Can a nuclear device be created that’s the size of say a bullet and delivered by the use of a gun type of device? I guess I’m asking what would a tactical nuclear weapon look like. Would it be hand held, concealable? Could it be small enough to say just blow up a house, and deliver fallout over say a one block radius?
Almost. The smallest nuclear bombs in the U.S. arsenal had a variable yield that went down to as low as either 10 tons or 100 tons of TNT. That’s still a big explosion, but the area of total destruction is less than a city block, I believe.
It needs to have the critical mass of whatever fissionable you are using, which will make it weigh several pounds, at least, and rule out bullet-sized bombs. Surprisingly, I had trouble finding figures for what critical mass is for various atomic fuels in a quick search of the internet, I did see a figure of 35kg for plutonium oxide crystals, but I know very little about atomics.
This is why antimatter bombs are the wave of the future - no problem in making a bullet-sized antimatter bomb, and it could be as devastating as our fusion weapons.
The Davy Crockett, which I believe is the device Sam Stone refers to.
Badtz Maru, search on “briefcase bomb”.
Here’s one discussion of briefcase bombs, but that says 10.5 kg. I’m sure I’ve seen smaller masses quoted.
I had a physics teacher once who had a flair for storytelling, and he described it like this: “Exactly what the critical mass for Uranium is, is a closely guarded secret. It’s known only to the government of the United States, China, Russia, India, and about a dozen other countries, the several thousand people who have worked on a project like this, and pretty much any physicist who wants to sit down and do a 30-minute calculation. It’s about 8½ kilograms.”
You must have known the question would come, Achernar. Cite? Or at least show your work.
Oh, sorry. I never did the calculation, and I really don’t know what’s involved. I didn’t mean to say that that’s the actual value; it’s only there to complete the joke. 
In order to make up for my shoddy posting, here’s the answer. For U-235, it’s about 8 lbs:
Just because I’ve not read all the links and the Wolfram site just gives a number, how would one go about calculating the critical mass for fissile isotope x?
(If it’s relevant, assume I have access to farily good computational systems (I’m a programmer and I have an HP-48GX ;)) and Google.)
Now, hang on a sec! Isn’t critical mass at least partially a function of the proximity of the nuclei? And that being the case, wouldn’t it also be a function of the purity of the isotope? And that being the case, couldn’t there be purer, denser isotopes we haven’t yet discovered? Huh?
Not likely. If you’re talking a fission bomb, Plutonium is about as “hot” as you can get, and even then producing the stuff requires some heavy-duty infrastructure. Anything higher (i.e. with an atomic number of 95 or more) is either too expensive, too toxic to carry around in a suitcase, or less effective than Plutonium at creating an Earth-shattering kaboom.
This page lists nuclear bomb fuels. Heavier metals than Plutonium are listed, specifically Americium, Curium and Californium, but none of them are producable in sufficient quantities to make them useful, nor does any of them promise anything better than a good ol’ thermonuke, as the second chart on this page indicates.
Man, there go my hopes of having a pack of Nuclear Black Cats to set off on Independence Day…
You want to celebrate the birth of your nation by blowing up a large part of it?
no, no…instead of having an explosive yeild equivalent to tons of TNT, I’d kinda like to see a nuke with a yeild equivalent to just a few milligrams of TNT, or a pinch of gunpowder or something. Instead of blowing up an ant mound with centuries-old technology (ho-hum), you could do it the modern way! Sure, it might leave your back yard an uninhabitable wasteland for millennia to come, but you’d be able to say that you literally nuked the ants…
Can always try hard radiation
Is anyone going to answer my question?
Urban Ranger: Hard radiation doesn’t go boom. 