How Dirty Bombs Work

A dirty bomb is an explosive designed to spread dangerous radioactive material over a wide area. When people hear "bomb" and "radioactive" in the ­same sentence, their minds jump to nuclear war pretty quickly. But it turns out that a dirty bomb's primary destructive power would probably be panic, not radiation damage.

A dirty bomb is much closer in power to an ordinary explosive than it is to the widespread destructive force of a nuclear bomb. But the fear of contamination could be debilitating, in the same way that 2001's anthrax scare in the United States terrorized much of the American populace, even though only a few people were infected.

In this article, we'll find out what dirty bombs are and what they do. We'll also explore what might happen if one actually went off in a public area, and consider some of the consequences of this sort of attack.

­Conceptually, a dirty bomb (or radiological dispersion bomb) is a very simple device. It's a conventional explosive, such as TNT (trinitrotoluene), packaged with radioactive material. It's a lot cruder and cheaper than a nuclear bomb, and it's also a lot less effective. But it does have the combination of explosive de­struction and radiation damage.

High explosives inflict damage with rapidly expanding, very hot gas. The basic idea of a dirty bomb is to use the gas expansion as a means of propelling radioactive material over a wide area rather than as a destructive force in its own right. When the explosive goes off, the radioactive material spreads in a sort of dust cloud, carried by the wind, that reaches a wider area than the explosion itself.

The long-term destructive force of the bomb would be ionizing radiation from the radioactive material. Ionizing radiation, which includes alpha particles, beta particles, gamma rays and X-rays, is radiation that has enough energy to knock an orbital electron off of an atom. Losing an electron throws off the balance between the atom's positively charged protons and negatively charged electrons, giving the atom a net electrical charge (the atom becomes an ion). The free electron may collide with other atoms to create more ions. (See How Atoms Work for more information on subatomic particles.)

If this happens in a person's body, the ion can cause a lot of serious problems, because an ion's electrical charge may lead to unnatural chemical reactions inside cells. Among other things, the charge can break DNA chains. A cell with a broken strand of DNA will either die or the DNA will develop a mutation. If a lot of cells die, the body can develop various diseases. If the DNA mutates, a cell may become cancerous, and this cancer may spread. Ionization radiation may also cause cells to malfunction, resulting in a wide variety of symptoms collectively referred to as radiation sickness. Radiation sickness can be deadly, but people can survive it, particularly if they receive a bone marrow transplant.

In a dirty bomb, the ionizing radiation would come from radioactive isotopes (also called radioisotopes). Radioactive isotopes are simply atoms that decay over time. In other words, the arrangement of protons, neutrons and electrons that make up the atom gradually changes, forming different atoms. This radioactive decay releases a lot of energy in the form of ionizing radiation. (See How Nuclear Radiation Works for details on radiation and radioactive isotopes.)

We're exposed to small doses of ionizing radiation all the time -- it comes from outer space, it comes from natural radioactive isotopes, it comes from X-ray machines. This radiation can and does cause cancer, but the risk is relatively low because you only encounter it in very small doses.

A dirty bomb would boost the radiation level above normal levels, increasing the risk of cancer and radiation sickness to some degree. Most likely, it wouldn't kill many people right away, but it could possibly kill people years down the road. ­

Dirty Bomb Possibilities

There is a huge range of possible dirty bomb designs. Different explosive materials, applied in different quantities, would generate explosions of varying sizes, and different types and quantities of radioactive material would contaminate an area to different degrees. Some designs include:

  • A small bomb, consisting of one stick of dynamite and a very small amount of radioactive material
  • A medium-size bomb, such as a backpack or small car filled with explosives and a greater amount of radioactive material
  • A large bomb, such as a truck filled with explosives and a good amount of radioactive material

The builders of these bombs wouldn't have much trouble getting their hands on high explosives -- dynamite is readily available, and TNT isn't too hard to come by. The main limitation on the bomb would be the available radioactive material.

It's not nearly as accessible as explosive material, but there are a number of sources for radioactive material around the world. For example:

  • Hospitals use small quantities of radioactive material, such as cesium-137, in nuclear medicine.
  • Universities use similar materials to conduct scientific research.
  • Food irradiation plants use radiation from cobalt-60 to kill harmful bacteria on food. (See CDC: Frequently Asked Questions about Food Irradiation for more information.)
  • Natural radioactive uranium isotopes are mined for use in nuclear energy. Terrorists could conceivably acquire uranium from various mines in Africa.
  • There are a number of abandoned "nuclear batteries" scattered around the former Soviet Union. These portable thermoelectric generators contain a sizable amount of strontium-90, a highly potent radioactive isotope. (Check out Makings of a 'Dirty Bomb' for more information.)
  • People could also collect spent radioactive fuel from Russian reactors, which have been abandoned in old nuclear submarines, among other places.
  • They could also put something together using various low-level radioactive materials available to anybody, such as the radioactive material in smoke alarms. Tale of the Radioactive Boy Scout is good evidence that this is a very real possibility.

The big question, of course, is what would actually happen if someone set off a bomb containing any of this material. As it turns out, there isn't a clear answer. Ask 10 different experts and you'll probably get 10 slightly different answers. In the next section, we'll explore the various possible scenarios.

Dirty Bomb Damage

It's difficult to predict the extent of a dirty bomb's damage because there are a huge number of variables at work. The type and quantity of the explosives and radioactive material make a big difference, of course, but completely random things like wind speed would also have an effect. There's also a lot of debate on what the long-term health effects would be.

The most likely dirty bomb would contain a small or medium amount of explosives (10 to 50 pounds [4.5 - 23 kg] of TNT, for example) with a small amount of low-level radioactive material (say a sample of cesium-137 or cobalt-60 from a university lab).

This sort of bomb wouldn't be terribly destructive. Most likely, any immediate deaths (and all property damage) would be from the explosive itself rather than the radiation. The explosive would act as a propellant force for the radioactive material. A radioactive dust cloud would extend well beyond the explosion site, possibly covering several square miles. Bombs containing radioactive waste from nuclear power plants or portable nuclear generators would inflict more damage, but terrorists would be less likely to use them because they are harder to handle. The bombers could die from exposure just building and transporting the bomb.

If people got rid of contaminated clothes, showered and evacuated the area within a day or so of a small or medium blast, they would probably be fine. The bomb would boost radiation levels above the normal, "safe" level, but not by a lot. In the short term, the human body could handle this increased exposure fairly easily. People very close to the blast could conceivably suffer radiation sickness and might require hospital care.

The main concern would be prolonged exposure. Many radioactive isotopes bind with other materials, including concrete and metal, extremely well. This would make it nearly impossible to completely remove the material without demolishing all contaminated structures. Clean-up crews could wash away a lot of the radioactive material, but a small amount would probably remain in the city for many years, even decades. Anybody living there would be exposed regularly to this radiation, which could conceivably cause cancer.

The question is, would this make a significant health difference? There are two schools of thought on this issue. Many experts have asserted that the health risks would be negligible if the government spent a few weeks or months on clean-up. The radiation level would be only marginally higher than normal, acceptable levels, and it would not significantly increase the risk of developing cancer. (See "Dirty Bombs" Much More Likely to Create Fear than Cause Cancer from the American Institute of Physics for more on this viewpoint.)

The other school of thought asserts that such an attack could make a city uninhabitable for years or decades. The Federation of American Scientists (FAS) recently prepared a report detailing three representative scenarios of a dirty bomb attack. In all three scenarios, the FAS asserts that the risk of cancer in some contaminated areas would be so high that the government would desert or demolish the area. These predictions are based on the Environmental Protection Agency's current guidelines for safe radiation levels. (Check out Dirty Bombs: Response to a Threat for the FAS' predicted scenarios.)

There's no precedent for a dirty bomb attack, but we can learn from other incidents of radioactive contamination. Nagasaki and Hiroshima were both exposed to a much larger amount of radioactive material, from an actual nuclear blast, and today, they're both considered completely safe for habitation. On the other hand, there are still areas around Chernobyl that are considered unsafe because of high radioactivity.

No matter their opinion on the long-term health risks, most experts agree that a dirty bomb would be more of a disruptive weapon than a destructive weapon. The news of radioactive contamination would probably cause widespread panic, and the rush to evacuate the targeted city could actually cause more damage than the bomb itself. A country's economy could also take a dive, especially if the bomb went off in major city. Even if the government did assure the public that the area was inhabitable, real estate values and tourism could plummet.

This is the precise reason dirty bombs are such an attractive weapon to terrorists. Their main goal is to get people's attention and inspire terror, two things a dirty bomb would certainly accomplish.

For much more information on dirty bombs, including possible scenarios for such an attack, check out the links on the next page.

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