So how good is the security around nuclear weapons? The United States, as one example, typically uses "barriers, guards, surveillance cameras, motion sensors and background checks on personnel" in any situation where an arsenal of weapons exists [source: CFR]. Human error or corruption is always a possibility, of course, so none of these precautions is infallible.
Nuclear weapons themselves are guarded with many safety measures. One of the main precautions is a sophisticated electronic system called a permissive action link, in which two correct codes must be inserted in order to arm the bomb. This uses a "two man rule" principle, making it nearly impossible for a person to detonate a weapon by himself.
Other countries may not have top-notch security, increasing the risk of material being stolen. Russia is constantly cited as an example of dubious security efforts -- the end of the Cold War and the breakup of the Soviet Union complicated matters because officials didn't keep proper records. Authorities are notorious for treating guards and other employees at weapons facilities very poorly by failing to pay on time. Instead of going home happy with a nice paycheck, workers might be tempted to make a quick buck by selling top-secret information or smuggling dangerous materials. The United States also has limited information on the safety devices within Russian nuclear weapons themselves, so it remains unclear what kind of steps developers have taken to safeguard the detonation of a nuclear device.
Another concern is the black market in nuclear materials, in which low-grade plutonium or uranium is smuggled for money. The chances of making an effective nuclear bomb out of this so-called "nuclear junk" are extremely low, but the material can still be used in dirty bombs -- typical explosives that might spread dangerous radioactivity in the event of an explosion.
For lots more information on nuclear bombs and their history, see the next page.