Dirty bombs: The terror and the truth

This is because a key aspect of dirty bombs is that they are self-limiting—the greater the dispersal, the lower the radioactive doses. A fist-sized amount of powdered cesium chloride (¹³⁷CsCl), for example, is lethal after about 1 hour of exposure at 1 meter (a dose of about 1,000 rem per hour, or 10 Sv per hour).

However, this amount of radiation is not dangerous if it is spread out by an effective car bomb over 10 square city blocks (a dose of < 1 rem a year, or 10 mSv a year), because the surface area of 10 square city blocks is over 1 billion square feet (93 million square meters).

The United States has performed detailed studies of dirty bombs for years and has come to the conclusion that such an attack would be ineffective and that the weapon does not pose a substantial danger beyond the conventional blast and the psychological effects. Other countries also have come to the same conclusion.

Fig. 1. RDDs are devices that disperse radioactive materials by any means possible—they are not nuclear weapons in any sense. The most effective means of deployment is a car bomb that incorporates easily dispersible, highly radioactive material, for example 137CsCl powder in an ammonium nitrate/fuel oil car bomb. (Photo: Middle East Intelligence Bulletin)

Only those people close enough to be hurt or killed by the explosive device itself would receive a significant dose of radiation. Those out of harm’s way from the blast should just go home, shower, and bag their clothes. Few people—if any—would die from the radiation of a dirty bomb, even a big one, although hundreds could die from the initial explosion.

But it would scare everyone. The resulting evacuation, disruption of services, shutdown of the local economy, and costly cleanup—not death and destruction—is the real point of a dirty bomb. Every day that lower Manhattan is closed would cost our economy $60 million. Cleanup could take years. This is why Sig Hecker, former director of Los Alamos National Laboratory, referred to RDDs as weapons of mass disruption.

Isotopes of plutonium, americium, and uranium are primarily alpha emitters; cobalt-60 and cesium-137 are gamma emitters; and strontium-90 is a beta emitter. Co-60 usually occurs as a metal (either pellets or small rods); Cs-137 as a powder; Sr-90 as a ceramic; and Pu, Am, and U as various oxides, salts, and nonmetallic solids.

Although the public generally thinks of plutonium and enriched uranium when hearing the word “radioactive,” these isotopes are not considered good dirty bomb materials, because they are primarily alpha emitters, costly, well tracked and secured (and therefore cannot be obtained in large amounts), and are more useful in the production of actual nuclear weapons than in being wasted in a dirty bomb. For the purposes of a dirty bomb, ¹³⁷CsCl powder is the most effective material—easily dispersible, hard gamma–emitting, and very cheap, at about $3 per curie.

So where can terrorists get radioactive materials for dirty bombs?

Fig. 2. The primary uses, specific isotope, and activity levels of radiological source materials around the world. The largest sources use only Co-60, Cs-137, or Sr-90, of which only Cs-137 is useful as a dirty bomb material. (Image: Greg van Tuyle)

Radioactive materials are used in many fields in many countries, particularly for medical, research, and industrial applications. Dozens of radiological source producers and suppliers are found on six continents, and about a billion sources exist worldwide—although most, like household smoke detectors, have such low activities that they pose no threat.

There are about 10,000 sources worldwide that exceed 1,000 curies. The largest sources use Co-60, Cs-137, or Sr-90, because the larger the source, the more cost-effective the application. Consider food and produce irradiation, for example. The USDA approved importation of irradiated food in 2002, and there are now greater incentives for deploying food irradiators in countries that export food into the United States, particularly in economically depressed regions like southern Asia. It is critical that regulations and oversight increase as this market increases, and that appears to be happening.

Sandia National Laboratories conducted substantial research on RDDs, along with New Mexico Tech’s Energetic Materials Research and Testing Center and LANL. An extensive amount of work has also been done by the Department of Homeland Security, universities, the Department of Energy, and at most of our national laboratories.

Fig. 3. 137CsCl powder, presently used in the irradiation industry, is the terrorist’s dirty bomb material of choice: it is inexpensive and emits a hard gamma ray at 0.66 mega-electron-volts, requiring about 18 centimeters of lead or three feet of concrete to shield. It has a high specific activity (87 Ci/g), so it takes only 2.5 kilograms to make a super RDD of over 200,000 curies. Its powdered form makes dispersal easy, and its 30-year half-life means it will be a problem if not cleaned up quickly. In the environment, cesium chloride dissolves easily into Cs+ and Cl− and behaves like potassium chloride. (Photo: J. Wischnewsky)

The only effective response to a dirty bomb attack is to deluge the affected area with water—immediately. One hundred fire hydrants going for 24 hours delivers about 100 million gallons of water, enough to wash the kilogram or so of ¹³⁷CsCl powder into the sewer system and out to sea or other sink, where it would be so diluted that the risk of radiological exposure would be vanishingly small. The response is the decision of the on-site incident commander, who most likely would be a local HAZMAT captain, since the official RDD incident commander—the FBI—wants nothing to do with radiation. But this is a good thing, since as long as the incident commander does not declare the emergency, they are free, legally, to make any decisions they want without having to go to committee, involve the Environmental Protection Agency, or worry about irrational fears of radiation.

In fact, the response to a dirty bomb incident could greatly impact the probability of such an event occurring again. If, for example, a dirty bomb attack shut down Manhattan for one year with an economic impact of $100 billion in cleanup costs and lost business, this would be deemed a great success by the perpetrators, and the likelihood is much greater of a second attack elsewhere. On the other hand, if we are even somewhat prepared, respond well and take appropriate measures in the week immediately following the event, and keep the economic impact below $100 million, the likelihood of a second attack presumably will be lower.

It is essential that responders understand the problem and have an effective and executable response plan. It is also critical that our society continues rational discussions of risk on a national level without sensationalizing particular scenarios beyond their actual likelihood.

Radiological risks are the most obvious subject of sensationalism. Knowledge about radiation and its effects is not intuitive. The subject is generally not taught in school and is not generally understood. In reality, no one ever dies in the nuclear power industry except for an occasional fall or non-nuclear-related accident every five years or so. On the other hand, in just one year, nearly 500,000 Americans die from smoking and almost 40,000 from car accidents. Even food poisoning kills 5,000 Americans every year. This misperception of how dangerous radiation is constitutes the number-one reason dirty bombs are so effective: How responders and the public react to an attack determines its success.

Short of a multiyear national public education initiative on radiation, how can we prepare for this type of attack? The answer is to develop simplified, practical guidance for responding to a radiological attack that does not depend on in-depth understanding of radiation, that dispels the fear that comes from misperceptions, and that fits into the National Incident Management System/Incident Command System framework so that it can be implemented during a crisis.

Below is an example of a 12-step guidance for first responders:

Assume all explosions, particularly vehicles, are the result of dirty bombs.

Set up an exclusion zone boundary (as appropriate, depending on the availability of dose or activity readings).

Personnel within the “hot zone” should wear full personal protective equipment with a particulate full face mask and updating, cumulative dosimeter.

Alert the appropriate secondary response teams (e.g., National Guard WMD Civil Support Team, Radiological Assistance Program, and FBI) as advised by regional protocols as soon as it is determined that the situation is radiological.

Establish an incident command center upwind of ground zero at the closest point outside of the affected zone.

Evacuate the affected area and exclude all nonessential personnel. Establish quick dose-rate screening or radiological monitoring, and advise the population to remove outer layers of clothing and bags; avoid eating, drinking, and touching their faces; and shower immediately with warm water and soap. Where possible, fire hose washdown curtains can be employed.

Do not decontaminate vehicles or structures during the initial phase, and encourage water to enter the municipal stormwater drainage system. Alert the wastewater treatment facility for possible diversion strategies.

Establish decontamination areas, and provide examination and follow-ups to those with obvious and/or heavy contamination. Countermeasures should be evaluated and administered promptly.

Perform surface decontamination for those requiring immediate medical assistance, and inform the receiving facility that there is little to no surface contamination, so admittance is not denied.

Map the affected area to obtain a rough dose profile and assess the magnitude of the event.

Essential personnel should continuously record cumulative dose.

Evacuate buildings along determined safe routes and away from the hot zone, using connected subsurface routes if available. Leave building ventilation systems running.

Options for post-event mitigation are still limited, but researchers have investigated spray-on fixatives to prevent secondary migration and make subsequent cleanup easier, and the DOE has developed spray-on solvents that can leach cesium out of concrete. These may be ideal for the most heavily affected areas and for cleanup of buildings where time is not a factor, but having sufficient materials to treat a billion square feet of surface area quickly is highly unlikely. The only solvent we have available in millions of gallons at a moment’s notice is water from fire hydrants—and because ¹³⁷CsCl powder is so soluble that it can be washed off surfaces with water, this is the best option.

However, washdown must be done quickly and completely—within days or even hours of the event—to preclude further effects such as diffusion into building materials and secondary migration. Diffusion rates are primarily a function of moisture content and will depend strongly on weather conditions and the porosity of materials.

There’s considerable debate over washdown approaches, but it’s unlikely that any other strategy could be implemented rapidly enough to be effective. One hundred fire hydrants operating for 24 hours would deliver about 100 million gallons of water, which is adequate to douse large areas and wash most of the cesium into the stormwater system, where it would be adequately diluted.

Alternatively, water used to wash away the radiological material could be treated at the outflow points using inexpensive materials, such as gabions of specific zeolitic gravel (at $80 per ton). The logistics would be difficult, but if solved this would also be effective.

Any RDD antiterror strategy must include serious efforts to communicate and inform the public about the relative dangers—and lack thereof—of dirty bombs. If the consensus is that the amount of radiation dispersed won’t do much harm, let’s say so—clearly and without qualification. Weasel words do not inspire confidence.

There should also be a significant federal investment in a renewed, focused low-dose radiation health effects research initiative. There is a growing consensus that the current radiation standards are overly conservative, with dangerous unintended consequences, like those seen at Fukushima.

Sure, an RDD with a category 3 source might still have significant socioeconomic costs, but the biggest concern is that we will wake up one day to the news that there has been an explosion and radiation has been detected. Then the experts start talking about billions of becquerels or picocuries, and before we know it, everyone is scared out of their gourds.

The American people aren’t stupid. They just need to have the facts. Together, we can find creative ways of having an honest conversation about this before something bad happens.

James Conca is a scientist in the field of earth and environmental sciences specializing in geologic disposal of nuclear waste, energy-related research, planetary surface processes, radiobiology and shielding for space colonies, and subsurface transport and environmental cleanup of heavy metals.