1. read Tom Clancy's The Sum of All Fears at about two-thirds of the way through the book. follow instructions therein.
  2. Choose target city to nuke.
  3. Hide bomb within city.
  4. Set explosive timer with generous countdown time
  5. Get the heck out of there

The hardest bit would be getting the fissionable material which is, to the detriment of potential nuclear terrorists worldwide, not readily available at your neighbourhood grocery store.

You'll also need a few kilograms of some type of conventional explosive. C4 is excellent, but regular old TNT will work just about as well. The basic principle is to surround the plutonium with the explosive, and have this enclosed in a metallic sphere. (usually made of beryllium) It must be set up so that all of the explosive detonates simultaneously. This causes the force of the detonation to be directed inward, bringing the plutonium to critical mass and triggering a chain reaction known as fission.

Let's be clear: while it may not be difficult to make explosives with radioactive components, which will spread radiation, it isn't quite that easy to build a device which causes an atomic detonation. At least, not unless you have a fairly large chunk of fissile material. If you have a lot of it, you can certainly build something that you can be reasonably sure will work; after all, the Manhattan Project scientists and engineers did just that. The uranium gun-type weapon dropped on Hiroshima (Little Boy) was an untested design of a type of weapon that had never been made - and yet, despite being designed with chalk, pencils and slide rules, worked fine. The caveat was that it required a (relatively) enormous amount of fissile materials to work; the reason the implosion weapon (Fat Man) became the standard is that while being more complicated it required a smaller amount of material.

In any case, getting enough fissionables to build a gun-type bomb would be really, really hard. Getting enough to build an implosion weapon wouldn't be easy; plus, the technology required isn't trivial. It can be done; however, doing it in a backyard is stretching credibility unless that backyard has some pretty high-quality machine tools, electronic components and explosives experts in it with the facilities to handle and cast radioactive metal.

Finally, while it's true that low-level radioactives can be found in everyday life, none of these are suitable for fission or even decent contamination levels.

Note: niaporte noted, quite properly, that I had misidentified Little Boy's target as Nagasaki. It was, in fact, Hiroshima. My error.

The two original Nuclear bombs, the "fat man" and "little boy" operated on the basic principle of sending a radioactive projectile to a larger target mass. They were an implosion type and gun type respectively.

The nodes above have discussed the implosion type rather throroughly, but nonetheless it operates on the principle that you propel a plutonium sphere into a beryllium-polonium core. This requires, however, a non-nuclear explosive.

The gun type bomb operates on the principle that one can send a uranium wedge into a larger uranium target thereby reaching critical mass. The uranium wedge would be projected by means of a conventional explosive ignited by a radio signal.

Again the principle behind each is relatively simple, create enough mass and density that the probability of neutrons from one nucleus impacting with another is high enough for a sustained chain reaction. In principle these are relatively simple, but they are also relatively low yield, with only a few kilotons. Apparently there are all sorts of ways to boost yield.

Its amazing what you can find from the internet and world book encyclopedia

One common misconception, and one that seems to have plagued even such esteemed scientists as Werner Heisenberg before the war, is that all you have to do is get a bunch on radioactive stuff in one place and run away real fast. This would indeed produce an explosion, but it would be a conventional explosion caused by the heat of the super-critical mass, rather than the release of heaping buttloads of neutrons. To get a real nuclear explosion, you have to bring the plutonium or enriched uranium together fast enough for the chain reaction to go through several generations before the heat blows it apart like an egg in the microwave.

As mentioned in previous writeups, there are two basic types of nuclear bombs:

The Little Boy design- Uranium 235 (U238 won't do) is formed into two parts that are brought together. This can be done as either a wedge into a sphere or a plug into a cylinder. By far the simplest design (though with a lower yield), this kind wasn't even tested before being dropped on Nagasaki, so confident were the designers of it's design. As mentioned above, however, the trick is to bring the two pieces fully together before the simple mechanical heat caused by the beginnings of a chain reaction blows the whole thing apart in a minor nuclear fizzle rather than a genuine nuclear explosion.

the Fat Man design- Plutonium formed into a spherical shell is imploded. The trick is that explosives aren't very good for forming things into dainty shapes. Generally something inside a spherical configuration of high explosives does indeed go inward, but not very precisely- bits of it tend to be ejected out, some areas are more compressed than others, all sorts of problems. The solution to this is to use different kinds of explosives in the outer shell. Like all materials, different explosives transmit force with different speeds. So high density explosives are packed in strategic locations throughout the shell that otherwise consists of lower density explosives. These high density nodes act as lense to focus the force on the center, so that instead of having these roughly interacting waves of force coming from every which way, the plutonium is instead hit by neatly focused force that pushes everything in to the center.

In both configurations, a neutron source is placed in the center of the configuration to ensure that the chain reaction actually starts. Chances are it would anyway, but why take the chance?

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