The U.S. Navy has its own name for the "seizure, theft or loss of a nuclear weapon or component" -- it's called a "Broken Arrow." (Anyone who's seen the 1996 John Woo movie "Broken Arrow," starring John Travolta, might already know this.) You might notice that the Navy's definition mentions both a nuclear weapon and a component of a nuclear weapon. Why is it important enough to draw a distinction between the parts of a bomb and the whole?
It's possible that someone could steal an entire nuclear bomb, but not very likely. As you can see in this article, intact nuclear weapons aren't something you just shove into your pocket or roll down the street. They're big and easily recognizable, so security at a nuclear arms stockpile would have to be extremely foolish and lax to let a bomb through.
The more likely scenario would involve a person or several persons stealing the different parts necessary for a bomb and putting them together to make a functional device. With the right information and materials, bombs of varying effectiveness can be built. A recent security breach in November 2006 at the Los Alamos National Laboratory, for instance, has officials worried that an employee passed on information concerning special access controls that would detonate a bomb. It's not a whole device, but it's an important piece of the puzzle [source: CBS News].
The most important element a terrorist would need would be the main ingredient to a nuclear bomb: highly enriched uranium (HEU) or plutonium (or both). Like an intact nuclear bomb, however, these two substances aren't something you can just shoplift. Neither HEU nor plutonium exists naturally in nature, and they're very expensive and difficult to produce. However, both the United States and Russia have held onto enormous amounts of plutonium from dismantled atomic weapons, and both countries have "excess" amounts of HEU. These massive amounts of nuclear material are used or planned for use in nuclear power plants or research centers, making the risk of stolen information or material greater if proper security isn't enforced.
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In order to get an amount of plutonium or uranium to a supercritical mass -- the point at which the material is compressed enough to create an uncontrolled nuclear reaction -- a certain amount of conventional explosives like TNT is needed. Acquiring TNT and its detonation device would probably be the easiest part of building a nuclear bomb. Building or manufacturing a metal case for the bomb's insides would be the last major basic step.
Although on some level it's possible for all of these steps and many more complicated ones to fall through, someone with the intent to acquire a bomb or parts of a bomb would need to be extremely powerful, knowledgeable and skilled to pull off such a stunt. It would also be impossible to do alone, as a team of technicians would be necessary for putting everything together. They may make it look easy in the movies, but in reality, it's much more difficult (and not something we recommend trying).
For more on security and the alternatives to a complete nuclear bomb, read the next page.