VenChar

Detecting nukes in transit: What can the newly-established DNDO do?

Just finished writing a paper with Sri and Tom Tisch - it's titled 'Nuclear Detection: Portals, fixed detectors, and NEST teams won't work on a national scale, so what's next?'. We analyze the *use* of nuclear detectors to help prevent terrorist nuclear attacks, and we conclude that fixed detector approaches (such as those currently being implemented) are unlikely to be that effective. Here's the executive summary of the paper:

Recognizing the need for detecting terrorist attempts to transport or use fissile nuclear materials, President Bush’s FY 2006 budget request includes $246 million to form a Domestic Nuclear Detection Office (DNDO) within the Department of Homeland Security (DHS). [1] “The DNDO will provide a single accountable organization with dedicated responsibilities to develop the global nuclear detection architecture, and acquire, and support the deployment of the domestic detection system…” [2] How can DNDO planners deliver a global nuclear detection architecture that works?

Nuclear detection systems, as architected and deployed today, leave loopholes in the transportation network that terrorists can easily exploit by making use of light road vehicles to private jets to oil tankers [3].  Progress can be made if we face up to three fundamental facts:

1. Terrorists will most likely try to use highly enriched uranium (HEU), not plutonium: assembly of a HEU bomb does not involve technically complex detonation as with a plutonium bomb.

2. Terrorists can circumvent a network of fixed detectors: fixed detectors not only lack sufficient proximity and exposure to the vehicle in transit but also do not screen many types of vehicles.

3. R&D breakthroughs cannot change the physics of detection: passive detection of HEU will always be limited by its natural rate of radioactivity, and the attenuation of radioactivity is very sharp with distance [4]. The gamma rays and neutrons useful for detecting shielded HEU permit detection only at short distances (2-4 feet or less) and require that there is sufficient time to count a sufficient number of particles (several minutes to hours).

Recommendation: Due to fundamental physical limits, the current trend toward a fixed detector infrastructure is a dead-end. The only way shielded HEU can be effectively detected is if commercially-available detector technology, rather than being kept at fixed locations, are directly integrated into vehicles themselves. Detectors would travel with vehicles and have enough time to record radioactivity before reporting their readings to a network of check-points (in the same way E-Z pass collects highway tolls).

Our paper, 'Nuclear Detection: Portals, fixed detectors, and NEST teams won't work on a national scale, so what's next?' explores tradeoffs in detecting HEU in transit, and analyzes its technical, operational, and economic feasibility.

---

[1] “R&D in the Department of Homeland Security”, AAAS, http://www.aaas.org/spp/rd/06pch12.htm

[2] “Fact Sheet: Domestic Nuclear Detection Office,” http://www.dhs.gov/dhspublic/display?content=4474

[3] Medalia, J., 2005, “Nuclear Terrorism: A Brief Review of Threats and Responses,” CRS Report for Congress, The Library of Congress http://fpc.state.gov/documents/organization/43399.pdf

[4] attenuation of radioactivity with distance is subject to an inverse-square law in free-space and is exponential with shielding

UN votes to outlaw nuclear terrorism

Sri sent me this link with the message: "OK, now we're safe."

The 191-member U.N. General Assembly on Wednesday unanimously approved a treaty outlawing the use of nuclear weapons by terrorists and their supporters.

The treaty, which governments will begin signing at the General Assembly session in September, criminalizes the possession or use of radioactive material or a nuclear device "to cause death or serious bodily injury." It also makes it a crime to use a nuclear device to damage property or the environment or to attack a nuclear facility.

It requires governments that ratify the treaty to amend national laws to prevent terrorists and their supporters from financing, planning or participating in nuclear terrorism. It also calls on governments to share information, ease extradition proceedings and pursue criminal prosecutions of suspects linked to such terrorist acts.

"It's a good thing" that they are making a concerted effort to grapple with the threat of nuclear terrorism, said Charles D. Ferguson II, an expert on terrorism at the Council on Foreign Relations. "But the bottom line is, it's not going to stop it."

Radiation detectors on buoys

The Lawrence Livermore National Labs site has an interesting write-up on trials of radiation detectors aboard buoys off the coast. The idea is to detect nuclear materials that might be carried on board boats and other vessels before they get close enough to land to be dangerous. The detectors are powered by wind- and solar-powered generators and are outfitted with wireless communications links.

Homeland security experts are evaluating a wide range of possible threats from terrorists. One of the more troubling scenarios is a small and crude nuclear device transported in and detonated from a boat located near a naval military base or a civilian shipping terminal. Thanks to a Livermore design, buoys outfitted with commercially available radiation detectors could soon play an important role by warning of the presence of nuclear materials in marine environments.

9/11 showed us that we needed to secure civilian transportation modalities (a shift away from the cold-war thinking of building missile shields, etc.). If the trials are successful, these detector systems might be deployed around busy ports to interdict and deter marine transport of nuclear materials and weapons. Apparently, proposals have already been submitted to deploy buoys with radiation detectors in the Oakland harbor.

Curious to see what the specs are on the detector system: how well detection at a distance works, how high the false positive rate is and how closely the buoys need to be spaced in order to be effective. As with any RF system, radiation has a power-law falloff (inverse-square law in this instance) with distance...

Radiation detection portals

Some of you might have seen the CBP (Customs and Border Patrol) announcement today re: the deployment of radiation detection portals at borders. The idea is to interdict trafficking of nuclear materials (among others) across US borders. If these are effective, border patrol have the ability to intercept nuclear weapons as they are brought across the borders. This would obviously be a good thing.

How might such a detection system work? I'm going to discuss this in very general terms because I have some misgivings on revealing potentially sensitive information. Consider a uranium bomb with, say, 12 kg of weapons-grade uranium and tungsten "tamper" that acts as a radiation shield. In a sense, this is a conservative weapons model (derived from Fetter, et al) - it is more likely that a terrorist group would use a gun-type bomb which would require about 50kg or more of highly-enriched uranium.

Such a bomb would emit neutrons and gamma rays, but the number of emissions observable at a detector may be smaller than the background rate of neutrons/gamma rays coming from cosmic rays, natural radioactivity, etc. So this presents an interesting problem of resolving signal from noise.

How can you make this detection problem easier? One obvious way is to move the detectors closer to the sources. Another is to increase the exposure time. To explain the latter point, consider a source that generates 20 neutrons/sec at the detector. The neutron background is 50/second with a standard deviation of 7/second (assuming a Poisson process with standard deviation equal to half the mean). Now if you see counts per second of 70, 75, 68, 75, 70..., you might notice a trend of 2-sigma events and conclude that there is a neutron source in your field emitting about 20 neutrons/sec. Well, the same goes for 1-sigma events, over a larger number of intervals, since the probabilities are multiplicative: a string of counts such as 59, 61, 64, 60, 59, 63, 56, 58, 60,... for instance, might lead you to conclude that what you're seeing is a smaller but still definite number of counts (perhaps 8-9 neutrons/second) above the background. So, given longer exposure times, it is possible to definitively detect weaker sources of radiation.

The truck or vehicle pulls up to or passes through the portal (a few meters wide) at pedestrian speeds (say 5 mph). This provides proximity and exposure time, aiding detection. Even so, this is a tricky problem, as noted earlier. Further, maximizing detection time is at odds with the goal of increasing throughput by reducing delays.

The above remarks primarily apply to passive detection, which consists of passively measuring gamma/neutron counts and registering counts that exceed a specified threshold. There is also active detection which involves actively probing the contents of a truck or car using gamma rays or x-rays and using the results to infer the presence of nuclear materials (this is conceptually similar to taking an x-ray image). This works quite a bit better, but obviously, since this is an invasive procedure that could affect any humans within the vehicle, this technique is not as popular as passive detection. However, this technique might be feasible at border checkpoints, where it might be feasible to require the passengers to step out of the vehicle for the duration of the inspection.