Types of Nuclear Weapons
The different types of nuclear weapons, delivery systems, and their basic purpose.
Nuclear Reactions
With the exception of the Dirty Bomb (below) all nuclear weapons rely on one or both of the following:
- Fission: This is the name for when a single large nucleus splits into two smaller nuclei. When this happens, some of the energy that had been holding the original nucleus together (and in some cases, left-over parts of the nucleus that end up in neither of the successor nuclei) is released. The materials used for fission bombs are unstable heavy elements that: a) undergo spontaneous fission which releases free neutrons, and b) undergo stimulated fission when exposed to free neutrons. Spontaneous fissions send neutrons into other nuclei, causing them to split and release more neutrons, and so on in a chain reaction.
In the vast majority of nuclear weapons, either U-235 or Pu-239 is used as the fissile material. Pu-239 is a synthetic element, essentially, and must be produced in particle accelerators or specialized plutonium-production reactors. Regular nuclear reactors produce it too, but they also produce other isotopes of plutonium that don't work as well. Uranium's quite common, but the vast majority of natural uranium is U-238, a stabler isotope than U-235. U-235 can be extracted from natural uranium to obtain weapons-grade material. This refers to uranium or plutonium which is something like 90-95% U-235 or Pu-239. It's possible to design weapons which use stuff that's enriched to only 80%, but that's a major challenge and only a nuclear power like the US could manage. And they kind of suck, too. By comparison, nuclear fuel in commercial power reactors is enriched to something like 5%.
Finally, a note on terminology: "fissile" nuclei can be split by low-energy 'thermal' neutrons (U-235 and Pu-239 are fissile) and can be 'burned' in reactors. They can sustain a chain reaction. 'Fissionable' materials can be fissioned, of course, but it's much more difficult. They can't be fissioned by low-energy neutrons, but require high-energy 'fast' neutrons to split them.
- Fusion: The reaction that powers stars. When two small nuclei collide with enough energy the two will combine into a single, larger nucleus, releasing even more energy in the process. It requires both vast pressure and immense temperature to get this kind of reaction started, because the positively charged nuclei must be forced against their electromagnetic repulsion until the strong nuclear force takes over (unlike fission, wherein an uncharged neutron can be made to smash into a nucleus more easily). In bombs or reactors, deuterium[1] would be made to fuse with tritium.[2] Depending on the nuclei used as "fuel", this may also release fast neutrons.
Fusion is more efficient than fission in converting mass to energy, but is much harder to initiate, sustain and control. Hence, we do not yet have fusion reactors, though there are ongoing efforts to create workable "tokamak" type reactors in which fusion plasma is contained inside an electromagnetic field, in a toroidal chamber. Fusion for bombs is somewhat simpler—you just need to initiate the reaction, and do not need to worry about sustaining it or containing it.
Just in case you're wondering, stars use the gravity exerted by their tremendous mass to initiate and sustain fusion—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, given that iron is "inert" when it comes to nuclear reactions). The most massive stars fuse elements above iron, but since this costs energy rather than releases it, they go through this rather quickly and explode brilliantly—what we call a supernova. This, by the way, is how almost all atoms of elements heavier than helium and a little bit of lithium (the former was produced in the Big Bang in quantity, and the latter in smaller amounts) originally came into being (leaving heaviest of them for later fission and decay).
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. Different elements have a different affinity for the two. 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. Larger elements decay very quickly, and therefore are absolute nightmares to amass or store sufficient quantities of, and smaller elements require far more effort to start the reaction. Fusion conversely works best with smaller elements: the typical fuel for fusion are two isotopes of Hydrogen (deuterium and tritium).
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. (E=mc^2) This energy might be in the form of gamma rays, highly energetic pions or possibly harmless neutrinos. Because antimatter is so hard to get, no matter-antimatter weapon has ever been built, and it seems improbable that any will be in the forseeable future. 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.
A slightly more practical use of this is 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 beyond present-day technology, but not nearly as far beyond as a pure annihilation device. Both classes of antimatter-based weapons would suffer from the same problem that plagues fission and fusion devices—the need to keep the (now exploding) device together long enough to fission/fuse/annihilate most of its fuel. Unlike fission and fusion, though, an antimatter device probably wouldn't involve large amounts of very dense uranium, making effective design quite difficult, even if enough antimatter could be procured.
Device Types
Atomic 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:
- Pure fission weapons. Obviously, only fission reactions occur when one of these babies detonates. Highly-enriched fissile material is used. Normally, it is kept in a subcritical state inside the bomb; when the time comes, conventional explosives are used to assemble the fissile materials so that they enter a supecritical state. After that comes a tremendous release of energy and a massive explosion. There are two ways to assemble the fissile material, and they're both implementations of this principle.
- The first, and simplest, is a gun-type nuclear device. Two subcritical masses of fissile material are brought together; one is a target, and one is a bullet. The bullet is fired down a tube at the target, conventional explosives acting as a propellant. (Yes, a nuclear bomb built upon the principle of More Dakka.) Only U-235 may be used as the fissile material in a practical device, because of predetonation problems with Pu-239.
The "Little Boy" device used on Hiroshima was a gun-type device using U-235; it was very simple and needed no testing. Gun-type weapons aren't as safe as implosion-type weapons because it is much easier to accidentally assemble the fissile material in such a way that there is significant nuclear yield; the Enola Gay may have gone up in a multi-kiloton fireball had it crashed carrying the device. They were useful, however, because of their form factor, which made them suited for tactical use. Before it was scrapped, the entire nuclear arsenal of South Africa consisted of gun-type devices.
- There are also implosion bombs. The fissile material is kept together in a single mass, typically a sphere. It may be hollow or solid, depending on the circumstances. This "pit" is surrounded by conventional explosives; there are numerous explosive "lenses". It is necessary to very, very carefully shape the shockwave in order to achieve proper compression of the pit. As such, the lenses must be manufactured to extremely tight tolerances, and the detonators must be triggered with precise timing. Special types of detonators and switches are used as a result. (Look up "explosive bridgewire detonators," "slapper detonators," and "krytrons.")
The Trinity device was an implosion-type weapon utilizing Pu-239. It had to be tested because there were far more doubts about implosion-type weapons than gun-type weapons. The Fat Man device, used on Nagasaki, was also an implosion-type weapon. Most nuclear weapons today are implosion-based, for safety and efficiency reasons, although for a time gun-type weapons were stockpiled.
- Boosted fission weapons: tritium is injected into the pit. As the fission reaction occurs, this is more than enough to induce fusion of the tritium; the extra neutrons allow more of the pit to be "burned" and converted to useful energy before the pit blows apart and the reaction stops. Efficiency can be increased enormously as a result. By varying the amount of tritium injected into the pit, the yield can be varied. That way, a single device can be made more flexible.
Hydrogen bomb (or H-bomb): These are properly known as "thermonuclear weapons." Even boosted-fission weapons can only be scaled up so much. Obviously, for a fission weapon, yield is a function of the amount of fissile material used. However, there is in practical use an upper limit on the size of the pit. The amount of fissile material in the weapon may exceed one critical mass. The British had trouble obtaining true thermonuclear devices and were desperate for a "megaton-class" weapon, and rushed to produce an extremely large pure-fission device (Green Grass) they could test before a test moratorium came into effect, so that they could prove that they were a proper nuclear power. This device was extremely clumsy and unsafe and rather funny. The British pulled off the deception quite well, however; even when it was tested, most everyone thought it was a thermonuclear device. (The bomb had several critical masses worth of U-235 in a hollow pit; it was a nightmare keeping it subcritical. The hollow space was supposed to be filled with a rubber bag housing several hundred thousand steel ball bearings, kept in place with a plug. It's as bad as it sounds. The people who developed it couldn't even guarantee that this safety mechanism could work, although they were "fairly sure" it would.)
Anyway, back when thermonuclear bombs were developed, delivery systems were inaccurate and only large, soft targets like airfields and cities could be hit. Also, bombers were much easier to intercept than ballistic missiles, so it was important to maximize destructive power.
Most extant thermonuclear weapons utilize the Teller-Ulam design. This involves a fission primary and a fusion secondary. The fission primary detonates; somehow this is conveyed to a fusion secondary. The exact mechanism has never been publicly revealed, though educated guesses have been made. The primary is a relatively conventional implosion-assembly fission device, often boosted. The secondary is usually cylindrical. There's a column of fissile material, known as a "spark plug," surrounded by layers of fusion fuel and a tamper of U-238. Everything is suspended in foam and surrounded by a heavy metal casing. What seems to happen is that the radiation from the detonating primary is reflected off of the casing walls and onto the tamper, which ablates extremely rapidly. Thanks to Newton's Third Law, the tamper is rapidly crushed as a result, initiating fission in the spark plug and then fusion in the fusion fuel. The fast neutrons from the fusion reaction cause many fission events in the U-238 tamper (or bomb casing) further contributing energy.
Early designs seem to have suggested using tungsten instead of uranium for the tamper. This is because tungsten is slightly denser than uranium, and the tamper's main job is to hold the exploding device together by sheer inertia while the primary and secondary suck up as much fuel as possible. This was changed to uranium when someone realized that the fusion neutrons would induce a bunch of fission (which might in turn cause even more fission) in a uranium tamper. This was also a very good place to use up the otherwise-useless U-238 produced as a byproduct of refining U-235. (Some of it also got made into bullets, but these are conventional weapons, so not otherwise addressed here.)
Because of the different energy distributions from fusion and fission reactions, a given amount of fission yield is more destructive than fusion yield. Please bear in mind that everything happens within an incredibly tiny fraction of a second. Also, this may all be a crock; for obvious reasons, the tricks of the trade are extremely closely-guarded secrets.
There is no upper limit on the explosive yield from a Teller-Ulam device; arbitrarily many stages may be added. Remember, when you merely wish to bury bombs, there is no limit to the size! However, scaling beyond a certain point is impractical, as the damage radius is approximately logarithmic with respect to the yield. The current power record is held by the Tsar Bomba, the original design of which was estimated to have a yield of 100 MT. For testing, the U-238 tamper was not used (so as to limit the fallout), thereby reducing the yield to "only" 50 MT. Size, weight and power make this weapon relatively useless. Not to mention the fact that had they dialed it up to the full 100 MT, it would have vaporized the plane that was dropping it.
Specialist Warheads
Dirty 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.
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. So, the tank will survive, but the crew will suffer a rapid onset case of fatal radiation poisoning out to half a mile.
Weapon Types
First, 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.
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.
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. 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.
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.
Delivery Systems
Aircraft: 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) and the Soviet/Russian Akula/"Typhoon" class (6 built, 3 still in service). The latter is a very distinctive design indeed- the largest submarines in the world, with the missile compartment at the front.
Anti-shipping cruise missiles: This are carried by aircraft and designed to engage naval groups from long distance (200 miles or more). A Russian speciality, 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.
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.