Detection and Radiological Threat Response

Feb. 24, 2005
Can building occupants be protected from an attack involving nuclear or radioactive material? The answers depend on the means of the attack.

by Mark L. Maiello, Ph.D.

Radioactive sources are in used routinely in industry, medicine and the military. It is believed that terrorists would find relatively intense gamma radiation or alpha radiation emitting sources the most desirable tools for a radiological attack.1

Gamma radiation is emitted over the longest distances. Heavy lead or other metal shielding is required to prevent dangerous human exposure and detection while transporting and using large sources. Alpha sources release radiation of extremely short range requiring no shielding and so, is difficult to detect remotely. To make it an effective terrorist weapon, the alpha source would have to be aerosolized. Inhalation could lead to a significant dose to the lungs, increasing the risk of developing lung cancer years after the exposure. Other forms of ionizing radiation include beta and neutron radiation.

Many radioactive sources are produced in nuclear isotope production reactors in Canada, France, Belgium, Russia, the United States, the Netherlands, South Africa and Argentina. Uses vary from industrial radiography (Iridium-192) to cancer therapy (Cobalt-60, Cesium-137), to food irradiation. Several terrorist scenarios have been postulated utilizing radiation sources, nuclear powered electric generating stations and nuclear weapons.

Implications of a Terrorist Attack on a Nuclear Reactor

The argument over nuclear power plant vulnerability will not be easily resolved. Therefore, it is necessary to consider the release of radioactivity from a damaged reactor as a potential inhalation hazard for building occupants in the path of the airborne plume.

Radioactive iodine-131 is the most important hazard in this scenario. It can enter the food chain via deposition on pasture where it then can be incorporated into the milk of cows and eventually ingested by the consumers. Iodine may also be inhaled directly from the airborne plume. The thyroid gland receives much of the radiation exposure from ingested radioiodine.

The sorbent method of charcoal filtration has long been used to capture radioiodine.2 It is possible to filter heating, ventilation and air-conditioning (HVAC) intake air using this technique. The National Institute of Occupational Safety and Health (NIOSH) recommends sorbent filtration installation down line from particulate filters to collect vapors formed on the filters from capture of the radioactive aerosol. Adsorbent capacity must be determined with consideration for the effects of humidity, temperature, and iodine buildup on the sorbent. The breakthrough point of the iodine vs. charcoal bed size will give some idea of the period of protection that charcoal filtration will afford.

At least one charcoal filtration supply-company manufactures a HVAC filtration system for chemical, biological and nuclear attack.3 The system uses a prefilter, a first stage high efficiency particulate air (HEPA) filter, a first stage carbon filter (V-bed arrangement using salt/metal impregnated "whetlerite" charcoal for reactivity with chlorine, phosgene and mustard gases; 12 x 30 mesh), a second stage carbon bed and finally a second stage HEPA filter. This system is self-contained, uses a "bag in/bag out" system for filter change-out and is provided with test ports for determining breakthrough from the 1st carbon stage. It can pull 9,000 ft3 per minute. Considering the installation costs and maintenance of such a system, it may be more efficient to stockpile potassium iodide pills for distribution to building occupants when radioiodine is released and sheltering in place is enforced or recommended by governmental authorities. Potassium iodide blocks the uptake of radioactive iodine but does not protect against other types of radiation or other toxic materials (see for guidance about use).

Detonation of a Nuclear Device

In light of the destructive power of nuclear weapons, it only makes sense to discuss the effects on surviving, functioning structures. The greater the distance from ground zero and the weaker the weapon, the higher the chances that a structure will survive the thermal and blast forces generated by the detonation. Occupants who are at a greater distance from ground zero will also have a lower probability of suffering thermal burns, blindness from the brightness of the explosion, and the initial ionizing radiation exposure.

When a nuclear device is large enough and detonated at ground level, it will carve out a crater and throw melted soil and debris into the air. Eventually, this radioactive material cools and falls back to ground level as solid particles with diameters of less than a micrometer to several millimeters. Deposited fallout will increase the intensity of the ground-level radiation field, potentially to levels where radiation burns could occur and radiation exposures could have a significant health impact.

Another nuclear detonation effect of consequence at a distance is the electromagnetic pulse (EMP).4 EMP arises from the interaction of the gamma rays emitted in the nuclear detonation with air molecules. The pulse contains electromagnetic energy of widely varying frequencies and amplitudes.5 EMP from a surface burst is confined to a radius around ground zero of about 2 to 5 miles depending on the weapon yield. Severe blast damage may still occur at these distances rendering concern about EMP within this range moot, but EMP may extend further than blast damage for low-yield weapons. Currents and associated high voltages can arise in metallic conductors in its path resulting in damaged electronics. The most susceptible equipment depends on solid-state semiconductors like those used in computers, computer-controlled building management systems, life support equipment, and communication systems. The least susceptible equipment includes 60 Hz motors, transformers, heavy-duty relays and breakers. Therefore, structures that survive the blast and thermal effects of a nuclear detonation may encounter both EMP and fallout effects under the proper circumstances.

Short of constructing sheltering for a nuclear attack, engineered fallout protection basically becomes a HVAC issue. Engineers should consider the tightness of the building envelope, the use of fast acting dampers and rapid HVAC shutdown because sheltering in place could be invoked as a survival method. Under that scenario, survival depends on moving people away from upper and lower floors where fallout deposition (roof and ground) will occur. In low-rise buildings (perhaps three floors or less), moving occupants to habitable, below-grade basements with concrete walls has been recommended.

The radiation from fallout decreases roughly by a factor of 10 for every seven-fold increase in time since the moment of detonation. An initial radiation exposure rate of 1 rad per hour will not decrease to a relatively low 0.01 rad per hour until 49 hours (7 x 7) have passed.6 Higher initial exposure rates will require more time to reduce to safe levels. This will strain all but the best-prepared internal resources especially under power blackout conditions with electronic communications knocked out.

Providing protection against EMP ("hardening") is a specialized practice of some electrical engineering firms. They may recommend the construction of shielded rooms using steel or copper sheets (Faraday cages) and grounding to couple important circuits to earth. Band-pass filters, spark gaps, circuit breakers, arresters and amplitude limiters also provide protection against the electrical transients produced in circuitry by EMP.

Hospitals & Radiological Events

Hospitals will have to deal with the casualties of terrorist actions. Medical radiological emergency response prioritizes injury treatment over patient radiation exposure and contamination issues in order to save life.7,8 This initial patient treatment is not expected to be a significant hazard to care-takers. A "hot" zone for receiving patients, a buffer zone for radiation surveys and the disposal of contaminated items such as street clothing, and finally, a clean area for admittance to the rest of the hospital are to be employed. Heavy-duty floor coverings such as Teflon coated, glue-on contact paper, can be applied to mitigate the clean up of contamination, should it occur.

Contamination control procedures primarily require personnel cooperation and training, rather than HVAC or other technologies to successfully protect the facility. Therefore, it is advisable for hospital designers and managers to observe emergency response drills and to have discussions with medical physics and health physics professionals concerning the contributions they can make in this area.

Detection of Illicitly Planted Radioactive Sources

Due to the silent, insidious nature of a covertly planted unshielded source, detection is the primary means of defense. The nuclear industry has long used portal monitors for real-time measurement of low-level radioactive contamination on personnel leaving or entering nuclear power plants. Similar in size to the metal detectors used to screen airline passengers, they employ sensitive beta/gamma detectors and audio alarms to indicate the presence of radioactivity. Typical prices for these devices are approximately $8,000 to $20,000, depending on capabilities.

Larger versions of these devices (approximately 15 feet high, 12 feet wide and 7 feet deep), called "gate monitors" can be employed to screen tractor-trailers and other vehicles. Costs range from about $9,000 to over $30,000. Related devices are currently in use at landfills and waste transfer stations to screen trash-hauling vehicles for improperly disposed radioactive waste (about $4,000 to $10,000).9

Wall-mounted area radiation detectors and "package transit monitors," can be installed in locations like mail rooms, shipment receiving docks or storage areas for continuous monitoring purposes (about $1,000 to $2,000). They can be set to alarm when a preset exposure level is exceeded. Some have network capability to provide the user with multiple area surveillance and continuous measurement documentation. Continuous Air Monitors (CAM) provide warning at predetermined alpha-particle air concentrations and can also be monitored by computer (roughly $12,000). Air monitors that specifically measure ambient beta-radioactivity can also be purchased.10

Portable radiation survey devices like Geiger counters (most under $1,000) have been the mainstay of radiological contamination control since the development of the atom bomb. Their small size and low weight make them ideal for surveying confined areas. However, non-technical personnel often find interpretation of survey measurements difficult. Devices worn at the belt that simply alarm when radiation is detected are often preferred. They have become commercially available to law enforcement and first responders since 9/11 (about $ 300 to $1,500).11

Radiological Dispersion Devices

A radiological dispersion device (RDD) is an aerosolized radioactive source made airborne or dispersed mechanically e.g., by a HVAC system or a crop dusting aircraft. The efficient production of aerosols with respirable particle diameters (roughly 0.001 to 10 microns12) by chemical explosion is not assured without proper engineering of the charge.13 There is also disagreement about whether radiation doses to the public from a detonated RDD will be low or high.14,15 Assuming aerosolization is possible, the primary routes of radiation exposure seem to be:

  • Inhalation of respirable alpha or beta/gamma-emitting radioactive particles
  • Close proximity to building materials covered with significant amounts of gamma-emitting contaminated dust and debris
  • Proximity to a fragment of a large gamma-source for a long period until rescued.

Protection against radioactive aerosols produced outdoors is not very different from chemical or biological toxin defense. Air filtration of intake air is key. Traditionally, HEPA filters are employed because of their high air-cleaning capacity. However, these filters create significant pressure drops that will require powerful fans and motors to overcome. The resulting initial and operating costs need to be weighed against the likelihood of a radiological attack on the structure. Other techniques include:

  • Eliminating air bypass around the filter holding racks to maintain efficient particulate filtration.
  • Tightening the building envelope to preclude outside air from entering except via the HEPA filtered air intakes.
  • Installing low leakage, fast acting dampers for the purposes of rapid shutdown of the HVAC system and limiting the infiltration of outdoor air.
  • Moving air intakes to a higher building level to prevent easy human access.

The defense against the implications of an indoor RDD detonation includes the following methods to isolate the affected areas and prevent the spread of radioactive contamination:

  • Physically isolating areas where an RDD can be introduced such as incoming mail and parcel rooms, loading docks and lobbies. The portals to these areas should be closed when not in use.
  • Physically isolating the HVAC systems (and the return air systems) of incoming mail and parcel rooms, loading docks and lobbies from the remainder of the return air system.
  • Maintaining the air pressure of incoming mail and parcel rooms, loading docks and lobbies negative relative to the rest of the building but positive relative to outdoors.

The sheltering of building occupants in place, a temporary defensive measure against an external RDD, also requires modifications to HVAC systems to make it truly effective. These include:

  • Modifying the HVAC system to minimize or eliminate mixing between air handling zones.
  • Tightening the building envelope and pressurizing the building to keep the radioactivity from infiltrating for as long as possible.
  • Modifying the HVAC system control to provide rapid shut down.


The economic and emotional detriment from a successful radiological incident will have serious consequences for those in the effected building(s) and the surrounding area (a nuclear weapons incident will of course, be devastating). The perception by politicians, regulators and the public that all the contamination will have to be totally removed is not a very realistic goal. Very effective radioactive decontamination can be achieved, but it will take a large expenditure of money. At any rate, a negative stigma will be associated with the target location for a long time.

Despite all our knowledge of radiation and our capabilities to control it, the moral of this story comes down to this: vigilance. Scientists and engineers, along with governments and policy-makers world wide, must do all they can to prevent malevolent use of radioactive sources and to safeguard and reduce the numbers of nuclear weapons.

(The reader will find more details concerning the defensive measures for building HVAC systems in several publications from NIOSH and in a draft document from the U.S. Army Corp of Engineers. 16,17,18)


1Ferguson, C.D., Tahseen, K., Perera, J. Commercial Radioactive Sources: Surveying the Security Risks. Occasional Paper No. 11, Monterey Institute of International Studies, Center for Nonproliferation Studies. January 2003.

2Stewart, D.C. Handling Radioactivity. John Wiley and Sons, New York, 1981.

3Flanders/CSC Corporation. Custom Engineered Systems. PB-2008-1099. Bath, North Carolina, 2002.

4Glasstone, S. & Dolan, P. The Effects of Nuclear Weapons 1977. U.S. Government Printing Office, Washington, DC, 1977.

5Hazardous Radiofrequency Radiation: Evaluation, Control, Effects and Standards. Course No. 588, George Washington University. Dr. Bernhard E. Kaiser. July, 1986.

6National Council on Radiation Protection and Measurements. Radiological Factors Affecting Decision-Making In a Nuclear Attack. NCRP Report No. 42. Bethesda, MD, November 15, 1974.

7Emergency Department Management of Radiation Casualties. Prepared by the Radiological Emergency Medical Preparedness & Management Subcommittee of the National Health Physics Society Ad Hoc Committee on Homeland Security (undated)

8American College of Radiology. Disaster Preparedness for Radiology Professionals. Version 2.1, Pearl River, NY 10965

9Ludlum Measurements, Inc. 2002 Catalog.

10Technical Associates. Nuclear Instrumentation Catalog.

11Will Whitehorn. Berkely Nucleonics Corporation. Personal Communication. September 23, 2004.

12Hinds, W.C. Aerosol Technology. John Wiley and Sons, New York. 1982.

13Public Protection From Nuclear, Chemical and Biological Terrorism. Proceedings of the Health Physics Society Summer School – 2004. Brodsky, A., Johnson, R, & Goans, R., Eds. pp. 71-72. Medical Physics Publishing, Madison, WI. 2004.

14American Institute of Physics. Inside Science News Service. "Dirty Bombs" Much more likely to create fear than cause cancer: radiation effects from such devices likely to be too low to calculate, health physicist says. March 12, 2002.

15Zimmerman, P. and Loeb, C. Dirty bombs: the threat revisited. Defense Horizons, pp. 1- 11, January 2004.

16NIOSH. Guidance for Protecting Building Environments from Airborne, Chemical, Biological, or Radiological Attacks. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. May 2002.

17NIOSH Filtration and Air-Cleaning Systems to Protect Building Environments From Department of Health and Human Services from Airborne, Chemical, Biological, or Radiological Attacks. Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. April 2003.

18U.S. Army Corp of Engineers. Protecting Buildings and Their Occupants Form Airborne Hazards (Draft). TI 853-01 Washington, DC. October 2001.

Mark L. Maiello, Ph.D. received his B.S. in physics from Manhattan College and master's and doctoral degrees from the New York University Institute of Environmental Medicine. As an employee of the Environmental Measurements Lab in NYC (then administered by the U. S. Dept. of Energy, now by the Dept. of Homeland Security), he pursued an interest in the measurement of radon gas and natural background radiation. Mark has published several peer-reviewed scientific articles on both subjects. He joined a New York consulting firm in 1990, performing radiation measurements and health physics training at radiological remediation sites in the New York metropolitan area. In 1996, he accepted the position of assistant radiation safety officer at Wyeth Research. Now radiation safety officer at the Wyeth Pearl River, N.Y. facility, Mark writes about radiation safety and the radiological implications of 9/11. He is a contributing editor to Health Physics News.

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