Before I get into an explanation of what a nuke is, I need make an analogy. Pretend you're on a range firing a pistol. It's bad to say you're aiming at the target, but one is out there. What are some ways to increase your odds of hitting the target? There are two ways of increasing your odds of hitting a target: putting more targets out there and moving the targets closer to you.
So what happens in atomic bombs that relates to that? Patience, grasshopper. You see when matter is converted to energy, a little bit of mass converts to a lot of energy (Famous equation E=MC^2). When a uranium nucleus splits into two smaller nuclei, a little mass is lost in the overall reaction - mass that is converted into energy. When a uranium nuclei splits, it also releases some neutrons. Tricky thing about these neutrons is that when they collide with another nuclei that nuclei will split as well and release more neutrons. They're like bullets aiming for other targets, only when you hit a target it starts shooting bullets as well.
Critical Mass or Critical Density?
I've always wondered about the term "critical mass". I think it should be refered to as "critical density". In the analogy above about shooting blindly at targets, I mentioned the two ways to increase your chances of a hit are to either bring the targets closer or to add more targets. Believe it or not, Hiroshima used one tactic while Nagasaki used the other.
Critical mass is when the nuclear explosion has been achieved. You got the core explosive that sits there until it is detonated. Critical mass is when the splitting of nuclei accelerates like an avalanche so that in one millionth of a second ten percent of the nuclei in a good bomb split, releasing a shit-ton of energy.
Hiroshima's bomb (Little boy) was a gun-type bomb. It brought two chunks of uranium together at incredible speeds - adding more targets on the firing range. Nagasaki's bomb (Fat Man) was imposion-type, which was bringing the targets in closer. Now I mentioned above that critical mass was when the splitting of nuclei was like an avalanche. You need to quit thinking of a chunk of uranium as a chunk of metal and think of it as a big collection of uranium atoms. Billions of trillions of uranium atoms. On a rare event, a uranium atom spontaneously splits and releases neutrons and energy. Usually when this happens, its neutrons might cause a small avalanche of splitting but in the end the majority of neutrons are lost out into the atmosphere without being able to propagate the avalanche very far - critical mass is not achieved. When you add more uranium, those neutrons that would be lost to the atmosphere are now finding greater odds of hitting another nucleus - increasing the chances of a propagating avalanche. Critical mass occurs when enough uranium is in the core that those spontaneous splittings will trigger a propagating avalanche that consumes a significant fraction of the atoms available.
What a Sabot Depleted Uranium Round Looks Like
Implosion-Type and Depleted Uranium
Implosion-Type bombs are ones where the uranium or plutonium is compacted to incredibly small volumes, bringing the nucei of the atoms closer together and thus increasing the odds of a propagating avalanche. Implosion bombs must have conventional explosives places precisely around the core so that when they detonate, a spherical pressure wave hits the core evenly at the exact same time and compresses it to critical mass. They talk about nuclear geometrists in the movie "Sum of all Fears"... a nuclear geometrist makes sure all the conventional explosives are shapes right and placed right to create that spherical wave.
You know, I've always wondered what the big hype was about depleted uranium shells in combat. What most people don't know is that uranium has multiple isotopes. Uranium 235 is the isotope used in bombs and fuel and makes up less than one percent of the uranium on the planet. It is available to pretty much any country on the planet. What they lack is the technology to isolate the 235 isotope - isolating the 235 is called enriching. Depleted uranium is the useless leftovers - the 99%+ uranium that can't be used in bombs and fuels. IT IS NOT RADIOACTIVE. When all the 235 is taken out of the natural uranium, what's left is safer than copper in terms of radioactivity. Since the useless uranium is the most dense natural element on the planet, it makes a great armor-piercing round and to this day they have not discovered a better tank-killer for its size.
Now one bit that I will say about the hype... it's not completely unfounded. When people hear of a depleted-uranium bomb or they see pictures of charred babies and other grotesque things... That is actually a dirty bomb. That is where the 235 has been left in the uranium in significant amounts, usually from spent fuel rods from power plants or enriching facilities. The US government does not make rounds that contain a significant amount of 235 - at least I pray to god they don't.