Take a fission bomb. (This is, fortunately, the hardest part.) Salt it – bolting cobalt jackets to the casing is one of the simplest and best ways – and, for preference, blow it somewhere nice and high up.* No matter where you set it up, though, you will kill a lot of people, due partly to the jackets producing years of massively radioactive fallout, and partly due to the fact you’ve just set off a goddamn fission bomb.
This is a scary device.
Take a briefcase full of TNT or some other conventional explosive. Pack some isotopes around it, the meanest, most radioactive shit you can obtain. (This is even more difficult than it sounds.) Leave it in the middle of a busy city; blow it when it’s surrounded by people, if you can, because blast aside, you will kill nobody.
This is a device capable of causing scares.
To be precise, this is a crappy, worthless weapon, capable of causing scares when gossip-mongering rags know what sells to gullible tech-illiterates who assume that anything involving “nuclear material” will cause a mushroom cloud and a sea of glass, rather than an expensive cleanup and a very mildly elevated cancer risk.
A “dirty bomb” is not a nuclear weapon. It is barely even a conventional weapon. It is a car bomb with media-supplemented delusions of grandeur. Even the most ferocious radioactive material you can put in one is incapable of doing serious damage to anyone or anything in the short time between detonation and mass evacuation, and it is not capable of causing any kind of nuclear reaction. It’s a spectacularly stupid, expensive and ineffectual way of poisoning people, worse than just dumping a wheelbarrow of caesium into the reservoir and watching it fizz. If you really seriously want to kill people, probably the cheapest, easiest and most effective way is a crate of AKs and a few men who can fire from the shoulder and aren’t afraid to die.
The world had pretty much forgotten about Osama bin Laden until today. His death changes precisely nothing; he could have died ten years ago and it would have made no difference whatsoever. This time around, can we have our bomb scares with a little less bullshit?
* Hell, if you can get it up a decent way in the sky you can get an EMP out of it and cause the ever-more-cliched, ever-more-potentially-crippling internetpocalypse in addition to your straight fission bomb and cobalt-salted storm of ions – though if you have both nuclear weapons and that sort of delivery system, why is this even a question, and why are you reading my blog?