Nuclear ReactionsWith the exception of the Dirty Bomb (q.v.), which isn't really a nuclear weapon at all, all nuclear weapons rely on one or both of the following two reactions for their effect:
Fusion for bombs is quite a bit simpler — you just need to initiate the reaction, which can easily be done by using a fission bomb to squash the necessary ingredients small enough and fast enough to fuse them; for obvious reasons, neither sustainability nor containment is of particular concern in thermonuclear weapons design.
If you're wondering, fusion in stars is both initiated and sustained by dint of the tremendous gravitational force exerted by so much mass; even smaller stars like our sun are capable of creating carbon via fusion, while more massive stars can go all the way up to iron, which is the end of the line since iron is both too massive to fuse, and too stable to fissionnote , at a positive energy budget. The most massive stars fuse elements above iron, but since this costs energy rather than releases it, they go through this phase rather quickly, and the eventual result is the spectacular explosion we call a supernova. These, incidentally, matter more than you might imagine, because stellar fusion is how almost all atoms of elements heavier than helium, which given helium's atomic number of 2 is to say almost all atoms period, originally came into being, and supernovae are the means by which such atoms escape the cores of stars, to be flung out into the universe and accrete into planets such as ours. Thus, to say, as Sagan did, that "we are star-stuff", is no more than a simple statement of literal truth.
Technically, in the correct circumstances any element can experience either fusion or fission. However, fusion only releases energy for elements of lower atomic number than iron and fission only for elements of higher atomic number than iron; inducing fission in elements lighter than iron, or fusion in elements heavier than same, requires more energy than it releases, making it useless for generating either power or boom. For both types of reaction, some elements release more energy than others. The best isotopes for use in a fission reaction tend to be Uranium-235 and Plutonium-239 due to their long half-lives and low spontaneous fission rate. Heavier elements decay very quickly, and therefore are absolute nightmares to amass or store sufficient quantities of, and lighter elements require far more energy input to induce a fission reaction. Fusion conversely works best with lighter elements: the typical fuels for fusion are deuterium and tritium, two isotopes of hydrogen, which with only one proton and one electron per atom is the lightest possible element.
(An "isotope", it should belatedly be noted, is a form of an element which differs from the "standard" sort, itself another isotope, only in its mass and in its nuclear properties; all isotopes of a given element are note chemically identical. This is why, for example, iodine-131 in fallout is so dangerous to human health — the human thyroid takes up iodine and stores it via chemical reactions, in which stable iodine and radioactive iodine-131 are indistinguishable, so your body will just as cheerfully store the sort that emits gamma rays which will kill you, as it will the sort which you can't live without. This is also why you often hear talk of centrifuges, and components of same, in discussion of e.g. Iran's nuclear ambitions — the isotope you want and the ones you don't are all mixed up together, and there's no chemical reaction that can separate one isotope from another, so you need to find another way of unmixing them. Isotopes do differ in mass, though, so you can spin down the mixture in a centrifuge, which will exert a stronger centripetal force on the heavier stuff than on the lighter, thus gradually moving them apart.) Finally, there is a third type of nuclear reaction: matter-antimatter annihilation. When a particle, like a proton, neutron or electron, collides with its antiparticle, the result is all of their mass converting to energy according to Einstein's mass-energy equivalence equation, the venerable E=mc^2. Following that equation, we find that the complete annihilation of one gram of protons with one gram of anti-protons would release about twice as much energy as the "Fat Man" bomb used on Nagasaki, whose core was about 3100 times as massive (~6.2kg vs 2g). Given the obvious advantages, in weapons design, of producing such an enormous release of energy from such a trivially tiny mass of reactants, it is trivial to explain science fiction's love affair with matter-antimatter bombs. Those who fear nuclear proliferation, though, need not worry; there are a few reasons why matter-antimatter bombs are almost certain to remain the exclusive province of SF for the foreseeable future — foremost of which is that actually producing antimatter takes so much energy that it's all but impossible, not just for modern technology, but for any plausibly predictable derivative of same. If they could be built, though, they would have the advantage of producing no radioactive fallout and practically no induced radioactivity. Technically a very small amount might occur due to the photoneutron effect, but it's really hard to find any hard numbers on this, primarily due to the aforementioned near-impossibility of producing enough antimatter to perform any kind of experiment with. Worse, the very characteristic that makes antimatter desirable for this purpose makes it almost impossible to use for same; after all, antiprotons (for example) will annihilate with any proton they encounter, including those which make up part of any container you could possibly put the stuff in — you'd need to find a way to store it without letting it come into contact with any normal matter at all, which would be almost as tough a problem as making enough antimatter to bother trying to store in the first place. (A slightly more practical use would be what's called antimatter-catalyzed fusion, which uses a minuscule amount of antimatter as a "spark plug" to set off a fusion reaction, allowing for a relatively clean, fusion-only bomb. This is also far beyond present-day technology, but not nearly as much so as a pure annihilation device.) Antimatter-catalyzed weapons would suffer from the same problem that plagues fission and fusion devices — the need to slow down the explosion enough to allow most of the fuel to react, rather than being uselessly scattered by the gargantuan energy release produced in the first few microseconds of the explosion. Unlike fission and fusion, though, an antimatter device probably wouldn't involve large amounts of very dense uranium, which would make effective design quite difficult even if obtaining the requisite antimatter weren't already a problem.
Device TypesAtomic bomb (or A-bomb): The original nuclear weapon, an atomic bomb is any explosive device where the majority of the energy output comes from a runaway nuclear fission reaction. They're more properly called "nuclear bombs." Two major types exist:
Specialist WarheadsDirty bomb: These weapons, properly known as radiological dispersion devices, don't actually involve nuclear explosions. Instead, this basically involves setting off a bomb with some radioactive material in. This would only cause direct destruction equivalent to the power of the device itself, but would cover a large area in radioactive nastiness and render much of it uninhabitable. They are a frequent terrorist device in fiction, and are a cause of some concern in the real world. For the most part, however, the threat they pose is vastly exaggerated by movies, TV shows and news media. With the most likely sources, one would need to stay in the affected area for several months or so before experiencing any ill-effects from radiation (and these would be minor) and decontamination would be a relatively simple affair. With nastier sources, the terrorists would probably fatally irradiate themselves and drop dead before they could deliver their bomb. Remember: the more active stuff is called this precisely because it decays that much quicker than less active one. The nastier it gets, the quicker it becomes safe. A dirty bomb is rather a weapon of mass distraction. It can be used as an area denial weapon, or causing a lot of economical damage by rendering area, resources and facilities useless. Cleansing the contaminated area is costly, and may take a lot of effort. Fortunately, a dirty bomb is likely to be as dangerous to its builders as to the intended target. Salted Bomb: The dirty bomb's bigger cousin, this is the same idea (spread nuclear material over a large area) applied to an actual nuclear warhead. This is achieved by surrounding the warhead with a suitable metal (such as cobalt or gold), which will be irradiated and scattered by the detonation. Unlike dirty bombs, these are a credible threat, bringing vastly increased fallout over a normal atomic bomb, at the cost of some of the damage the initial explosion would have otherwise achieved. Thankfully, these are far less likely to be built by terrorists than their smaller cousins. Neutron Bomb: Any regular nuclear warhead that has been constructed so as to give most of its energy output as neutron radiation rather than explosive force. (There is still an explosion, but smaller than other varieties of nuke.) Exceptionally good at killing people via radiation poisoning without seriously damaging fortified infrastructure, though a neutron bomb's explosion is still powerful enough to level any non-fortified civilian building within the range of its lethal radiation. It should be noted that the primary intended use of neutron bombs is as a tactical last line of defense against enemy tanks. Among the other uses for depleted uranium is tank armor thick enough to be blast proof, heat proof, EMP proof, impact proof (from small arms or debris), gamma ray proof and gas impermeable. Sounds invincible, right? Wrong. Besides the fact that a powerful enough blast will overwhelm any armor (tactical nukes can take out tanks within about 20 meters of ground zero), depleted uranium is worse than useless at stopping fusion neutrons — consider the above discussion of uranium enrichment by neutron activation, in the context of a tank crew surrounded by a heavy mass of uranium under bombardment by hot, fast neutrons. The tank itself will hardly notice a difference, but its crew will suffer a rapid onset case of fatal radiation poisoning out to half a mile. Which is why Soviet tanks were armored with tungsten, which doesn't exhibit the activation effect, along with thick "anti-neutron" linings made of ballistic polyethylene, that stopped most of the neutrons which the tungsten didn't.
Weapon TypesFirst, there's the tactical/strategic dichotomy. It can be hard to discern the difference between the two when it comes to nuclear weapons, which is part of why there is so much controversy over this. Generally, though, a tactical nuclear weapon is designed to be used on a battlefield, directly against enemy forces. That's fairly broad, though; it may be on land, at sea, or in the air. A strategic nuclear weapon is most everything else. In addition, nuclear devices need to be weaponized; many early nuclear devices, or at least prototype nuclear devices, weren't very practical weapons. Perhaps they were far too big and clumsy (up to several hundred tons, and let's not forget the refrigeration plant) to be deliverable, or else they were far too complex and fussy. Weaponization involves miniaturizing and simplifying them until they're practical and making them acceptably reality-proof. And let's not forget the safety interlocks... Nuclear gravity bombs: This was the first type of nuclear weapon and is still one of the most common. It's a bomb. An airplane carries it into the air, flies over some target, and drops it on, or at least close enough to, the target. That's simple enough, right? If the nuclear power in question has large strategic bombers, generally it isn't too hard miniaturizing a first-generation device enough to be deliverable by such an aircraft. The US had B-29s, for instance. Nuclear gravity bombs need aerodynamic casings, impact fuses and barometers. If they're big enough and if it's important that the delivering aircraft get away in time, they might be parachute-retarded; that is, they're equipped with parachutes to delay their impact (or dropping to detonation altitude in case of an airburst) long enough for the aircraft to get away. Sometimes they're even laydown-delivered, which involves the bomb landing on the ground and coming to rest before detonating. That's different from a simple impact detonation. They can be miniaturized further, so tactical aircraft (light bombers, fighters, attack aircraft) can carry and deliver them. Often, they're not even dropped, but somehow tossed or lofted, involving a steep climb. This is useful to prolong its fall and to give the delivery plane some altitude to turn into speed and separation afterwards. Missile warheads: Nuclear and thermonuclear weapons can be made small enough to fit on rockets and missiles, or else the missiles in question have large throw-weights. This is a very broad category. It ranges from very small warheads, mounted on missiles with a range of several hundred kilometers at most and intended for tactical use, to megaton-class strategic warheads, designed for long-range (intercontinental, in fact) suborbital, or even orbital, delivery. They can be used to destroy everything from an armored column to a city, or perhaps even an entire nation if used in sufficient quantities. Some nuclear missile warheads have even been intended for use against various types of aerial or even orbital targets. Many, many SAM systems had nuclear-tipped versions, and both the US and USSR hoped to use nuclear-tipped missiles against incoming ICBMs (Russia maintains an ABM system, A-135, but now with conventional warheads only, while the US NMD system will also be conventional). There were even some nuclear-tipped air-to-air rockets and missiles. These were fielded by the US early in the Cold War, when bombers were still the primary delivery system. The Americans assumed Soviet bombers would carry their nukes in an armed, "fail-deadly" state, able to detonate even if their plane was shot down. Thus the goal was not to knock bombers down but obliterate them in the air. A special nuclear air-to-air missile, code-named "Genie", was developed to be used against Soviet bomber formations. In Real Life, it proved to be Too Awesome to Use. Exactly what is required for weaponization varies substantially. Obviously, various degrees of miniaturization are required. Sometimes miniaturization can be very challenging indeed, especially if the package is small enough or a certain form-factor is desired. Ballistic missile warheads are housed in special "reentry vehicles," designed to protect them from the stresses of atmospheric reentry. ICBMs often carry multiple RVs. Some RVs can even maneuver during their flight, either to strike their targets with greater accuracy or to confound the defenders. Artillery shells: Nuclear warheads can be made small enough to be placed in special artillery shells and fired from artillery tubes. Initially they were big and clumsy; the first nuclear artillery shell, the US W-9 (its 1953 test, shot Grable in test series Operation Upshot-Knothole, is very famous) required a special 280mm piece. Soviet Union had actually beaten it in 1957 by the enormous 406mm Kondensator self-propelled howitzer, but it proved, ahem, a bit impractical and only four were built. Later nuclear artillery shells could be fired from 155mm howitzers, and there are rumors about a 105mm nuclear artillery shell. Early nuclear artillery shells were gun-type devices, as previously mentioned; these are inherently long and thin, just right for an artillery shell. Later nuclear artillery shells utilized a form of implosion, but with a non-spherical primary and an unusual lens arrangement. This is called "linear implosion." It's much more inefficient and a lot of fissile material is required, but at least the warheads have the right shape. Both the US and USSR constructed and stockpiled large numbers of nuclear artillery shells; many were stored in European depots, for easy use in a hypothetical third world war there. Israel is also known to have developed and stockpiled such weapons. The "Free World" developed nuclear artillery shells in 406mm, 280mm, 175mm and 155mm for certain. The 406mm nuclear artillery shells were intended to be fired from the 406mm (16-inch) guns of USN battleships... none of which are now in service. Oops. Nuclear torpedoes: Many submarines are armed with torpedoes that have nuclear warheads. These are designed to destroy large enemy targets such as aircraft carriers and typically have a yield of a few kilotons. In design they are similar to nuclear missiles, except that they tend to be a lot smaller and are, well, torpedoes instead of missiles. These can be especially effective if used against a group of submarines, such as a missile sub bastion. If the subs are close enough together, one torpedo detonated in the pack can damage or destroy most of them, since water is non-compressible and will transmit large shockwaves with great enthusiasm. And then there's that Sakharov-proposed 15-meter long torpedo, which was less of a torpedo in a traditional sense, and more of an unmanned demolition sub, carrying a multimegaton warhead close to the target ashore and destroying it with a radioactive man-made tsunami. Demolition charges and suitcase bombs: Nuclear devices may be miniaturized sufficiently that they become man-portable. The US and USSR both developed such devices, but Soviet developments have been described in far less detail. These devices were intended for use by special operations units in event of a war; they would sneak past enemy lines and use their man-portable nukes to destroy bridges and similar targets. Some devices were even designed to be deliverable by swimmers. There are videos on YouTube, declassified government films, describing swimmer delivery of nuclear demolition devices. The most famous highly-miniaturized US device was the W54. A version of it was used in something called the SADM, or Special Atomic Demolition Munition. Another version could be fired using an M-388 recoilless rifle; this arrangement was the famous "Davy Crockett," widely known as the "nuclear bazooka." There were far fewer reservations about nuclear weapons use in the 1950s, of course. Note that the Davy Crockett weapon system was very much a failure as a weapon, as were most other "nuclear bazooka" designs. This category also sort of includes nuclear land mines. This troper knows of only one specific design, however: the British Blue Peacock. This became famous as the "chicken-powered nuclear bomb" when the relevant documents were declassified in 2004. If it was buried in winter, the thing might have frozen over, so one proposal involved stuffing live chickens into the casing, with a supply of food and water. They'd live for a week, which was the intended buried lifetime of the mine anyway. In the interim, they'd keep everything at operating temperature by virtue of their body heat. There are also rumors that most large highway interchanges in Western Europe, particularly, in Germany, have provisions for the nuclear mines to be installed, with a view to block the incoming waves of Soviet tanks rolling on German autobahns.
Delivery SystemsAircraft: A wide variety of aircraft can deliver nuclear weapons, and historically, aircraft delivery was the first method. It is also the only method that has actually been used operationally. As mentioned previously, these aircraft can be anything from huge multi-engined strategic bombers, to supersonic one-man fighter-bombers. ICBMs: Intercontinental ballistic missiles, or ICBMs, are ballistic missiles that have more than 5,500 km range. What makes a ballistic missile? Well, initially there is a powered phase of flight, but after that it acts like a really big rock that's been thrown very hard. It flies on a parabolic suborbital trajectory until it reaches the target. Various types of ICBMs are, today, the strategic nuclear delivery systems of choice. They're a diverse group, however; for example, there are many possible launch environments. An ICBM may be launched from a fixed land base... of course, it may require a launch pad and large, soft aboveground facilities, it may be stored horizontally in a hardened concrete "coffin" before being raised to the vertical for launch, or it may be stored upright in a hardened underground silo, a large massive silo door sliding open when the time comes. A land-based ICBM may even be mobile; it might be launched from a heavy truck or a railcar. An ICBM may even be air-launched, although air-launched ICBMs have never gotten past the design study phase. ICBMs may be launched from submarines, surfaced or submerged, or even from surface ships. Modern ICBMs generally have solid-fuelled lower stages, for the sake of quick reaction times (the flight time from the US to Russia or vice versa is about half an hour), and liquid-fuelled upper stages, as liquid-fuelled engines have superior performance and may be throttled and restarted. They're much smaller than their predecessors, but at the same time so much more capable; they can be on their way within two or three minutes of the launch order being given, they can carry multiple miniaturized high-yield warheads, and they carry penetration aids and countermeasures to defeat anti-ballistic missile defenses. SRBMs: Short-range ballistic missiles, or SRBMs, are ballistic missiles designed for use at a tactical or theater level. They tend to far more mobile then their ICBM counterparts, focusing on targets like troop concentrations. The 1987 Intermediate Nuclear Forces Treaty between the US and USSR resulted in them scrapping all land-based ballistic and cruise missiles with a range between 500 and 5,500 km. Missile submarines: These are, simply put, submarines capable of launching ballistic nuclear missiles. Their role is not necessarily to launch in the first part of nuclear exchange (although some were, like the Soviet "Yankee" class, intended for that role), but as a "second-strike" weapon. Since "boomers" (the US navy slang term for them) are very hard to find, they could wait out for months if need be. They can usually be recognised by a pronounced hump behind the sail, which is where the missiles are stored. The ultimate examples of this type are the US Ohio class (18 built, 14 still in a nuclear role with the others now converted for conventional Tomahawk launching), which carries 24 Trident ICB Ms. The next tier down is the British Vanguard class, the Russian Borei class and the French Triomphant class, all of which carry 16 ICB Ms each. One design that pops up a lot in fiction is the Soviet/Russian Akula/"Typhoon" class. Whether they are still operation is unclear, of six ships, three have been decommissioned and one (Dmitr Donskoy) has been modified to serve as a test platform for the Russia "Bulava" ("Mace") ICBM. The fates of the remaining two, (Archangelsk and Severstal) are unclear. The Kremlin seems unable to make up its mind as to what to do with them - minelaying, special operations insertion and cruise missile launch have been variously suggested. Latest reports seem to indicate that the submarines are still operational, but their missiles are not. The Typhoon is a very distinctive design indeed- the largest submarines in the world, with the missile compartment at the front, which is probably what accounts for its popularity in fiction. Anti-shipping cruise missiles: This are carried by aircraft and designed to engage naval groups from long distance (200 miles or more). A Russian specialty, these weapons tend to be supersonic, but fly high to seek out their targets. These missiles will need be guided to their targets by a radar source — which can provide mid-course corrections, as ships can move a fair distance in the ten minutes or so it can take to reach a target. This role usually goes to a submarine or maritime patrol aircraft, which are vulnerable. To answer that problem Soviet Union even employed a special satellite constellation for recon and targeting of its naval cruise missiles. Land-attack cruise missiles: Similar to the above, except these missiles are used to attack land-based targets, usually with much greater range. The target doesn't move, but much of Earth's land surface is rather less flat than the ocean, so most modern weapons of this type have some variety of terrain-following guidance, which also enhances their ability to fly low enough to stay below the radar horizon of the defenders until (theoretically) it's too late for them to mount an effective defense. Like their anti-shipping cousins, many can be adapted for use as conventional weapons; although by far not the only example, the "Tomahawk" cruise missile is probably the most famous one, thanks to its heavy use in the 1991 campaign against Iraq, and subsequent engagements in and around southwestern Asia. Guns: As mentioned above, nuclear warheads have been packed inside artillery shells, too. These are usually pretty small, a dozen or so kilotons at most (up to maybe 45 kilotons in the case of the massive 406mm nuclear battleship rounds the US built in the 1970s), intended to destroy massed tank or truck formations as a last-ditch effort to thwart an invasion. The earliest examples of the type were gun-type warheads, making them an odd example of a gun that shoots smaller guns that turn into bombs.