A dirty bomb or radiological dispersal device is a radiological weapon that combines radioactive material with conventional explosives. The purpose of the weapon is to contaminate the area around the dispersal agent/conventional explosion with radioactive material, serving primarily as an area denial device against civilians. It is not to be confused with a nuclear explosion, such as a fission bomb, which produces blast effects far in excess of what is achievable by the use of conventional explosives. Unlike the rain of radioactive material from a typical fission bomb, a dirty bomb's radiation can be dispersed only within a few hundred meters or a few miles of the explosion.

Dirty bombs have never been used, only tested. They are designed to disperse radioactive material over a certain area. They act through the effects of radioactive contamination on the environment and related health effects of radiation poisoning in the affected populations. The containment and decontamination of victims, as well as decontamination of the affected area require considerable time and expenses, rendering areas partly unusable and causing economic damage. Dirty bombs might be used to create mass panic as a weapon of terror.

Effect of a dirty bomb explosion

When dealing with the implications of a dirty bomb attack, there are two main areas to be addressed: the civilian impact, not only dealing with immediate casualties and long term health issues, but also the psychological effect; and the economic impact. With no prior event of a dirty bomb detonation, it is considered difficult to predict the impact. Several analyses have predicted that radiological dispersal devices will neither sicken nor kill many people.

;Differences between dirty bombs and fission bombs

{| class="wikitable" style="width: 50%;"

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|width="50%"|Dirty bomb

  • Explosives combined with radioactive materials
  • Detonation vaporizes or aerosolizes radioactive material and propels it into the air
  • Not a nuclear detonation

|width="50%"|Fission bomb

  • Caused by an uncontrolled nuclear chain reaction with highly enriched uranium or weapons-grade plutonium
  • Sphere of fissile material (pit) surrounded by explosive lenses
  • Initial explosion produces imploding shock wave that compresses pit inward, creating supercritical mass by increasing the density of fissile material. Neutrons from either modulated neutron initiator or external neutron generator start chain reaction in compressed pit
  • Resulting fission chain reaction causes bomb to explode with tremendous force: nuclear detonation

|}

Source: Adapted from Levi MA, Kelly HC. "Weapons of mass disruption". Sci Am. 2002 Nov;287(5):76-81.

Accidents with radioactives

The effects of uncontrolled radioactive contamination have been reported several times.

One example is the radiological accident occurring in Goiânia, Brazil, between September 1987 and March 1988: Two metal scavengers broke into an abandoned radiotherapy clinic and removed a teletherapy source capsule containing powdered caesium-137 with an activity of 50 TBq. They brought it back to the home of one of the men to take it apart and sell as scrap metal. Later that day both men were showing acute signs of radiation illness with vomiting and one of them had a swollen hand and diarrhea. A few days later one of the men punctured the thick window of the capsule, allowing the caesium chloride powder to leak out and when realizing the powder glowed blue in the dark, brought it back home to his family and friends to show it off. After two weeks of spread by contact contamination causing an increasing number of adverse health effects, the correct diagnosis of acute radiation sickness was made at a hospital and proper precautions could be put into procedure. By this time 249 people were contaminated, 151 exhibited both external and internal contamination, of whom 20 people were seriously ill and five people died.

The Goiânia incident to some extent predicts the contamination pattern if it is not immediately realized that the explosion spread radioactive material, but also how fatal even very small amounts of ingested radioactive powder can be. This raises worries of terrorists using powdered alpha emitting material, that if ingested can pose a serious health risk, as in the case of Alexander Litvinenko, who was poisoned by tea with polonium-210. "Smoky bombs" based on alpha emitters might be just as dangerous as beta or gamma emitting dirty bombs.

Public perception of risks

Although the exposure might be minimal, many people find radiation exposure especially frightening because it is something they cannot see or feel, and it therefore becomes an unknown source of danger. When United States Attorney General John Ashcroft on June 10, 2002, announced the arrest of José Padilla, allegedly plotting to detonate such a weapon, he said: