CFATS and the Nashville Bombing

I have been waiting patiently since the bombing in Nashville on Christmas day on information about the explosive used to talk about its potential impact on the Chemical Facility Anti-Terrorism Standards. The use of ammonium nitrate or some other ‘conventional’ improvised explosive would have called into question the effectiveness of the CFATS program in preventing the theft/diversion of IED precursor chemicals. Well, the unofficial word appears to be out that the CFATS program has no worries, but maybe the rest of us do.

The Article

I ran across an article on thedailybeast.com that posits that the bomb appears to have been a fuel-air bomb (or thermobaric weapon), an explosion created by the detonation of a large mass of flammable gas. The military likes this type of fuel-air explosive (FAE) for destroying things because they create a large area of over-pressure that has a propensity for collapsing structures and is useful in detonating mine fields. It is the former capacity that would have apparently been the intended use of the Nashville bomber.

According to the article, the bomber apparently released a flammable gas from a pressurized cylinder (propane would be a likely choice) inside the motor home used as the vehicle in the vehicle-borne improvised explosive device (VBIED) and then detonated it when it had reached an appropriate fuel-air mix within the vehicle. The problem posited by the article is that the bomber would probably already be unconscious or dead from asphyxiation by the time the proper concentration were reached.

Background

To understand the limitations of an FAE you have to understand two flammability concepts well known to chemical process safety professionals, LEL and UEL. LEL is the ‘lower explosive limit’ for a chemical, the minimum concentration of the chemical in air that can be ignited; below that limit there is too much distance between the molecules of that chemical in the air to sustain combustion. Similarly, the UEL is the ‘upper explosive limit’; above that limit there is not enough oxygen in the air to sustain combustion.

The LEL and UEL is well known for all industrial chemicals. There is a nice table over on the engineeringtoolbox.com that provides the UEL for common industrial gasses. The table shows the limits for propane (for instance) as 2.1 to 10.1% respectively.

Now the total energy released in a FAE is directly related to the total weight of the material that will be oxidized in the conflagration, typically carbon, nitrogen and hydrogen. So, generally, the higher the UEL the more material available for combustion and the more energy released. Simplistically, the more stuff, the bigger the boom.

Hmmm. Looking at the table acetylene would be a much better flammable gas than propane; hardly surprising to anyone that has seen fires at acetylene facilities.

One last technical issue, you can get bigger, faster explosions by adding more oxygen to the atmosphere.

Why No More IFAEs?

Okay, if these improvised fuel-air explosives (IFAEs) are so impressive why haven’t they been more prevalent? Why is this possibly the first one in a terror attack? There are a number of technical problems with IFAEs. The first is mixing. If the flammable gas is not adequately mixed in with the atmosphere you will have portions of the containment (the motor home in this case) where you are below the LEL or above the UEL. If your detonator (flame source) is in either of those areas, you will not get a detonation. If your concentration (even if well mixed) is too close to the LEL you will get a fire that burns out too quickly due to lack of fuel; same for being too near the UEL but for lack of oxygen.

You also have to be very careful to remove any unintended sources of ignition, including static discharges from releasing the gas into the containment area. Any premature ignition will lead to underperforming explosives due to the mixing issues described above.

The military solved these problems with their thermobaric weapons through lots of engineering, trial and error and lots of repetition. Terrorist bomb makers are going to have to go through the same type development process. It is much easier to just use the readily available conventional IEDs.

No CFATS Problem?

If it is confirmed that the Nashville bombing was an IFAE, there will be no finger pointing at the CISA CFATS program because the program does not attempt to regulate flammable gasses as theft/diversion security threats. While many of the chemicals listed in the table mentioned above are on the Appendix A, 6 CFR Part 27, list of DHS chemicals of interest, they are all regulated as release-flammable security threats. Security measures are put into place to ensure that industrial storage tanks of these materials are not ignited or turned into local FAEs.

Could they be regulated as theft/diversion chemicals of interest? Not likely. DHS has not been able to come up with a cost-effective way to regulate the sale of ammonium nitrate, a much less common but better understood IED precursor. How would they be able to regulate the sale of propane cylinders or even acetylene cylinders?