The real terrorist nuclear threat

As I mentioned in my post last week, the US government seems to be overly concerned with the possibility of terrorists using a “dirty bomb” to spread radioactive material over a wide area.

While I do think the threat of a dirty bomb is overblown, the one threat I do not think is overblown is a terrorist building and using a real nuclear bomb. A real nuclear weapon is much easier to construct than most people think: the only hard part is obtaining the material for it.

Modern nuclear weapons are typically an implosion design: with specially-shaped explosive charges called "lenses" surrounding a sphere of fissile material, with the entire assembly resembling a soccer ball with wires sticking out of it. When the weapon detonates, the explosive lenses explode simultaneously, compressing the sphere of fissile material into a supercritical mass. This type of weapon is very complicated to make, since it requires specially shaped explosive charges and precision electronics and detonators to ensure the explosive charges detonate at exactly the same moment to compress the fissile material evenly. If the timing is even slightly off, or if the explosive charges are not shaped quite right, they would only succeed in pulverizing the fissile matter into dust rather than compressing it and making it explode.

However, there is a much simpler design, known as a gun-barrel. In this design, there are two subcritical pieces of nuclear material: one stationery piece, and one moving piece, with the pieces at opposite ends of a gun barrel. To detonate the weapon, an explosive charge at one end of the gun barrel pushes one piece (the "bullet") into the other piece (the "target") forming a supercritical mass, which then explodes. The atomic bomb known as "Little Boy" that was dropped on Hiroshima in World War II was a gun-barrel design, and as the citizens of Hiroshima can attest, this design can be quite effective. A gun-barrel weapon is physically bigger and heavier than one with the same yield that is based on an implosion design, but if you are a terrorist, you don’t care about that.

The problem with terrorists is that some are willing to commit suicide during their attack, and if that is the case, the gun-barrel design could theoretically be simplified by using gravity instead of explosives. All the terrorist needs to do is have two specially-shaped pieces of fissile material and when he is ready to detonate the bomb, drop Piece A into Piece B, and ka-boom.

In short, the technical knowledge needed to build a basic nuclear weapon is widely available - many people who have studied chemistry, chemical engineering, or nuclear engineering could come up with a simple design for a workable nuclear weapon. We should not delude ourselves into thinking terrorists are not smart enough to do this - the 9/11 tragedy was proof to the contrary. In fact, the only difficult thing for a terorrist is getting his hands on enough fissile material to build the weapon. Fortunately for all of us, this is much easier said than done.

To explain the difficulty behind this, let me explain the term “fissile material”. Fissile material is a substance where each atom is inherently unstable. If one of these atoms is exposed to a certain type of radiation, it may absorb an extra neutron causing it to become very unstable and undergo nuclear fission: breaking apart, and releasing large amounts of energy and radiation.

As is widely known, there are two substances that can be used as the fissile material in a bomb: uranium 235 (U-235), and plutonium 239 (Pu-239). Plutonium does not occur naturally: it is produced as a by-product in a nuclear reactor, so is very difficult to obtain. Uranium does occur naturally, but in a mixture of U-238 and U-235 (with a much greater quantity of U-238). U-238 and U-235 are both isotopes of uranium, which means the two behave exactly the same in chemical reactions, except that one is heavier. U-238 is not fissile, so in order to make an atomic bomb, it is necessary to “enrich” the uranium (increasing the concentration of U-235 relative to U-238) to a high degree of purity.

Refining uranium is not easy (and we should all be thankful for this). Since U-235 and U-238 behave the same chemically, it is necessary to separate them by weight. One way to do this is to react the uranium with fluorine gas to produce uranium hexafluoride gas (UF6). UF6 is then spun in a centrifuge, causing the gas containing U-238 to move closer to the outside of the centrifuge, while the lighter gas containing U-235 will remain closer to the inside. However, since there is less than a 1% difference in weight between the U-235 based UF6 and U-238 based UF6, this whole process requires specialized equipment and must be repeated many, many times to produce weapons-grade enriched uranium.

It is for this reason that we should all experience a collective shudder whenever we hear stories about nuclear material going missing in places like the former Soviet Union, because in reality, obtaining this fissile material is the only barrier to a terrorist building a nuclear bomb.

So, while I do not think a “dirty bomb” is a major threat, I do think a real nuclear weapon is. Terrorists are often, unfortunately, quite intelligent people and we should not underestimate their ability to assemble a workable nuclear weapon if they are able to obtain sufficient quantities of fissile material. Thus, the US government is right to be worried about terrorists obtaining nuclear material and smuggling this material into the United States. We should all be very concerned about this possibility.