Security Studies Program Seminar

Detecting Nuclear and Radiological Weapons

Dwight L. Williams, PhD PE
Massachusetts Institute of Technology Martin Luther King Visiting Professor
Department of Nuclear Science and Engineering

April 4, 2007


The Basics

Dr. Williams began his talk by reviewing the basics of atomic structure. An atom is made of protons and neutrons contained in the nucleus. Electrons circle the nucleus. Protons define an element. Neutrons define an isotope. Every isotope has different nuclear properties, but all isotopes have the same chemical properties. Some of these isotopes are radioactive, but some are not.

Example of Isotopes

Types of Radiation

Nuclear Weapons

Radiological Weapons

There is no such thing as a representative radiological weapon.

A radiological weapon could be any of the following: a radiological source strapped onto a regular bomb, a more elaborate device with radiological material inside weapon casing. It does not have to be explosive. It could be powdered Po-210 or an emplaced radiological source such as Cs-137 or Co-60.

In the early 1990s weapon of mass destruction (WMD) meant Nuclear, Biological, Chemical (NBC). Then it meant Chemical, Biological, Radiological, Nuclear (CBRN). Then it meant Chemical, Biological, Radiological, Nuclear, Explosive (CBRNE). In the future WMD might be limited to Nuclear and Biological (NB) because some in the government are questioning whether CRE cause “mass” destruction.

Detecting Uranium Weapons

Detecting uranium weapons is very difficult and a big concern especially if detection is based on gamma emissions.

Detecting Plutonium Weapons

Detecting plutonium weapons is a lot easier than uranium weapons.

Detecting Radiological Weapons

Detecting radiological weapons is possibly easier than uranium and probably harder than plutonium weapons, where large error bars are present. There is no radiological design blueprint because so there are so many design options. As such, there is no governing template. Characterization is likely time consuming. Detecting pre- and post- employment of nuclear weapons (including Po-210, planted sources and dirty bombs) is much easier, and this is always the case.

How Do You Know What's Out There?

Consider the Analogy of the Old AM Radio

There is never just dead silence; the static is the background. There is still signal strength, but it has to be above the background. The background is not constant; it is varying.

So if you want no false alarms, you must set the threshold for deviation high enough above the baseline. If you want to make sure nothing slips through the cracks, the baseline must be lowered. You cannot do both of these things simultaneously. You have to choose.

There is an option to optimize between the two paths. The acceptable false alarm rate must be decided. The optimal rate is principally a matter of convenience. However, it can quickly become an operational issue. Consider police who had sensitive detection pagers which they shut off after a high incidence of false alarms. So the false alarm rate must be reconciled with the level of acceptable risk that the “big one” is missed and a catastrophe occurs. Today both risks are non-zero.

Is There Hope?

Mainly passive detection has been addressed so far. Active interrogation, which identifies materials based on neutron-induced output, offers several benefits. It enables signature/signal detection even in non-radiological materials. It also enables strong and more identifiable signals to be generated. But, operational issues exist including Occupational Safety and Health Administration and Department of Energy regulations of techniques related to the use of active interrogation.

Policymakers must be realistic. There are not perfect solutions to detection. They must try to ask questions and differentiate the realities of technologies offered from unrealistic claims. They need to ask in what situation certain technologies work, if technologies detect or characterize, etc. In conclusion, new paradigms may be needed to solve new problems.

Rapporteur: Nina Fahy

back to Wednesday Seminar Series, Spring 2007