Terrorist Dirty Bombs: A Brief Primer
CRS Report for Congress
Terrorist “Dirty Bombs”: A Brief Primer
Specialist in National Defense
Foreign Affairs, Defense, and Trade Division
Many fear a terrorist attack with a radiological dispersal device (RDD).1 RDDs
may scatter radioactive material with an explosive (a “dirty bomb”) or other means.
Radioactive atoms are unstable; as they decay, they emit electromagnetic radiation or
subatomic particles that can damage cells. Many legitimate activities worldwide use
such material. Dealing with RDDs involves controlling sources, detecting radiation, and
preparing for and responding to an attack. This report will be updated. “Nuclear and
Radiological Terrorism,” in the CRS electronic briefing book on terrorism, tracks
developments. This report does not address nuclear power-related issues; see CRS
Report RS21131, Nuclear Powerplants: Vulnerability to Terrorist Attack.
RDDs vs. Nuclear Weapons. In nuclear weapons, fission and fusion of certain
slightly radioactive materials release energy in a huge explosion. RDDs simply scatter
radioactive material; their main physical effect is contaminating an area. A terrorist group
could create an RDD much more easily than a nuclear weapon.
Radiation. Most atoms are stable: they remain in their current form indefinitely.
Unstable, or radioactive, atoms “disintegrate” or “decay” into other elements, mainly by
emitting an alpha particle (two neutrons and two protons) or a beta particle (an electron
or positron). Emission of photons (typically gamma rays, or high-energy x-rays) often
accompanies decay. The emitted particles and photons are radiation. All elements have
multiple isotopes, or forms with the same chemical properties but different numbers of
1 Useful documents include Roger Eckhardt, “Ionizing Radiation — It’s Everywhere,” Los
Alamos Science, no. 23, 1995, a primer on radiation; Charles Ferguson et al., Commercial
Radioactive Sources: Surveying the Security Risks, Center for Nonproliferation Studies, January
[http://eed.llnl.gov/ans]; U.S. Nuclear Regulatory Commission, “Medical, Industrial, and
Academic Uses of Nuclear Materials,” [http://www.nrc.gov/materials/medical.html]; and Gregory
Van Tuyle et al., “Reducing RDD Concerns Related to Large Radiological Source Applications,”
September 2003 [http://www.nti.org/e_research/official_docs/labs/LAUR03-6%202.pdf].
Congressional Research Service ˜ The Library of Congress
neutrons. Each radioactive isotope decays by steps to isotopes of other elements, ending
as a stable atom. While the instant when one atom will decay cannot be predicted, each
isotope has a “half-life,” the time for half the atoms in a mass of that isotope to decay.
The faster an isotope decays, the faster it releases, and exhausts, its radiation. The
radioactivity of a mass of material is measured in Curies (Ci; 1 Ci = 3.7 x1010
disintegrations per second). Cobalt-60 (the number is the number of neutrons plus
protons in the atom’s nucleus), with a half-life of 5.3 years, is highly radioactive;
uranium-235, with a half-life of over 700 million years, is not.2 Each isotope has a unique
decay fingerprint (e.g., gamma radiation energy) that can be used to identify it.
Biological Effects. Radiation strikes people constantly, but most of it, like radio
waves and light, is not “ionizing”: it does not have enough energy to damage cells
significantly. The biological effects of ionizing radiation depend on the amount of energy
deposited in the body, called the absorbed dose. Higher doses produce direct clinical
effects including tissue damage, radiation sickness and, at very high levels, rapid death.
With chronic low-level exposure, no clinical effects are observed, but the exposed
individual may have an increased lifetime risk of developing cancer. Absorbed dose
depends on several factors. Some are straightforward, such as source strength, distance,
shielding, time of exposure, and energy per particle or photon. Others are more complex.
Type of radiation: A layer of dead skin or a few inches of air stops alpha particles, more
material is needed to stop beta particles, and much more is needed to block gamma rays,
which are more penetrating. Form of material: Alpha and beta emitters do little harm
outside the body because they are easily stopped. Inside the body, they can do much
damage. One can with few ill effects pick up a lump of plutonium-239, an alpha emitter,
because the dead skin layer stops alphas, but a speck of the same material deep in the
lungs bombards tissue with alphas and can cause lung cancer. An RDD thus poses a
greater health threat if its material is finely powdered — and thus more readily dispersed
and inhaled — rather than granular. Chemical behavior of the element in the body:
Certain organs concentrate particular elements. Strontium concentrates in bone;
radioactive strontium-90 can cause bone cancer, breast cancer, and leukemia. The thyroid3
gland concentrates iodine; radioactive iodine-131 can cause thyroid cancer.
Sources of Radioactive Material. Radioactive sources have many beneficial
uses; millions are used worldwide. Sources with a tiny fraction of a Curie, such as
household smoke detectors, do not pose a terrorist threat, but a source with even a few
Curies may be of use for an RDD. While hundreds of radioactive isotopes exist, only a
few isotopes, all produced in nuclear reactors, are of concern for RDDs. Isotopes of
special concern, typical sources, and Curies per source, include cesium-137 (half-life 30.2
years), used in external beam radiation devices to treat cancers (13,500 Ci) and equipment
to monitor wells for oil (0.027-2.7 Ci); and cobalt-60 (half-life 5.3 years), used in
industrial radiography (3-250 Ci) and cancer therapy (0.0014-0.27 Ci). Such sources
often have little security because they are small, have modest amounts of shielding so they
2 U.S. Department of Energy. Office of Environmental Management. “Characteristics of
Important Radionuclides.” [http://www.em.doe.gov/idb97/tabb1.html]
3 Potassium iodide protects against radioactive iodine by saturating the thyroid with stable
iodine-127; it does not protect against other elements. Terrorists are unlikely to use iodine-131
because they could obtain it only from a nuclear reactor, its half-life is so short that much of it
would decay before they could use it, and its intense radioactivity makes it hazardous to handle.
can be used in the field, and do not have enough radiation to be self-protected. They are
sometimes abandoned. In contrast, terrorists would find isotopes with very short half-
lives (hours or less) of little use because the radiation could decay to low levels before the
material could be used, while those with long half-lives (millions of years) emit radiation
very slowly and would do little damage unless inhaled. There is legitimate global
commerce in radioactive materials of concern, but also potential for fraudulent purchases
and theft during shipment or use, and problems of disposing of sources no longer wanted.4
Radiological Dispersal Devices
Alternative Designs. The term “dirty bomb” may have led the media to focus on
a device in which powdered radioisotope surrounds chemical explosive. Many terrorist
groups would have the skill and materials to make the explosive part of the device; it
would be somewhat harder for them to obtain the radioactive material and convert it to
powdered form. Terrorists could also scatter radioactive material without an explosive.
Effectiveness. An RDD’s effectiveness depends on many factors. (1) Some
isotopes do more harm than others, and some elements (including their radioisotopes),
such as cesium, bond strongly to concrete and asphalt. (2) Smaller particles disperse more
easily and are more readily inhaled, but may be harder to make. (3) Using more material
increases physical effects. (4) More explosive would disperse the material more widely.
(5) Weather would play a large role. Higher wind speed would disperse the material more
widely, and wind direction would determine where it would fall. Thermal currents, more
prevalent on a summer’s day than a winter’s night, would also disperse material. Rain or
snow would wash material out of the air but concentrate it in rivers, lakes, and seacoasts.
Greater dispersion would increase the number of people affected while reducing the effect
on each; less dispersion would inflict more effects but on fewer people.
Several estimates have appeared on radiation levels from dispersal of radioactive
material. For example, the Federation of American Scientists calculated that the cesium-
137 in a medical gauge, a small amount, detonated in an RDD at the National Gallery of
Art in Washington, would cover about 40 city blocks with radiation that would exceed
Environmental Protection Agency (EPA) contamination limits (a one in 10,000 chance
of getting cancer). This area might, depending on wind direction, include the Capitol,
Supreme Court, and Library of Congress. “If decontamination were not possible, these5
areas would have to be abandoned for decades,” by one estimate. Others feel that such
scenarios exaggerate the effectiveness of RDDs by assuming that material disperses well
and by downplaying the ability to decontaminate affected areas. EPA guidelines magnify
RDD effectiveness. Steven Koonin, Provost of California Institute of Technology, stated
that 3 curies of an appropriate isotope, a fraction of a gram, dispersed over a square mile
“would make the area uninhabitable, according to the maximum dose currently
recommended for the general population.” However, “the health effects of such
contamination would be minimal. For every 100,000 people exposed to that level of
radiation, four lifetime cancers would be induced, which would take place on top of the
4 Much of the material in this paragraph is from Ferguson et al., Commercial Radioactive
Sources, p. vi, 3, 12, 13, 43-44.
5 U.S. Congress. Senate. Committee on Foreign Relations. Dirty Bombs and Basement
Nukes: The Terrorist Nuclear Threat, hearing, 107th Congress, 2nd Session, 2002, p. 39-40.
20,000 cancers already expected to arise from other causes.”6 Even such low-level effects
are debated; some argue that these effects are extrapolations from higher doses with no
conclusive evidence to support their existence.7
Terrorists could try to achieve several goals with RDDs in the following sequence.
Most depend on public fear of any radiation rather than actual levels of radiation. (1)
Deaths and injuries. Any prompt casualties would most likely come only from the
explosion of a dirty bomb; many experts believe these would be few in numbers.8 (2)
Panic. Small amounts of radioactive material might cause as much panic as larger
amounts. (3) Recruitment. The worldwide media coverage of an RDD attack would be
a powerful advertisement for a terrorist group claiming responsibility. (4) Asset denial.
Public concern over the presence of radioactive material might lead people to abandon a
subway system, building, or university for months to years. (5) Economic disruption. If
a port or the central area of a city were contaminated with radioactive material, commerce
there might be suspended. (6) Long-term casualties. Inhalation of radioactive material
or exposure to gamma sources could lead to such casualties, probably in small numbers.
Prevention and Response
Securing Radioactive Sources. Prior to September 11, 2001, safe handling of
sources was the chief concern. They were used worldwide in medical equipment, oil well
gauges, etc., with little security. Some were abandoned, becoming “orphan sources.”
After the attacks, attention shifted to securing them. Various measures seek to control
U.S. radioactive materials. The Nuclear Regulatory Commission (NRC) regulates the use
and transportation of most radioactive sources for nuclear reactors, for medical, industrial,9
and academic uses, and related facilities. Many states share in regulation. An NRC-
Department of Energy (DOE) working group to increase the security and regulatory
oversight of high-risk radioactive sources has proposed verifying the legitimacy of
applicants for licenses, preventing insiders from diverting sources, and controlling imports10
and exports of sources. Several programs provide for the disposal of unwanted
radioactive sources, which can be difficult. EPA’s Orphan Sources Initiative will
establish a national system to retrieve radioactive sources from non-nuclear facilities like
6 Senate Foreign Relations Committee, Dirty Bombs and Basement Nukes, p. 17.
7 See U.S. General Accounting Office. Radiation Standards: Scientific Basis Inconclusive, and
EPA and NRC Disagreement Continues. RCED-00-152 June 30, 2000.
8 Richard Meserve, former Chairman, Nuclear Regulatory Commission, held that an RDD might
cause “deaths on the order of tens of people in most scenarios.” Senate Foreign Relations
Committee, Dirty Bombs and Basement Nukes, p. 8.
9 U.S. Nuclear Regulatory Commission, “Medical, Industrial, and Academic Uses of Nuclear
Materials” [http://www.nrc.gov/materials/medical.html]; and “How We Regulate,”
[http://www.nrc.gov/what-we-do/regulatory.html#evaluating]. For legislation establishing and
governing NRC, see [http://www.nrc.gov/who-we-are/governing-laws.html].
10 Richard Meserve, Chairman, “Statement Submitted by the United States Nuclear Regulatory
Commission to the Subcommittee on Oversight and Investigations, Committee on Energy and
Commerce, United States House of Representatives Concerning Nuclear Security,”Mar. 18, 2003.
scrap yards and dispose of them.11 The Off-Site Source Recovery Project, operated by
Los Alamos National Laboratory, gathers sources owned by, or the responsibility of, the
Department of Energy from around the United States, transports them to Los Alamos, and
stores them there.12 The National Nuclear Security Administration’s (NNSA’s) Nuclear
Radiological Threat Reduction Task Force is consolidating that and other programs to
secure high-risk radioactive material worldwide.13
Some international efforts seek to secure sources. This goal is important; according
to one expert, over 100 countries in 1999 were “known or thought to lack effective control
over radiation sources and radioactive materials.”14 In March 2003, the International
Atomic Energy Agency (IAEA) held an International Conference on Security of
Radioactive Sources.15 In June 2002, the G-8 committed itself to “six principles to
prevent terrorists or those that harbour them from acquiring or developing” radiological
and other WMD.16 NNSA has identified 35 large radiological waste sites and over 1,000
orphan or surplus radioactive sources in the former Soviet Union, and has initiated a
cooperative program with the IAEA and these republics to locate and secure these sites
and sources.17 The IAEA has begun discussions with source manufacturers and suppliers
to address alternate sources, possible fraudulent purchases, and source disposal options.
Avoiding the Use of Radioactive Sources. For some uses, radioactive
material is the only way to achieve the desired result. For others, alternatives exist, such
as x-ray machines or particle accelerators. These machines use electric power to generate
radiation, have no radioactive material, and are not radioactive when the power is off.
Detection. RDDs are the least difficult WMD to detect. Chemical or biological
agents in airtight containers have no signatures by which they could be detected. RDD-
suitable material is more detectable than the highly enriched uranium or plutonium-239
used in nuclear weapons because it is much more radioactive. Hiding radioactive material
would require much shielding that could raise suspicions if seen on an x-ray inspection
machine, and infrared detectors can detect the heat generated by large radioactive sources
despite shielding. Many sensors can detect radioactive material, such as Geiger counters,
gamma-ray detectors, and (at short range) pager-size radiation detectors used by Customs
and Border Protection (CBP) agents. Portal monitors detect radiation in nearby sources,
11 U.S. Environmental Protection Agency. “Orphan Sources Initiative.”
ht t p: / / www.epa.gov/ r a di at i on/ cl eanme t a l s / or phan.ht m
12 For further information on this program, see [http://osrp.lanl.gov].
13 U.S. Department of Energy. National Nuclear Security Administration. “NNSA Forms New
Task [Force] to Address Nuclear and Radiological Threats,” press release, November 3, 2003.
14 Abel Gonzalez, “Strengthening the Safety of Radiation Sources & the Security of Radioactive
Materials: Timely Action,” IAEA Bulletin, 41/3/1999: 9.
15 See [http://www.iaea.org/worldatom/Press/Focus/RadSources/index.shtml].
16 G8, “The G8 Global Partnership Against the Spread of Weapons and Materials of Mass
Destruction,” June 27, 2002, [http://www.g7.utoronto.ca/summit/2002kananaskis/arms.html].
17 U.S. Department of Energy. Office of Management, Budget and Evaluation/CFO. FY 2004
Congressional Budget Request: National Nuclear Security Administration, DOE/ME-0016, vol.
such as vehicles or containers. CBP is installing them nationwide at sea, land, and air
ports.18 NNSA is deploying them in Russia and elsewhere through its Second Line of
Defense program, and in other ports through its Mega-Ports program.19 Detecting RDDs,
though, is not simple. Material might be smuggled across unguarded stretches of coasts
or borders or obtained within the United States, so a system to detect RDDs inside this
nation might be needed to complement border detection efforts. The difficulty of finding
RDD material emphasizes the value of eliminating or securing it.
Advance Steps to Minimize Effects of an RDD Attack. As noted earlier,
most such effects flow from fear of radiation. A large-scale public education program,20
available for use in the event of attack, could help quell panic. Other steps might
include deploying radiation detectors in large cities, and developing and applying coatings
to prevent radioactive material from bonding to streets and buildings, though it is not clear
that the benefit of coatings would merit the cost.
Response to an Attack. The initial response would likely involve detecting an
attack, evacuating areas that might receive radiation or keeping people indoors until
respirable material had dispersed, treating people who might be exposed, and sheltering
evacuees. The Federal Radiological Emergency Response Plan21 would come into play.
DOE’s Nuclear Emergency Support Teams, among others, could assist.22 Harry Vantine,
of Lawrence Livermore National Laboratory, suggests having the prompt ability to predict
dose to the population from an RDD attack, and exercising decontamination procedures.23
A public education program could be implemented promptly. Longer-term responses
would include monitoring radiation levels, defining and decontaminating affected areas,
and decontaminating or demolishing affected buildings. Promulgating standards that
permitted exposure to somewhat higher levels of radiation while having few adverse
health effects, as noted above, would greatly reduce the area to be abandoned and the
decontamination required. Public acceptance of such standards would be uncertain.
18 U.S. Department of Homeland Security. Customs and Border Protection. “Radiation and
Portal Monitors Safeguard America from Nuclear Devices and Radiological Materials.” c. 2004.
19 U.S. Department of Energy. Office of Management, Budget, and Administration/CFO. FY
DOE/ME-0032, February 2004, p. 447, 454, 455.
20 For information on coping with RDD attack and other emergencies, see U.S. Federal
Emergency Management Agency, Are You Ready?: A Guide to Citizen Preparedness, revised
September 2002, 101 p. [http://www.fema.gov/areyouready/]; National Council on Radiation
Protection and Measurements, “Management of Terrorist Events Involving Radioactive
Material,” 2001, 232 p., NCRP report 138, [http://www.ncrp.com/ncrprpts.html]; and RAND,
Individual Preparedness and Response to Chemical, Radiological, Nuclear, and Biological
Terrorist Attacks, 2003, 232 p. [http://www.rand.org/publications/MR/MR1731].
21 See [http://www.au.af.mil/au/awc/awcgate/frerp/frerp.htm].
22 See Jeffrey Richelson, “Defusing Nuclear Terror,” Bulletin of the Atomic Scientists, March-
April 2002: 39-43; and U.S. Department of Energy. Order DOE 5530.2, “Nuclear Emergency
Search Team,” Sept. 20, 1991, at [http://www.fas.org/nuke/guide/usa/doctrine/doe/o5530_2.htm].
23 Senate Foreign Relations Committee, Dirty Bombs and Basement Nukes, p. 55.