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Frequently Asked Questions

General Questions about Nuclear Weapons

  1. What is a nuclear weapon?
  2. What is a nuclear warhead?
  3. What types of nuclear weapons are there?
  4. What is the difference between nuclear fission and nuclear fusion?
  5. Can you briefly discuss how nuclear fission actually happens?
  6. What is radiation?
  7. How big and heavy is a nuclear weapon?
  8. How long does it take to build a nuclear weapon?
  9. How expensive is a nuclear weapon?


What is a nuclear weapon?

Any weapon that uses a nuclear reaction (fission of an atom or fusion of two atoms) for its explosive power. Nuclear weapons include missiles, bombs, artillery rounds, and mines.

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What is a nuclear warhead?

A nuclear warhead is the part of an armament system containing the explosive charge, in this case one whose explosive power comes from a nuclear reaction. Warheads are mounted in the forward portion of a projectile, such as a ballistic missile, artillery round, or bomb.

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What types of nuclear weapons are there?

  • Atomic bomb - A nuclear weapon that is powered by nuclear fission, and therefore produces a quick release of energy and great destruction.
  • Hydrogen bomb - A nuclear weapon with enormous explosive power, fueled by nuclear fusion, in which atoms of hydrogen combine to form atoms of helium. A hydrogen bomb is 10 to over 100 times more powerful than the atomic bomb dropped on Hiroshima. A hydrogen bomb also is about 1/10 the size of an atomic bomb.
  • Neutron bomb - a particular form of a fusion device; it is designed to produce lethal neutrons but less blast and thermal damage than other nuclear weapons.

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What is the difference between nuclear fission and nuclear fusion?

Nuclear fission is a nuclear reaction in which a single large nucleus splits into two or more smaller nuclei, resulting in a total mass that is less than the mass of the original nucleus. As a result (described by Albert Einstein in his famous equation, E=mc2), enormous amounts of energy are released in this reaction. The fission of uranium-235, an isotope of uranium, supplies energy for nuclear reactors and atomic bombs.

Nuclear fusion is the combining of two small atomic nuclei to form a larger nucleus. The fusion of hydrogen into helium releases huge amounts of energy and is the main energy source of stars, including the sun.

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Can you briefly discuss how nuclear fission actually happens?

To understand how fission works in a nuclear weapon, you need to understand a little bit about uranium atoms. Uranium has different isotopes. Isotopes are atoms that have the same number of protons but a different number of neutrons in the nucleus. The most common form of uranium is U-238, which means it has 238 protons and neutrons (119 of each) in its nucleus. This is a HUGE atom. U-235 is much rarer and is the isotope used in nuclear weapons because it is fissionable. (U-238 is nonfissionable.)

Protons do not want to be right next to each other. The force that keeps them together in the nucleus of an atom is called the strong force, which simply means that if you can get two protons close enough so that they stick together, it is really hard to get them apart. Because the uranium nucleus is so large, however, the forces that repel protons from each other start to counteract the effects of the strong force (affecting protons on opposite sides of the atom).

In U-235, which has three fewer neutrons than U-238, this tension between the repellent force and the strong force creates a very unstable situation. When you hit the uranium core of a nuclear weapon with slow neutrons, the U-235 atom rips in half and releases a huge amount of energy. When the atom does split apart, neutrons start shooting out in all directions. When one of those neutrons hits another U-235 atom, that atom splits apart, and more neutrons shoot out, and so on, creating a chain reaction. The trick with a nuclear weapon is to get enough uranium-235 together in such a way that you can get this chain reaction going and sustained so that it releases all the energy at one time.

Another note about U-235--because it is much rarer than U-238, it must be separated out from U-238 to produce enough to be used in a weapon. In fact, uranium separation was one of the biggest early challenges of making nuclear weapons.

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What is radiation?

Radiation is energy radiated in the form of waves or particles. Radiation is a natural part of the environment, and everything emits some type of radiation. The kind of radiation we commonly associate with nuclear weapons is ionizing radiation.

There are four types of ionizing radiation produced by nuclear explosions that can cause significant injury: neutrons, gamma rays, beta particles, and alpha particles. Gamma rays are energetic, short-wavelength photons (as are X-rays). Beta particles are energetic (fast-moving) electrons. Alpha particles are energetic helium nuclei. Neutrons are damaging whether they are energetic or not, although the faster they are, the worse their effects. They all share the same basic mechanism for causing injury: the creation of chemically reactive compounds called "free radicals" that disrupt the normal chemistry of living cells.

The different types of radiation present different risks. Neutrons and gamma rays are very penetrating types of radiation; that is, they are the hardest to stop with shielding. They can travel through hundreds of meters of air and the walls of ordinary houses. Therefore, they can deliver deadly radiation doses even if an organism is not in immediate contact with the source. Beta particles are less penetrating; they can travel through several meters of air, but not walls, and can cause serious injury to organisms that are near the source. Alpha particles have a range of only a few centimeters in air and cannot even penetrate skin; they can only cause injury if swallowed or inhaled.

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How big and heavy is a nuclear weapon?

The bomb that was dropped on Hiroshima was about 6 feet long, about 3 feet in diameter around, and weighed several tons. The bomber that carried it had to be stripped bare of any extra equipment or supplies so that it was not carrying any extra weight. Even then, it was barely able to take off. Now, weapons such as a modern reentry vehicle, is about 3 feet long with a small diameter, and weighs about 200 pounds. It also has about 10 times the force of Fat Man or Little Boy (the bombs dropped on Hiroshima and Nagasaki).

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How long does it take to build a nuclear weapon?

If the assembly line is up and going, probably a few months. (The US is not currently building any new nuclear weapons.)

Building a nuclear weapon is kind of like building a car; the US has different production facilities around the country. Some of them make the electronic parts, some of them machine out the bomb casing, some do the work on the plutonium parts, and all these parts come together in Pantex, Texas. The final product is assembled on an assembly line, with all kinds of safety and security checks. The completed weapon is then shipped to the Air Force or Navy. The process could go faster or slower, depending on how many people you have working on it, but generally, it takes a few months.

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How expensive is a nuclear weapon?

When you consider all the research and development (including the national laboratories), production (including refining the nuclear materials in the bomb--plutonium and uranium--which are very expensive processes, as well as building the weapon), storage (including the security maintained by the armed services), and maintenance costs , you are probably looking at about $2 million for one bomb. Of course, its like any manufacturing process; if you have done the research, development and design for a product, and you set up an assembly line to make the product (in this case a nuclear weapon), the more products you make, the lower the overall cost is for each. If you are only making a few of them, the cost per weapon increases.

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For more information, see "Nuclear Weapons Frequently Asked Questions," by Carey Sublette, May 15, 1997.

Page Owner: Science Education and Outreach
Judie Hurtz
Last Modified: March 10, 1998

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