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A dirty bomb is a speculative 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 however not to be confused with a nuclear explosion, such as a fission bomb, which by releasing nuclear energy produces blast effects far in excess of what is achievable by the use of conventional explosives.
Though a radiological dispersal device (RDD) would be designed to disperse radioactive material over a large area, a bomb that uses conventional explosives and produces a blast wave would be far more lethal to people than the hazard posed by radioactive material that may be mixed with the explosive. At levels created from probable sources, not enough radiation would be present to cause severe illness or death. A test explosion and subsequent calculations done by the United States Department of Energy found that assuming nothing is done to clean up the affected area and everyone stays in the affected area for one year, the radiation exposure would be "fairly high", but not fatal. Recent analysis of the nuclear fallout from the Chernobyl disaster confirms this, showing that the effect on many people in the surrounding area, although not those in close proximity, was almost negligible.
Since a dirty bomb is unlikely to cause many deaths by radiation exposure, many do not consider this to be a weapon of mass destruction. Its purpose would presumably be to create psychological, not physical, harm through ignorance, mass panic, and terror. For this reason dirty bombs are sometimes called "weapons of mass disruption". Additionally, containment and decontamination of thousands of victims, as well as decontamination of the affected area might require considerable time and expense, rendering areas partly unusable and causing economic damage.
Since the 9/11 attacks the fear of terrorist groups using dirty bombs has increased significantly, which has been frequently reported in the media. The meaning of terrorism used here, is described by the U.S. Department of Defense's definition, which is "the calculated use of unlawful violence or threat of unlawful violence to inculcate fear; intended to coerce or to intimidate governments or societies in the pursuit of goals that are generally political, religious, or ideological objectives". There have only ever been two cases of caesium-containing bombs, and neither was detonated. Both involved Chechnya. The first attempt of radiological terror was carried out in November 1995 by a group of Chechen separatists, who buried a caesium-137 source wrapped in explosives at the Izmaylovsky Park in Moscow. A Chechen rebel leader alerted the media, the bomb was never activated, and the incident amounted to a mere publicity stunt.
In December 1998, a second attempt was announced by the Chechen Security Service, who discovered a container filled with radioactive materials attached to an explosive mine. The bomb was hidden near a railway line in the suburban area Argun, ten miles east of the Chechen capital of Grozny. The same Chechen separatist group was suspected to be involved. Despite the increased fear of a dirty bombing attack, it is hard to assess whether the actual risk of such an event has increased significantly. The following discussions on implications, effects and probability of an attack, as well as indications of terror groups planning such, are based mainly on statistics, qualified guessing and a few comparable scenarios.
When dealing with the implications of a dirty bomb attack, there are two main areas to be addressed: (i) the civilian impact, not only dealing with immediate casualties and long term health issues, but also the psychological effect and then (ii) 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 RDDs will neither sicken nor kill many people.
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 the men had a swollen hand and diarrhea. A few days later one of the men punctured the 1 mm 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 2 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 which 20 people were seriously ill and 5 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 deceased former K.G.B. spy Alexander Litvinenko, who either ate, drank or inhaled polonium-210. "Smoky bombs" based on alpha emitters might easily be just as dangerous as beta or gamma emitting dirty bombs.
For the majority involved in an RDD incident, the radiation health risks (i.e. increased probability of developing cancer later in life due to radiation exposure) are comparatively small, comparable to the health risk from smoking five packages of cigarettes on a daily basis. The fear of radiation is not always logical. 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. Dealing with public fear may prove the greatest challenge in case of an RDD event. Policy, science and media may inform the public about the real danger and thus reduce the possible psychological and economic effects.
Statements from the U.S. government after 9/11 may have contributed unnecessarily to the public fear of a dirty bomb. 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:
[A] radioactive "dirty bomb" (...) spreads radioactive material that is highly toxic to humans and can cause mass death and injury.
— Attorney General John Ashcroft
This public fear of radiation also plays a big role in why the costs of an RDD impact on a major metropolitan area (such as lower Manhattan) might be equal to or even larger than that of the 9/11 attacks. Assuming the radiation levels are not too high and the area does not need to be abandoned such as the town of Pripyat near the Chernobyl reactor, an expensive and time consuming cleanup procedure will begin. This will mainly consist of tearing down highly contaminated buildings, digging up contaminated soil and quickly applying sticky substances to remaining surfaces to adhere the radioactive particles before radioactivity penetrates the building materials. These procedures are the current state of the art for radioactive contamination cleanup, but some experts say that a complete cleanup of external surfaces in an urban area to current decontamination limits may not be technically feasible. Loss of working hours will be vast during cleanup, but even after the radiation levels reduce to an acceptable level, there might be residual public fear of the site including possible unwillingness to conduct business as usual in the area. Tourist traffic is likely never to resume.
There is also a psychological warfare element to radioactive substances, visceral fear is not widely aroused by, for example, the daily emissions from coal burning, although, as a National Academy of Sciences study found, this causes 10,000 premature deaths a year in the US population of 317,413,000. Medical errors leading to death in U.S. hospitals are estimated to be between 44,000 and 98,000. It is "only nuclear radiation that bears a huge psychological burden — for it carries a unique historical legacy".
In order for a terrorist organization to construct and detonate a dirty bomb, they must acquire radioactive material by stealing it or buying it through legal or illegal channels. Possible RDD material could come from the millions of radioactive sources used worldwide in the industry, for medical purposes and in academic applications mainly for research. Of these sources, only nine reactor produced isotopes stand out as being suitable for radiological terror: americium-241, californium-252, caesium-137, cobalt-60, iridium-192, plutonium-238, polonium-210, radium-226 and strontium-90, and even from these it is possible that radium-226 and polonium-210 do not pose a significant threat. Of these sources the U.S. Nuclear Regulatory Commission has estimated that within the U.S., approximately one source is lost, abandoned or stolen every day of the year. Within the European Union the annual estimate is 70. There exist thousands of such "orphan" sources scattered throughout the world, but of those reported lost, no more than an estimated 20 percent can be classified as a potential high security concern if used in a RDD. Especially Russia is believed to house thousands of orphan sources, which were lost following the collapse of the Soviet Union. A large but unknown number of these sources probably belong to the high security risk category. Noteworthy are the Russian very strong beta emitting strontium-90 sources used as radioisotope thermoelectric generators for beacons in lighthouses in remote areas. In December 2001, three Georgian woodcutters stumbled over such a power generator and dragged it back to their camp site to use it as a heat source. Within hours they suffered from acute radiation sickness and sought hospital treatment. The International Atomic Energy Agency (IAEA) later stated that it contained approximately 40 kilocuries (1.5 PBq) of strontium, equivalent to the amount of radiation released immediately after the Chernobyl accident (though the total radiation release from Chernobyl was 2500 times greater at around 100 MCi (3,700 PBq)).
Although a terrorist organization might obtain radioactive material through the "black market", and there has been a steady increase in illicit trafficking of radioactive sources from 1996 to 2004, these recorded trafficking incidents mainly refer to rediscovered orphan sources without any sign of criminal activity, and it has been argued that there is no conclusive evidence for such a market. In addition to the hurdles of obtaining usable radioactive material, there are several conflicting requirements regarding the properties of the material the terrorists need to take into consideration: First, the source should be "sufficiently" radioactive to create direct radiological damage at the explosion or at least to perform societal damage or disruption. Second, the source should be transportable with enough shielding to protect the carrier, but not so much that it will be too heavy to maneuver. Third, the source should be sufficiently dispersible to effectively contaminate the area around the explosion.
An example of a worst-case scenario is a terror organization possessing a source of very highly radioactive material, e.g. a strontium-90 thermal generator, with the ability to create an incident comparable to the Chernobyl accident. Although the detonation of a dirty bomb using such a source might seem terrifying, it would be hard to assemble the bomb and transport it without severe radiation damage and possible death of the perpetrators involved. Shielding the source effectively would make it almost impossible to transport and a lot less effective if detonated.
Due to the three constraints of making a dirty bomb, RDDs might still be defined as "high-tech" weapons and this is probably why they have not been used up to now.
The present assessment of the possibility of terrorists using a dirty bomb is based on cases involving one terrorist organization, namely Al-Qaeda. This is because the attempts by this group to acquire a dirty bomb are the most well-described in the literature, in part due to the attention this group received for their involvement in the 9/11 attacks.
On 8 May 2002, José Padilla (a.k.a. Abdulla al-Muhajir) was arrested on suspicion that he was an Al-Qaeda terrorist planning to detonate a dirty bomb in the U.S. This suspicion was raised by information obtained from an arrested top Al-Qaeda official in U.S. custody, Abu Zubaydah, who under interrogation revealed that the organization was close to constructing a dirty bomb. Although Padilla had not obtained radioactive material or explosives at the time of arrest, law enforcement authorities uncovered evidence that he was on reconnaissance for usable radioactive material and possible locations for detonation. It has been doubted whether José Padilla was preparing such an attack, and it has been claimed that the arrest was highly politically motivated, given the pre-9/11 security lapses by the CIA and FBI.
Later, these charges against José Padilla were dropped. Although there was no hard evidence for Al-Qaeda possessing a dirty bomb, there is a broad agreement that Al-Qaeda poses a potential dirty bomb attack threat because they need to overcome the alleged image that the U.S. and its allies are winning the war against terror. A further concern is the argument, that "if suicide bombers are prepared to die flying airplanes into building, it is also conceivable that they are prepared to forfeit their lives building dirty bombs". If this would be the case, both the cost and complexity of any protective systems needed to allow the perpetrator to survive long enough to both build the bomb and carry out the attack, would be significantly reduced.
Several other captives were alleged to have played a role in this plot. Guantanamo captive Binyam Mohammed has alleged he was subjected to extraordinary rendition, and that his confession of a role in the plot was coerced through torture. He sought access through the American and United Kingdom legal systems to evidence he was tortured. Guantanamo military commission prosecutors continue to maintain the plot was real, and charged Binyam for his alleged role in 2008. However they dropped this charge in October 2008, but maintain they could prove the charge and were only dropping the charge to expedite proceedings. US District Court Judge Emmet G. Sullivan insisted that the administration still had to hand over the evidence that justified the dirty bomb charge, and admonished United States Department of Justice lawyers that dropping the charge:
...raises serious questions in this court's mind about whether those allegations were ever true.
In 2006, Dhiren Barot from North London pleaded guilty of conspiring to murder innocent people within the United Kingdom and United States using a radioactive dirty bomb. He planned to target underground car parks within the UK and buildings in the U.S. such as the International Monetary Fund, World Bank buildings in Washington D.C., the New York Stock Exchange, Citigroup buildings and the Prudential Financial buildings in Newark, New Jersey. He also faces 12 other charges including, conspiracy to commit public nuisance, seven charges of making a record of information for terrorist purposes and four charges of possessing a record of information for terrorist purposes. Experts say if the plot to use the dirty bomb was carried out "it would have been unlikely to cause deaths, but was designed to affect about 500 people."
In January 2009, a leaked FBI report described the results of a search of the Maine home of James G. Cummings, a white supremacist who had been shot and killed by his wife. Investigators found four one-gallon containers of 35 percent hydrogen peroxide, uranium, thorium, lithium metal, aluminum powder, beryllium, boron, black iron oxide and magnesium as well as literature on how to build dirty bombs and information about cesium-137, strontium-90 and cobalt-60, radioactive materials. Officials confirmed the veracity of the report but stated that the public was never at risk.
In April 2009, the Security Service of Ukraine announced the arrest of a legislator and two businessmen from the Ternopil Oblast. Seized in the undercover sting operation was 3.7 kilograms of what was claimed by the suspects during the sale as plutonium-239, used mostly in nuclear reactors and nuclear weapons, but was determined by experts to be probably americium, a "widely used" radioactive material which is commonly used in amounts of less than 1 milligram in smoke detectors, but can also be used in a dirty bomb. The suspects reportedly wanted US$ 10 million for the material, which the Security Service determined was produced in Russia during the era of the Soviet Union and smuggled into Ukraine through a neighboring country.
In July 2014, ISIS militants seized 88 pounds of uranium compounds from Mosul University. The material was unenriched and so could not be used to build a conventional fission bomb, but a dirty bomb is a theoretical possibility. However, uranium's relatively low radioactivity makes it a poor candidate for use in a dirty bomb. 
The term has also been used historically to refer to certain types of nuclear weapons. Due to the inefficiency of early nuclear weapons, only a small amount of the nuclear material would be consumed during the explosion. Little Boy had an efficiency of only 1.4%. Fat Man, which used a different design and a different fissile material, had an efficiency of 14%. Thus, they tended to disperse large amounts of unused fissile material, and the fission products, which are on average much more dangerous, in the form of nuclear fallout. During the 1950s, there was considerable debate over whether "clean" bombs could be produced and these were often contrasted with "dirty" bombs. "Clean" bombs were often a stated goal and scientists and administrators said that high-efficiency nuclear weapon design could create explosions which generated almost all of their energy in the form of nuclear fusion, which does not create harmful fission products.
But the Castle Bravo accident of 1954, in which a thermonuclear weapon produced a large amount of fallout which was dispersed among human populations, suggested that this was not what was actually being used in modern thermonuclear weapons, which derive around half of their yield from a final fission stage. While some proposed producing "clean" weapons, other theorists noted that one could make a nuclear weapon intentionally "dirty" by "salting" it with a material, which would generate large amounts of long-lasting fallout when irradiated by the weapon core. These are known as salted bombs; a specific subtype often noted is a cobalt bomb.