States attempting to build a nuclear arsenal have historically faced a number of barriers. Some of these barriers are more robust than others and remain in effect to this day, and others have been overcome. First, you had to have the requisite knowledge. This was  the first barrier to topple. Khidhir Hamza, author of Saddam’s Bomb-maker said that the Iraqis were able to take great advantage of the Manhattan Project Reports that were made available to Iraq as part of the Atoms for Peace technical cooperation program. In the information age, it has become easy to rapidly distribute even sizable quantifies of information, and any information once in the public domain is likely to remain therein.

Second, the would-be proliferant had to have an advanced technological base consisting of both skilled labor and specialized machines. With export control regimes in place, it is not necessarily a simple matter to obtain what is needed from abroad. Though there is always potential for rogue traders exemplified by the A.Q. Khan network to bring such technologies to would-be proliferators. It is also true that it is no simple matter to classify technologies and equipment that are dual-use. Some readers may remember the controversy over whether certain high strength aluminum tubes ordered by Iraq were suitable for another task besides for centrifuges used in the process of enriching uranium. (They apparently did- in the manufacture of conventional rockets.) Trigger lists that are too restrictive risk setting off trade wars, reducing the efficiency of global trade, or creating the impression that the advanced industrial nations are attempting to keep the industrializing nations down; while ones that are not restrictive enough may be circumvented.

Thirdly, and closely related to the former, the state had to have a link to raw materials used to produce fissile material  and the aforementioned technical base to convert said raw material into weapons usable material. This capacity varies from country to country. North Korea apparently has large quantities of high grade uranium ore indigenously, but Iran has only small amounts of low grade uranium. Because of the specialized materials and equipment needed,  building the fuel cycle technologies will most certainly require some level of supply and / or technical assistance from abroad.

Of course, the discussion to present has not addressed the question at hand. What does a non-state actor like al-Qaeda need for a bomb, and how likely is it to be able to obtain it? The resources of a terror group are considerably less than that of even a small and economically poor nation, however, as it turns out, so are such a group’s needs with respect to developing a nuclear capability. For the most part, states are seeking to produce relatively small and relatively powerful nuclear weapons that can tip a missile or be delivered by somewhat fast-moving planes. This requires technological capabilities, supply chains, and a level of scientific understanding that are far greater than if the actor just wants a bomb that will fit in a shipping container or a U-haul. The terrorist just wants to create a high degree of disruption, fatalities, and destruction, and is willing to do so at a level that would be far below the level of optimality achievable by even the least capable of the nuclear weapon states.

If you are al-Qaeda you face one major barrier, getting an adequate quantity of highly enriched uranium [HEU] (i.e. preferably in the 90 percent uranium-235 range or higher) to violently assemble a critical mass of fissile material out of two sub-critical masses, and getting it into usable shapes. You probably need a smart individual or two who can do the calculations to facilitate construction of your crude device. You may be able to your hands on some instructions. You will need some other materials that you cannot get at your local five-and-dime, such as a gun-barrel-like tube and high explosives. However,  these are not exceedingly difficult to obtain. The effectiveness of roadside bombs in Iraq and Afghanistan should attest to the ability of non-state actors to get their hands on or manufacture potent explosives.

With this you get a big clunky crude weapon of relatively low yield. However, if your plan is to put it in a rental truck, drive it into densely populated area, and do an unprecedented amount of damage for a terror group while striking terror into the general public, it will likely suit your needs. Even if it fizzles it may serve as a dirty bomb (a radioactivity dispersal device.)

 Herein lies the emphasis on material control of highly enriched uranium and plutonium (though I am told Plutonium is unsuitable for the crude type of weapon envisioned in this article) that are put front-and-center today. There have been efforts to encourage converting research reactors that currently operate utilizing HEU to run off low enriched uranium (LEU.) As efforts are being pursued to expand recycling of plutonium in order to reduce the volume of radioactive waste and to squeeze more energy out of the non-renewable resource of uranium, new processes are being considered that do not separate out Plutonium in a form that would be readily usable for bomb production. While Plutonium may not be useful for gun-type devices, there is evidence that given an adequate amount of plutonium, a smart enough person or people [knowing the weapons physics and the  chemistry of explosives, etc.], a non-state entity might be able to produce a technically more sophisticated implosion-design device. It should be noted that, unlike a state building an arsenal for deterrence, a terrorist that produces a defective design that looks feasible could cause a great deal of damage. Terrorists following the proviso of Northern Irish terrorists that they [the terrorists] only have to have success once, while the forces of counter-terrorism have to be successful every time, may not be overly put off if their device does not work perfectly as planned. One can imagine the damage to the economy resulting from even a credible-looking failure.