A nuclear weapon releases destructive energy produced by fission or fusion reactions at a subatomic level among nuclei, rather than by the atomic- and molecular-level chemical reactions that produce the energy from conventional explosives. Fission weapons are also called nuclear, atomic, or A-bombs, while fusion weapons are called thermonuclear, fusion, or hydrogen bombs. There is an intermediate type called boosted fission. "Nuclear weapon" is also the most general term, as all fusion weapons are at least "two-stage", with a fission Primary triggering a fusion Secondary. For weapons-specific design of fission and fusion reactions, see the subpages. A particular Primary design may be used with different Secondaries.
"Bomb" often is used generically, but nuclear weapons, not experimental or improvised devices, fall into the general categories of gravity bombs and warheads. "Warhead" is the set of nuclear weapons components that go into a guided missile, round of artillery ammunition, a torpedo. Warheads also describe the weapon component of a backpack (e.g., W54 Special Atomic Demolition Munition). In U.S. practice, for a number of years, the "B" prefix has meant that a given numbered weapon (e.g., B61) is a gravity bomb, while a "W" prefix means it is a warhead. The number defines the physics package, the actual fission or fusion components of a bomb or warhead, exclusive of fuzing, power supplies, etc.; it can be ambiguous if "physics package" refers to the specialized explosives that trigger fusion. The power supplies, arming system, etc., that initiate the conventional explosion are definitely not included in "physics package" A given physics package may be used in both bomb and warhead versions. 
Nuclear weapons development and potential use, beyond the scientific and engineering details needed to build the large industrial infrastructure and detailed technical knowledge to make usable weapons, involve additional issues. Weaponization covers the disciplines needed to convert laboratory principles to a weapon, such as miniaturizing it to a size that can be carried by available means. Weapons effects deal with the events that occur when a weapon detonates. Weapons surety programs ensure both that a weapon will explode when commanded to do so, but be safe against accidental detonation or triggering by unauthorized persons.
The most difficult part of making a weapon is obtaining the special radioactive materials in which fission or fusion occur; manufacturing these takes nation-state level investment. Once those materials were available, however, the next most difficult part is the design of the high explosive system that triggers the fission Primary. It is virtually impossible that a non-national actor could get the special nuclear materials, but, given adequate knowledge, a non-national group, willing to accept inefficiencies and hazardous development, could produce a fission weapon. A fusion weapon is probably not within the capabilities of non-national actors; it is much more complex.
During the Second World War, the United States of America, with British cooperation, set up what was, at the time, the largest industrial and research program in history, the Manhattan Project. This established the basic principles of designing and building fission weapons. In the Trinity Test on 16 July 1945, the U.S. detonated the first nuclear weapon, of the same design that was to be used on Nagasaki. The next two bombs detonated were the only ones ever used in combat: Little Boy on Hiroshima, 6 August 1945,and Fat Man on Nagasaki, 9 August 1945. The United States was constructing additional bombs to be used in the planned October invasion of southern Japan. After the Japanese surrendered, the U.S. tested more bombs in Operation Crossroads in the summer of 1946.
While the Soviet Union had done theoretical research on nuclear weapons in 1940, they concluded they were not feasible. Only after receiving espionage information from the British and U.S. programs, did they begin a serious development program in 1943. Nevertheless, their program was not reverse engineering; they approached several technical problems in a manner different from the U.S. The first Soviet First Lightning, which the U.S. called "Joe-1", was tested on 29 August 1949.
Compared with the amount of high explosives and incendiaries that would have been needed to produce equivalent damage, the bombs were extremely small. As physical objects, however, they were large objects that could only be carried by a large aircraft. While the next stages of development, in the postwar period, focused both on reducing the physical size while increasing the power of the weapons, the technology was originally seen as at its limits and an exclusive U.S. and British secret. Vannevar Bush, WWII science adviser to the President of the United States, wrote, in 1949, that while the Germans had developed early V-2 ballistic missiles, the devices were far too large to ever be carried by a missile.
Nuclear weapons have gone through several generations of development, in which one of the goals has been to make them physically smaller, while increasing yield.
By the summer of 1942, theoreticians in the Manhattan Project, notably Hans Bethe and Edward Teller, believed fission conceptual design was moving well under Robert Serber and his colleagues, and they turned their attention to fusion, considered a distraction by some. By the end of the summer, J. Robert Oppenheimer merged the theoretical group, called the "luminaries", with Serber's, to focus on what was becoming seen as a major effort. Teller, however, continued to have fusion as his major interest.
The U.S. tested the first thermonuclear "device" (i.e., not weaponized), IVY MIKE on 31 October 1952, and the first droppable bomb, CASTLE BRAVO on 28 February 1954. The Soviet Union tested a thermonuclear device, using a different principle than the Teller-Ulam design all subsequent thermonuclear weapons have used, on 12 August 1953. A weaponized Soviet bomb, apparently based on the Teller-Ulam design, was tested on 22 November 1955.
There is no theoretical limit to the number of stages in a thermonuclear weapon; the U.S. developed only one three-stage bomb of 25 Mt yield, the B41. The B41 has been retired; the 9 Mt B53 bomb is not deployed, but stockpiled until replaced, probably by the ground-penetrating B61-11.
From wartime organizations, the Strategic Air Command of the United States Air Force was formed as the primary U.S. nuclear delivery force, although joint planning with the United States Navy began in 1962 and continued until SAC was replaced by the United States Strategic Command in 1992. SAC had control of land-based intercontinental ballistic missiles, while theSoviet Union formed the Strategic Rocket Forces as a separate branch of service, their bombers being a subcommand of the Red Air Force. Britain and France maintained small nuclear forces; Britain now has only submarine-launched ballistic missiles while France is more diversified. China's ICBMs are under the Second Artillery, effectively a separate branch of service.
South Africa and North Korea built nuclear weapons, but the former definitely disarmed and the North Koreans may have done so. India and Pakistan have demonstrated nuclear weapons, and it is widely accepted that Israel has a substantial nuclear arsenal although it maintains a position of strategic ambiguity.
The highest-yielding bomb ever tested was the Soviet Tsar Bomba, which had a yield of 58 megatons, possibly with more than one Secondary. It has been estimated that had it had a 238uranium (U) Tertiary, the yield would have been on the order of 100 Mt.
Effects of nuclear weapons
There are several different kinds of effects from a nuclear explosion, effects which vary with the altitude at which the bomb explodes. For there to be widespread thermal or immediate radiation effect, it must detonate in air, at least 2000 feet/620 meters. This means it must be delivered by aircraft or missile, since that is above the height of the tallest buildings. 
Regardless of the height of the burst, the spot on the ground directly below the center of the explosion is called [actual] ground zero. Designated ground zero (DGZ) is a term used in planning attacks with nuclear weapons; the DGZ is the point on the ground, either below an air burst or the actual point of a surface burst, where the weapon is aimed.
|Type of effect
|Bomb yield and burst altitude
|Symmetrical around ground zero, decreasing by inverse cube [Note ]
|Immediate ionizing radiation
|Biologically significant absorbed radiation (SI units of Grays)
|Bomb design (i.e., radiation enhancement), yield, burst height
|Symmetrical around ground zero, decreasing by inverse square
|Delayed ionizing radiation (fallout)
|Biologically significant absorbed radiation (SI units of Grays)
|Bomb design ("clean" vs. "dirty"), yield, burst height (or subsurface depth)
|Wind patterns; radioactivity is greatest downwind of burst
|Bomb yield and burst altitude, clouds and precipitation
|Symmetrical around ground zero, decreasing by inverse square
Blast, stated as an equivalent number of kilotons or megatons of the conventional explosive, trinitrotoluene (TNT), is the most common way to state the power of a nuclear weapon. If the conversion of mass to energy were to be perfect, an efficiency which no bomb approaches, the fission of 1 pound of uranium or plutonium would produce energy equivalent to that produced by the explosion of 8,000 tons (8 kt) of TNT. 
For example, the Little Boy bomb used on Hiroshima weighed 8,900 pounds had a yield of approximately 15-16 kilotons. The W87 warhead of a Minuteman III intercontinental ballistic missile has an estimated weight of 300-400 pounds (perhaps 800 pounds including the reentry vehicle case), but a yield of 300 kilotons. 
Describing blast power in terms of TNT equivalent can be quite misleading, even before considering weapons effects other than blast. At the most basic, the physical damage done by an explosion, conventional or nuclear, is done by shock waves, usually in the form of air pressure, or overpressure of so many units of force per unit area. One reason that tonnage of TNT is misleading is that a nuclear weapon can cause immensely higher overpressures than a conventional weapon, overpressures only needed to destroy extremely rugged construction such as underground missile silos and command post. To do damage even to factories, a larger number of lower-yield weapons is apt to have a greater military effect, as adequately destructive overpressure spreads over a larger area.
Even overpressure is highly dependent on burst altitude, weapon yield, and the geography of the target area. For even more complexity introduced by the geometry of the shock wave and building type, see Chapter IV of Glasstone.
As opposed to the general experience with conventional explosives, the overpressure caused by an explosion is not a simple inverse cube relationship. Instead, the potential of superheated air produces two potential overpressure patterns from a weapon of a given yield, detonated at different altitudes, according to the Mach Effect.  The Mach effect allows a wider area of lower overpressure to be created by detonating the bomb as an air burst, so the shock wave can travel, faster and farther, through less dense air superheated by the fireball. Surface or subsurface bursts, however, do not create this area of fast shock wave movement, so their blast is more concentrated on a smaller area. The level of overpressure achievable from the Mach effect would destroy civilian buildings and many military structures, but an extremely high overpressure is needed for a structure specifically intended to withstand blast (e.g., a missile silo).
High overpressures are essential in counterforce attacks against what may be superhardened targets, while ordinary buildings and factories, the targets of countervalue attacks, would be destroyed by much lower overpressures.
|Window glass shatters. Light injury from fragments.
|Residential structures collapse, with many serious injuries and some blast/fragmentation deaths
|Most buildings collapse. All within this overpressure are injured and many are killed.
|Reinforced concrete buildings are severely damaged or demolished. Most die.
|Heavily built concrete structures are severely damaged or destroyed, and deaths approach 100%
To put this into perspective with respect to military structures, however, some missile silos are known to have been designed to withstand overpressures into the hundreds or low thousands of PSI.
Blast, however many it kills, still will tend to underestimate damage because there may be as many or more injuries and deaths due to thermal and radiation effects, especially in areas affected by firestorms.
Immediate ionizing radiation effects
Immediate radiation (i.e., straight-line radiation from the fireball) is a relatively small proportion of the casualties, with caveats for such special cases as tank crews, mentioned below. Most people close enough to die of immediate radiation would also be in lethal areas for blast or fire. Acute radiation syndrome will result if individuals are exposed, over a short period of time, to 75 rad/0.75 Gray of penetrating radiation; the threshold may be lower if neutrons rather than gamma or X-ray source.
Enhanced radiation weapons (ERW), or "neutron bombs", were intended as a tactical weapon, principally to blunt Soviet tank attacks against Western Europe. Such devices, while still producing enormous blast and heat, produce relatively more immediate ionizing radiation than would the larger-yield weapons delivered to strategic targets. The idea was abandoned due to political concerns, and the realization these weapons would still do massive damage to one's own side. Now that extremely effective antitank precision-guided munitions that can attack large formations, such as the AGM-154 Joint Standoff Weapon (JSOW), are available, the last arguments for ERW are gone.
Some anti-ballistic missiles, had a different type of ERW warhead design, intended to damage incoming warheads with X-rays.  This kill mechanism relied on detonating the ABM warhead physically near the incoming reentry vehicle; it is not a so-far-theoretical technique called an X-ray laser. The arms control problems created by nuclear weapons in the atmosphere or in space, however, coupled with far more advanced missile guidance systems, led to current ABMs using a "kinetic kill" mechanism -- literally colliding with the incoming warhead.
Nuclear explosions produce varying intensities of electromagnetic pulse (EMP), which has the potential to damage electronic equipment. Effective power, coverage, and frequencies of the electromagnetic pulse are dependent, at a minimum, on the yield of the nuclear weapon and the altitude of the burst.
While general U.S. planning and engineering documents specify means of EMP protection,  no unclassified references suggest that any weapons, targeted under SIOP, are intended principally to produce EMP.
Delayed radiation effects
Delayed and continuing ionizing radiation comes from fallout products of the explosion. In general, the higher the burst altitude, the less fallout; the more surface material that comes in contact with the fireball, the more fallout.
As opposed to the several thousand degrees of heat generated by a conventional explosion, the intense energy release, in a small space, produces temperatures of millions of degrees near the center of the burst. The initial release is of soft X-rays, which superheat the adjacent air; the blast also results indirectly from X-ray heating. Since larger bombs release head more slowly than smaller ones, there is less damage done by each heat calorie; the area of thermal damage increases fairly linearly with increased yield. 
An analysis by Stanford University historian Lynn Eden uses the example of a 300-kiloton weapon bursting, on a clear day, 1500 feet above the Pentagon. Blast would destroy the Pentagon, which is not a hardened facility, and nearby buildings. According to Eden, a much larger area would be set afire thermal energy released by the bomb. 
Since much of the thermal effect is from straight-line infrared radiation, clouds, rain, etc. could attenuate the thermal effect. In clear air, thermal energy decreases from the DGZ by an inverse square law, and is not significantly attenuated by dry air.
Another area of complexity would come from target areas that would have be struck by multiple weapons at different Designated Ground Zero (DGZ) points. For example, in an attack on Moscow, the Kremlin and the Special Purpose Command headquarters of the Russian Air Force might be targeted separately. A fire storm centered on each DGZ might eventually merge.
As mentioned, there have been multiple generations of nuclear weapons. There has always been a goal of improving the ratio of yield to bomb weight, for which fusion or boosted fission has always proven more efficient in real weapons. The practical limit, achieved in the B41 bomb, is 6 kt/pound of bomb weight. 
Among nuclear states, there also have been efforts to both improve reliability (i.e., making sure the bomb goes off when told to so) and make it difficult to detonate a bomb by accident or without authorization.
Reliability and survivability
These measures are in addition to controls over human access to nuclear weapons and systems that issue orders for nuclear weapon deployment and use.
- Personnel reliability program: all personnel with access to weapons, or weapons control system, are under scrutiny for psychological stability. People under intense stress, on medication, etc., may be temporarily removed from access without damaging their careers.
- Positive control with two-man rule: No weapon or weapons control system can ever be accessed by a single person; at least two people must agree on an action. For the most critical actions, the controls are physically separated so that one person could not activate both.
- Permissive Action Links with limited retry
- Environmental Sensing Device
- Possible remote disable
No one measure is considered adequate to provide safety over these most destructive of devices.
- One-point safe criterion
- Weak link-strong link
- Thermal insulation in the case with "Fire-resistant pits" (FRPs)
- Insensitive high explosives (IHE)
- Insulating containers may be used to reduce the influx of heat from a fire,
Tactical vs. strategic
Especially in the 1950s and 1960s, major powers distinguished between "tactical" and "strategic" weapons. See Single Integrated Operational Plan for the principles of U.S. strategic nuclear war planning.
To a significant extent, the difference was more in the targets and delivery systems than in the weapons themselves; the bombs that destroyed two Japanese cities were relatively low-yield, and well within the range of the larger weapons planned for battlefield use.
While there were some literal battlefield applications, especially at sea, tactical nuclear weapons tended to be targeted at facilities behind enemy lines, or on troops moving to, but not in, the battle area. Using nuclear weapons in defense, close to the battle line, had severe "use it or lose it" concerns; it was entirely possible that permission to use a nuclear artillery shell might not be granted before the firing battery was about to be overrun, and its crew destroyed the shell.
The U.S. no longer has any dedicated tactical nuclear weapons (TNW), although the B61 and B83 bombs could be delivered by tactical aircraft. Since both these weapons have variable yield, their effects could be limited, but the political consequences would still be immense. There have been no serious suggestions that the U.S. plans the use of battlefield weapons, in part because precision-guided munitions (PGM), using conventional explosives or even simple kinetic energy, can carry out the missions for which battlefield nuclear weapons were intended. Swedish military researchers, in a 2005 report, have suggested that Russia may still consider tactical nuclear weapon use, "...in response to insufficient conventional capabilities. The TNWs retained obviously have a defensive as well as an offensive role. A large fraction of Russia’s TNWs seem to have been developed to defend “the homeland” against NATO strategic strikes by destroying incoming bombers and cruise missiles as well as eliminating NATO’s naval nuclear assets. The attempt to protect Moscow against incoming missiles could also be mentioned in this regard. Primarily offensive TNWs can be found within the Ground Forces and the Tactical Air Force" 
Types of weapons
For a variety of reasons, nuclear weapons have a finite storage life. Part of nuclear surety programs is making a reliable estimate of that life. Some limits have to do with the degradation, over time, of radioactive components such as tritium, essential to fusion weapons but also a means of reducing the size while increasing the power of "boosted-fission" weapons. The ability to inject different amounts of tritium before detonation is probably the "dial-a-yield" mechanism used to select different explosive yields, especially important in tactical methods.
Some nuclear weapons, under the rules of arms control agreements, are not kept at operational bases or in ready-to-use format. One reason for doing this is as a "hedge" against a breakdown in nuclear arms limitations. There are anecdotes that the U.S. and Russia have agreed to keep their largest weapons, no longer needed in operational use, available for possible use against an asteroid or other space object in a collision course with the earth; the 9-megaton W53 is such a U.S. weapon.
The United States has two types of gravity bombs, and warheads specific to each operational model of intercontinental ballistic missile (ICBM), cruise missile, and submarine-launched ballistic missile (SLBM). Since Russia and the U.S. exchange information as part of arms control programs, it is known the Russian types are roughly similar.
Britain only has nuclear warheads for its SLBMs. France has warheads for SLBMs and air-launched cruise missiles. China and Israel have, at least, bombs and ballistic missile warheads. India and Pakistan appear only to have ballistic missile warheads.
U.S. gravity bombs are of two types, the "tactical" B61 and the "strategic" B83, although the distinction is quite blurry. Both can be set for different yields. Depending on the model of the B61, its yield can be as low as 300 tons and as high as 170 kt. One version, the B61-11, has a limited earth-penetrating capability, for use against hardened underground structures. The -11 will generate fallout, but by being even slightly underground when it detonates, a lower yield might be adequate than a higher-yield surface burst. The B83 varies from a "low" range to 1.2 Mt.
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