There have been 15 known cases of stolen nuke material over the past 20 years. Every time the goods were traced back to a source, the Soviet bloc was on the return address. In all of those cases, the recovered nuclear material wasn't known to be missing until it was found, and the bust itself came about mostly by luck.
That means if you're al-Qaeda, and you're out looking for HEU, it's only a matter of time before you find it, and just a matter of bad luck if you get caught.
Russia, by the way, holds onto about 200 tons of plutonium. It only took six kilograms of the stuff to level Nagasaki in 1945.
Even before you track down your nuclear material, decide whether you want a gun assembly or implosion device.
An implosion-triggered fission bomb creates criticality by compressing fissile masses together through an explosive charge. A gun assembly device is the no-frills way to get 'er done.
If you're a terrorist looking for maximum yield with minimal effort, you'll love what a gun assembly can do for you. They're easy to make, easy to use, and while relatively inefficient, they can leave a very nasty impression.
But don't take our word for it, just take a peek at history: the nuke that destroyed Hiroshima at the end of the Second World War was a gun assembly device. In fact, the brains behind the Manhattan Project were so confident in their simple design that they skipped testing the thing altogether and went straight to dropping it on Japan, killing 150,000 people.
Making life at once easier for terrorists and impossible for the rest of the world, all the ingredients for a gun assembly device are commercially available, save the nuclear material itself (which is always HEU in gun assemblies, never plutonium).
For the motivated terrorist, you'll need a lathe, a furnace, a surplus artillery gun, and about 15 friends who know a thing or two about weapons design, machining, metallurgy, ballistics, electronics and physics.
You'll also need a workshop, and about 150 acres of open space to blast some artillery tests. For the unmotivated terrorist, that's significantly larger than your parents' basement; so don't even ask.
As far as budget, the New American Foundation's Jeffrey G. Lewis figures on spending upward of $10 million; that's about $1.4 million to cover parts, pay, and facilities, with a ballpark $9 million leftover for HEU.
How much HEU can a terrorist get for nine large?
In 1994, Osama bin Laden bought a three-foot cylinder of weapons-grade uranium from a Sudanese military officer for $1.5 million. Fortunately for mankind, bin Laden got bamboozled on the sale, and the cylinder turned out to be a fake. The price, however, serves a good indicator of what terrorists would cough up for a little nuke material.
Comparing costs to 9/11, which ran bin Laden about $500,000 and killed 3,000 people, spending $10 million to kill 100,000 is a bargain. Grim math to be sure, but trust that bean-counting terrorists have already crunched these numbers. For them, maximizing casualties while minimizing cost is key.
The basic premise behind a gun assembly device is, well, pretty basic: if you shoot one piece of HEU at another, you take out Toledo.
The process is a little like getting a ball of HEU, removing the core as you would an apple's, bolting the pitted end to the muzzle of an artillery gun, then firing the core back into its center.