,neutrons and electrons. The center of an atom, called thenucleus, is composed of protons and neutrons. Protons are positively charged, neutrons have no charge at all and electrons are negatively charged. The proton-to-electron ratio is always on e to one, so the atom as a whole has a neutral charge. For example, a carbon atom has six protons and six electrons.

An atom's properties can change considerably based on how many of each particle it has:

• The number of protons in an atom determines the type of element. Elements are classified by their atomic number, which is simply the number of protons in an atom's nucleus. Some common elements on Earth are oxygen, carbon and hydrogen. You can see the elements on the periodic table here.
• There are different types of atoms called isotopes. These isotopes look and act the same in nature -- the only difference is the number of neutrons in the nucleus.
• You can calculate the “mass” of an atom by counting the number of protons and neutrons inside the nucleus. This number is called the atomic mass. Carbon has three isotopes, for example -- carbon-12 (six protons + six neutrons), carbon-13 (six protons + seven neutrons) and carbon-14 (six protons + eight neutrons).

Nuclear Energy

Two important concepts in physics explain how massive amounts of energy can come from very small particles -- Einstein's famous equation E = MC² and nuclear radiation.

E = mc²
An atom's nucleus and the structure of certain isotopes make it possible to release incredible amounts of energy when the atom splits. You can understand how much energy this process releases by looking at Einstein's equation E = mc², where E is energy, m is mass and c is the speed of light (approximately 300,000 kilometers per second). Although you may have heard of this equation without knowing what it really means, the concept behind it is pretty simple. Matter and energy are essentially interchangeable -- matter can be converted into energy, and energy can be converted into matter, and the numbers involved are enormous. The speed of light is a huge number -- if you multiply a large amount of mass by the speed of light, you get an extreme amount of energy. And even though atoms are small -- they don't have a lot of mass -- it takes a vast number of them to make a substance.

Substances like uranium, which are commonly used in nuclear bombs, have a very high atomic number-- the atoms themselves are larger and contain more particles than the atoms of other naturally-occurring substances. Because of this additional nuclear material, uranium has the power to release a lot of energy. If you multiplied 7 kilograms of uranium by the speed of light squared, you would get about 2.1 billion Joules of energy. By comparison, a 60-watt light bulb uses 60 joules of energy per second. The energy found in a pound of highly enriched uranium is equal to something on the order of a million gallons ofgasoline. 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.

Nuclear Fission

You might wonder why fission bombs use uranium-235 as fuel. Uranium is the heaviest naturally occurring element on Earth, and it has two isotopes - uranium-238 and uranium-235, both of which are barely stable. Both isotopes also have an unusually large number of neutrons. Although ordinary uranium will always have 92 protons, U-238 has 146 neutrons, while U-235 has 143 neutrons.

Both isotopes of uranium are radioactive, and they eventually decay over time. U-235, however, 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. Instead of waiting more than 700 million years for uranium to naturally decay, the element can be broken down much faster if a neutron runs into a U-235 nucleus. The nucleus will absorb the neutron without hesitation, become unstable and split immediately.

Fission Bombs

To bring the subcritical masses together into a supercritical mass, two techniques are used:

• Gun-triggered
• Implosion

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.

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.

This figure shows a uranium-235 nucleus with a neutron approaching from the top. As 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. There are a couple of things about this induced fission process that makes 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. It helps to think of a big circle of marbles as the protons and neutrons of an atom. If you shoot one marble -- a single neutron -- in the middle of the big circle, it will hit one marble, which will hit a few more marbles, and so on until a chain reaction continues.
• The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (0.000000000001 seconds).

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.

Critical Mass
In a fission bomb, the fuel must be kept in separate subcritical masses, which will not support fissio n, to prevent premature detonation. Critical mass is the minimum mass of fissionable material required to sustain a nuclear fission reaction. Think about the marble analogy again. If the circle of marbles are spread too far apart -- subcritical mass -- a smaller chain reaction will occur when the "neutron marble" hits the center. If the marbles are placed closer together in the circle -- critical mass -- there is a higher chance a big chain reaction will take place. 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.