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Encyclopedia > Nuclear weapon design
The first nuclear weapons, though large, cumbersome and inefficient, provided the basic design building blocks of all future weapons. Here the Gadget device is prepared for the first nuclear test: Trinity.

Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package[1] of a nuclear weapon to detonate. There are three basic design types. In all three, the explosive energy is derived primarily from nuclear fission, not fusion. Setting up the gadget at the Trinity site, 1945. ... Setting up the gadget at the Trinity site, 1945. ... The gadget, partially assembled on the shot tower for the Trinity test. ... Preparation for an underground nuclear test at the Nevada Test Site in the 1980s. ... The Trinity test was the first test of a nuclear weapon, conducted by the United States on July 16, 1945 at , thirty miles (48 km) southeast of Socorro on what is now White Sands Missile Range, headquartered near Alamogordo, New Mexico. ... The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 km (11 mi) above the epicenter. ... For the generation of electrical power by fission, see Nuclear power plant. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing sustainable fusion power. ...

• Pure fission weapons were the first nuclear weapons built and the only type ever used in warfare. The active material is fissile uranium (U-235) or plutonium (Pu-239), explosively assembled into a chain-reacting critical mass by one of two methods:
• Gun assembly, in which one piece of fissile uranium is fired down a gun barrel to a fissile uranium target at the end of the barrel (plutonium can use this design, but it has proven to be wildly impractical), or
• Implosion, in which a fissile mass of either material (U-235, Pu-239, or a combination) is surrounded by high explosives which compress it, turning the sub-critical mass into a critical mass.
• Fusion-boosted fission weapons improve on the implosion design. The high temperature and pressure environment at the center of an exploding fission weapon compresses and heats a mixture of tritium and deuterium gas (heavy isotopes of hydrogen). The hydrogen fuses to form helium and free neutrons. The energy release from fusion reactions is relatively negligible, but each neutron starts a new fission chain reaction, greatly reducing the amount of fissile material that would otherwise be wasted. Boosting can more than double the weapon's fission energy release.
• Two-stage thermonuclear weapons are essentially a daisy chain of fusion-boosted fission weapons, with only two daisies, or stages, in the chain. The second stage, called the "secondary," is imploded by x-ray energy from the first stage, called the "primary." This radiation implosion is much more effective than the high-explosive implosion of the primary. Consequently, the secondary can be many times more powerful than the primary, without being bigger. The secondary could be designed to maximize fusion energy release, but in most designs fusion is employed only to drive or enhance fission, as it is in the primary. More stages could be added, but the result would be a multi-megaton weapon too powerful to be useful. (The United States briefly deployed a three-stage 25-megaton bomb, the B41, starting in 1961. Also in 1961, the Soviet Union tested, but did not deploy, a three-stage 50-megaton device, Tsar Bomba.)

All innovations in nuclear weapon design originated in the United States;[3] the following descriptions feature U.S. designs.

In early news accounts, pure fission weapons were called atomic bombs or A-bombs, a misnomer since the energy comes only from the nucleus of the atom. Weapons involving fusion were called hydrogen bombs or H-bombs, also a misnomer since their destructive energy comes mostly from fission. Insiders favored the terms nuclear and thermonuclear, respectively.

The term thermonuclear refers to the high temperatures required to initiate fusion. It ignores the equally important factor of pressure, which was considered secret at the time the term became current. Many nuclear weapon terms are similarly inaccurate because of their origin in a classified environment. Some are nonsense code words such as "alarm clock" (see below).

Nuclear weapons

History of nuclear weapons
Nuclear warfare
Nuclear arms race
Weapon design / testing
Effects of nuclear explosions
Delivery systems
Nuclear espionage
Proliferation / Arsenals The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 kilometers (11 mi) above the hypocenter A nuclear weapon derives its destructive force from nuclear reactions of fusion or fission. ... Image File history File links A picture of a mockup of the Fat Man nuclear device, from http://www. ... A nuclear fireball lights up the night in a United States nuclear test. ... The Titan II ICBM carried a 9 Mt W53 warhead, making it one of the most powerful nuclear weapons fielded by the United States during the Cold War. ... U.S. and USSR/Russian nuclear weapons stockpiles, 1945-2006. ... Preparation for an underground nuclear test at the Nevada Test Site in the 1980s. ... An American nuclear test. ... // Nuclear weapons delivery is the technology and systems used to place a nuclear weapon at the position of detonation, on or near its intended target. ... Nuclear espionage is the purposeful giving of state secrets regarding nuclear weapons to other states without authorization (espionage). ... World map with nuclear weapons development status represented by color. ... This is a list of nuclear weapons ordered by state and then type within the states. ...

Nuclear-armed countries

US · Russia · UK · France
PR China · India · Israel
Pakistan · North Korea
(South Africa) Nations that are known or believed to possess nuclear weapons are sometimes referred to as the nuclear club. ... The United States of America was the first country in the world to successfully develop nuclear weapons, and is the only country to have used them in war against another nation. ... The Peoples Republic of China is estimated to have an arsenal of about 400 nuclear weapons stockpiled as of 1999, although this number is questionable because the Chinese government releases little information regarding nuclear weapons other than stating that China possesses the smallest nuclear arsenal amongst the five nuclear...

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Nuclear fission splits the heaviest of atoms to form lighter atoms. Nuclear fusion bonds together the lightest atoms to form heavier atoms. Both reactions generate roughly a million times more energy than comparable chemical reactions, making nuclear bombs a million times more powerful than non-nuclear bombs.

In some ways, fission and fusion are opposite and complimentary reactions, but the particulars are unique for each. To understand how nuclear weapons are designed, it is useful to know the important similarities and differences between fission and fusion. The following explanation uses rounded numbers and approximations.[4]

### Fission

Fission can be self-sustaining because fission produces more neutrons of the speed required to cause new fissions. When a free neutron hits the nucleus of a fissionable atom like uranium-235 ( 235U), the uranium splits into two smaller atoms called fission fragments, plus more neutrons.

The uranium atom can split any one of dozens of different ways, as long as the atomic weights add up to 236 (uranium plus the extra neutron). The following equation shows one possible split, namely into strontium-95 ( 95Sr), xenon-139 ( 139Xe), and two neutrons (n), plus energy:[5]

${}^{235}mathrm{U} + n = {}^{95}mathrm{Sr} + {}^{139}mathrm{Xe} + 2n + 180 mathrm{MeV}$

The immediate energy release per atom is 180 million electron volts (MeV), i.e. 74 TJ/kg, of which 90% is kinetic energy (or motion) of the fission fragments, flying away from each other mutually repelled by the positive charge of their protons (38 for strontium, 54 for xenon). Thus their initial kinetic energy is 67 TJ/kg, hence their initial speed is 12,000 kilometers per second, but their high electric charge causes many inelastic collisions with nearby nuclei. The fragments remain trapped inside the bomb's uranium pit until their motion is converted into x-ray heat, a process which takes about a millionth of a second (a microsecond). An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ...

This x-ray energy produces the blast and fire which are the purpose of a nuclear explosion.

After the fission products slow down, they remain radioactive. Being new elements with too many neutrons, they eventually become stable by means of beta decay, converting neutrons into protons by throwing off electrons and gamma rays. Each fission product nucleus decays between one and six times, average three times, producing radioactive elements with half-lives up to 200,000 years.[6] In reactors, these products are the nuclear waste in spent fuel. In bombs, they become radioactive fallout, both local and global.

Meanwhile, inside the exploding bomb, the free neutrons released by fission strike nearby U-235 nuclei causing them to fission in an exponentially growing chain reaction (1, 2, 4, 8, 16, etc.). Starting from one, the number of fissions can theoretically double a hundred times in a microsecond, which could consume all uranium up to hundreds of tons by the hundredth link in the chain. In practice, bombs do not contain that much uranium, and, anyway, just a few kilograms undergo fission before the uranium blows itself apart.

Holding an exploding bomb together is the greatest challenge of fission weapon design. The heat of fission rapidly expands the uranium pit, spreading apart the target nuclei and making space for the neutrons to escape without being captured. The chain reaction stops.

Materials which can sustain a chain reaction are called fissile. The two fissile materials used in nuclear weapons are: U-235, also known as highly enriched uranium (HEU), oralloy (Oy) meaning Oak Ridge Alloy, or 25 (the last digits of the atomic number, which is 92 for uranium, and the atomic weight, here 235, respectively); and Pu-239, also known as plutonium, or 49 (from 94 and 239). This article or section should include material from Fissile material In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. ... Enriched uranium is uranium whose uranium-235 content has been increased through the process of isotope separation. ...

Uranium's most common isotope, U-238, is fissionable but not fissile. Its aliases include natural or unenriched uranium, depleted uranium (DU), tubealloy (Tu), and 28. It cannot sustain a chain reaction, because its own fission neutrons are not powerful enough to cause more U-238 fission. However, the neutrons released by fusion will fission U-238. This reaction produces most of the energy in a typical two-stage thermonuclear weapon. Depleted uranium storage yard. ...

### Fusion

Fusion cannot be self-sustaining because it does not produce the heat and pressure necessary for more fusion. It produces neutrons which run away with the energy. In weapons, the most important fusion reaction is called the D-T reaction. Using the heat and pressure of fission, hydrogen-2, or deuterium ( 2D), fuses with hydrogen-3, or tritium ( 3T), to form helium-4 ( 4He) plus one neutron (n) and energy:[7]

$^2mathrm{D} + ^3! mathrm{T} = ^4! !mathrm{He} + n + 17.6 mathrm{MeV}$

Notice that the total energy output, 17.6 MeV, is ten times less than that with fission, but the ingredients are almost fifty times less massive, so the energy output per kilo is greater. However, in this fusion reaction 80% of the energy, or 14 MeV, is in the motion of the neutron which, having no electric charge and being almost as massive as the hydrogen nuclei that created it, can escape the scene without leaving its energy behind to help sustain the reaction – or to generate x-rays for blast and fire. Image File history File links D-T_fusion. ...

The only practical way to capture most of the fusion energy is to trap the neutrons inside a massive bottle of heavy material such as lead, uranium, or plutonium. If the 14 MeV neutron is captured by uranium (either type: 235 or 238) or plutonium, the result is fission and the release of 180 MeV of fission energy, which will produce the heat and pressure necessary to sustain fusion, in addition to multiplying the energy output tenfold.

Fission is thus necessary to start fusion, to sustain fusion, and to optimize the extraction of useful energy from fusion (by making more fission). In the case of a neutron bomb, see below, the last-mentioned does not apply since the escape of neutrons is the objective.

### Tritium production

A third important nuclear reaction is the one that creates tritium, essential to the type of fusion used in weapons and, incidentally, the most expensive ingredient in any nuclear weapon. Tritium, or hydrogen-3, is made by bombarding lithium-6 ( 6Li) with a neutron (n) to produce helium-4 ( 4He) plus tritium ( 3T) and energy:[7] Lithium (Li) Standard atomic mass: [6. ... This article or section does not adequately cite its references or sources. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ...

$^6mathrm{Li} + n = ^4!!mathrm{He} + ^3!mathrm{T} + 5 mathrm{MeV}$

A nuclear reactor is necessary to provide the neutrons. The industrial-scale conversion of lithium-6 to tritium is very similar to the conversion of uranium-238 into plutonium-239. In both cases the feed material is placed inside a nuclear reactor and removed for processing after a period of time. In the 1950s, when reactor capacity was limited, for the production of every atom of tritium the production of an atom of plutonium had to be dispensed with.

The fission of one plutonium atom releases ten times more total energy than the fusion of one tritium atom, and it generates fifty times more blast and fire. For this reason, tritium is included in nuclear weapon components only when it causes more fission than its production sacrifices, namely in the case of fusion-boosted fission.

However, an exploding nuclear bomb is a nuclear reactor. The above reaction can take place simultaneously throughout the secondary of a two-stage thermonuclear weapon, producing tritium in place as the device explodes.

Of the three basic types of nuclear weapon, the first, pure fission, uses the first of the three nuclear reactions above. The second, fusion-boosted fission, uses the first two. The third, two-stage thermonuclear, uses all three.

## Pure fission weapons

The first task of a nuclear weapon design is to rapidly assemble, at the time of detonation, more than one critical mass of fissile uranium or plutonium. A critical mass is one in which the percentage of fission-produced neutrons which are captured and cause more fission is large enough to perpetuate the fission and prevent it from dying out. For other uses of critical mass, see critical mass (disambiguation). ...

Once the critical mass is assembled, at maximum density, a burst of neutrons is supplied to start as many chain reactions as possible. Early weapons used an "urchin" inside the pit containing non-touching interior surfaces of polonium-210 and beryllium. Implosion of the pit crushed the urchin, bringing the two metals in contact to produce free neutrons. In modern weapons, the neutron generator is a high-voltage vacuum tube containing a particle accelerator which bombards a deuterium/tritium-metal hydride target with deuterium and tritium ions. The resulting small-scale fusion produces neutrons at a protected location outside the physics package, from which they penetrate the pit. This method allows better control of the timing of chain reaction initiation. General Name, Symbol, Number polonium, Po, 84 Chemical series metalloids Group, Period, Block 16, 6, p Appearance silvery Standard atomic weight (209) gÂ·molâˆ’1 Electron configuration [Xe] 4f14 5d10 6s2 6p4 Electrons per shell 2, 8, 18, 32, 18, 6 Physical properties Phase solid Density (near r. ... General Name, symbol, number beryllium, Be, 4 Chemical series alkaline earth metals Group, period, block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... For the DC Comics Superhero also called Atom Smasher, see Albert Rothstein. ... This article is about the electrically charged particle. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing sustainable fusion power. ...

The critical mass of an uncompressed sphere of bare metal is 110 lb (50 kg) for uranium-235 and 35 lb (16 kg) for delta-phase plutonium-239. In practical applications, the amount of material required for critical mass is modified by shape, purity, density, and the proximity to neutron-reflecting material, all of which affect the escape or capture of neutrons. Nuclear weapon designs are often divided into two classes, based on the dominant source of the nuclear weapons energy. ...

To avoid a chain reaction during handling, the fissile material in the weapon must be sub-critical before detonation. It may consist of one or more components containing less than one uncompressed critical mass each. A thin hollow shell can have more than the bare-sphere critical mass, as can a cylinder, which can be arbitrarily long without ever reaching critical mass.

A tamper is an optional layer of dense material surrounding the fissile material. Due to its inertia it delays the expansion of the reacting material, increasing the efficiency of the weapon. Often the same layer serves both as tamper and as neutron reflector. This article is about inertia as it applies to local motion. ...

### Gun assembly

1. Explosive 2. Gun barrel 3. Hollow uranium "bullet" 4. Cylinder target
Main article: Gun-type fission weapon

Little Boy, the Hiroshima bomb, used 140 lb (64 kg) of Uranium with an average enrichment of around 80%, or 112 lb (51 kg) of U-235, just about the bare-metal critical mass. (See Little Boy article for a detailed drawing.) When assembled inside its tamper/reflector of tungsten carbide, the 140 lb was more than twice critical mass. Before detonation, it was separated into two sub-critical pieces, one of which was later fired down a gun barrel at the other. About 1% of the uranium underwent fission; the remainder, representing 98% of the entire wartime output of the giant factories at Oak Ridge, scattered uselessly.[citation needed] Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the gun method: shooting one piece of sub-critical material into another. ... Little Boy was the codename of the atomic bomb which was dropped on Hiroshima, on August 6, 1945 by the 12-man crew of the B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets (Tibbets, age 92, died Nov. ...

The inefficiency was caused by the speed with which the uncompressed fissioning uranium expanded and became sub-critical by virtue of decreased density. Despite its inefficiency, this design, because of its shape, was adapted for use in small-diameter, cylindrical artillery shells (a gun-type warhead fired from the barrel of a much larger gun). Such warheads were deployed by the U.S. until 1992, accounting for a significant fraction of the U-235 in the arsenal. Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the gun method: shooting one piece of sub-critical material into another. ...

### Implosion

Fat Man, the Nagasaki bomb, used 13.6 lb (6.2 kg) of Pu-239, which is only 39% of bare-metal critical mass. (See Fat Man article for a detailed drawing.) When surrounded by its tamper of U-238, the 13.6 lb was just short of one critical mass. In detonation, critical mass was achieved by imploding, or squeezing, the plutonium pit to increase its density. About 20% of the plutonium underwent fission; only 11 lb were scattered. Image File history File links Implosion_Nuclear_weapon. ... This article is about the nuclear weapon used in World War II. For other uses, see Fat Man (disambiguation). ...

An implosion shock wave might be of such short duration that only a fraction of the pit is compressed at any instant as the wave passes through it. A pusher shell made out of low density metal—such as aluminium, beryllium, or an alloy of the two metals (aluminium being easier and safer to shape and beryllium for its high-neutron-reflective capability) —may be needed. The pusher is located between the explosive lens and the tamper. It works by reflecting some of the shockwave backwards, thereby having the effect of lengthening its duration. Fat Man used an aluminum pusher. Image File history File links Implosion_bomb_animated. ... For other uses, see Density (disambiguation). ... This article is about metallic materials. ... Aluminum redirects here. ... General Name, symbol, number beryllium, Be, 4 Chemical series alkaline earth metals Group, period, block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... An alloy is a homogeneous hybrid of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ...

The key to Fat Man's greater efficiency was the inward momentum of the massive U-238 tamper (which did not undergo fission). Once the chain reaction started in the plutonium, the momentum of the implosion had to be reversed before expansion could stop the fission. Holding everything together for perhaps a few hundred more nanoseconds made the difference.

#### Plutonium pit

The core of an implosion weapon – the fissile material and any reflector or tamper bonded to it – is known as the pit. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium,[8] but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.

Casting and then machining plutonium is difficult not only because of its toxicity, but also because plutonium has many different metallic phases. As plutonium cools, changes in phase result in distortion. This distortion is normally overcome by alloying it with 3–3.5 molar% (0.9–1.0% by weight) gallium which causes it to take up its delta phase over a wide temperature range.[9] When cooling from molten it then suffers only a single phase change, from epsilon to delta, instead of the four changes it would otherwise pass through. Other trivalent metals would also work, but gallium has a small neutron absorption cross section and helps protect the plutonium against corrosion. A drawback is that gallium compounds themselves are corrosive and so if the plutonium is recovered from dismantled weapons for conversion to plutonium dioxide for power reactors, there is the difficulty of removing the gallium. Not to be confused with Galium. ... In chemistry, valency is the power of an atom of an element to combine with other atoms measured by the number of electrons which an atom will give, take, or share to form a chemical compound. ... This article is about metallic materials. ... Electromagnetic radiation may be characterised by its wavelength. ... For the hazard, see corrosive. ... Prepared during the reprocessing of nuclear fuel by calcination of plutonium(IV) oxalate, Pu(C2O4)2. ... Core of a small nuclear reactor used for research. ...

Because plutonium is chemically reactive and toxic if inhaled or enters the body by any other means, for protection of the assembler, it is common to plate the completed pit with a thin layer of inert metal. In the first weapons, nickel was used but gold is now preferred.[10] For other uses, see Nickel (disambiguation). ... GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ...

### Levitated-pit implosion

The first improvement on the Fat Man design was to put an air space between the tamper and the pit to create a hammer-on-nail impact. The pit, sitting on a hollow cone inside the tamper cavity, was said to be levitated. The three tests of Operation Sandstone, in 1948, used Fat Man designs with levitated pits. The largest yield was 49 kilotons, more than twice the yield of the unlevitated Fat Man.[11] Operation Sandstone was the third American series of nuclear weapon tests conducted in 1948 at Eniwetok Atoll. ...

It was immediately clear that implosion was the best design for a fission weapon. Its only drawback seemed to be its diameter. Fat Man was 5 feet wide vs 2 feet for Little Boy.

Eleven years later, implosion designs had advanced sufficiently that the 5 foot-diameter sphere of Fat Man had been reduced to a 1 foot-diameter cylinder 2 feet long, the Swan device.

The Pu-239 pit of Fat Man was only 3.6 inches in diameter, the size of a softball. The bulk of Fat Man's girth was the implosion mechanism, namely concentric layers of U-238, aluminum, and high explosives. The key to reducing that girth was the two-point implosion design.

### Two-point linear implosion

A very inefficient implosion design is one that simply reshapes an ovoid into a sphere, with minimal compression. In linear implosion, an untamped, solid, elongated mass of Pu-239, larger than critical mass in a sphere, is imbedded inside a cylinder of high explosive with a detonator at each end.[12]

Detonation makes the pit critical by driving the ends inward, creating a spherical shape. The shock may also change plutonium from delta to alpha phase, increasing its density by 23%, but without the inward momentum of a true implosion. The lack of compression makes it inefficient, but the simplicity and small diameter make it suitable for use in artillery shells and atomic demolition munitions - ADMs - also known as backpack or suitcase nukes. Suitcase with hypothetical nuclear weapon mock-up inside A suitcase bomb is a bomb which uses a suitcase as its delivery method. ...

All such low-yield battlefield weapons, whether gun-type U-235 designs or linear implosion Pu-239 designs, pay a high price in fissile material in order to achieve diameters between six and ten inches.

### Two-point hollow-pit implosion

A more efficient two-point implosion system uses two high explosive lenses and a hollow pit.

A hollow plutonium pit was the original plan for the 1945 Fat Man bomb, but there was not enough time to develop and test the implosion system for it. A simpler solid-pit design was considered more reliable, given the time restraint, but it required a heavy U-238 tamper, a thick aluminum pusher, and three tons of high explosives.

After the war, interest in the hollow pit design was revived. Its obvious advantage is that a hollow shell of plutonium, shock-deformed and driven inward toward its empty center, would carry momentum into its violent assembly as a solid sphere. It would be self-tamping, requiring a smaller U-238 tamper, no aluminum pusher, and less high explosive. The hollow pit made levitation obsolete.

The Fat Man bomb had two concentric, spherical shells of high explosives, each about 10 inches thick. The inner shell drove the implosion. The outer shell consisted of a soccer-ball pattern of 32 high explosive lenses, each of which converted the convex wave from its detonator into a concave wave matching the contour of the outer surface of the inner shell. If these 32 lenses could be replaced with only two, the high explosive sphere could become an ellipsoid (prolate spheroid) with a much smaller diameter. The truncated icosahedron is an Archimedean solid. ...

The best illustration of these two features is a 1956 drawing from the Swedish nuclear bomb program. The program was terminated before it produced a test explosion. The drawing shows the essential elements of the two-point hollow-pit design.

There are similar drawings in the open literature that come from the post-war German nuclear bomb program, which was also terminated, and from the French program, which produced an arsenal.

The mechanism of the high explosive lens (diagram item #6) is not shown in the Swedish drawing, but a standard lens made of fast and slow high explosives, as in Fat Man, would be much longer than the shape depicted. For a single high explosive lens to generate a concave wave that envelops an entire hemisphere, it must either be very long or the part of the wave on a direct line from the detonator to the pit must be slowed dramatically.

A slow high explosive is too fast, but the flying plate of an "air lens" is not. A metal plate, shock-deformed, and pushed across an empty space can be designed to move slowly enough.[13][14] A two-point implosion system using air lens technology can have a length no more than twice its diameter, as in the Swedish diagram above.

## Fusion-boosted fission weapons

The next step in miniaturization was to speed up the fissioning of the pit to reduce the amount of time inertial confinement needed. The hollow pit provided an ideal location to introduce fusion for the boosting of fission. A 50-50 mixture of tritium and deuterium gas, pumped into the pit during arming, will fuse into helium and release free neutrons soon after fission begins. The neutrons will start a large number of new chain reactions while the pit is still critical. Boosted fission weapons are a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. ...

Once the hollow pit is perfected, there is little reason not to boost.

The concept of fusion-boosted fission was first tested on May 25, 1951, in the Item shot of Operation Greenhouse, Eniwetok, yield 45.5 kilotons. The Greenhouse Item booster device. ... Operation Greenhouse was the fifth American nuclear test series, the second conducted in 1951 and the first to test thermonuclear weapons (hydrogen bombs). ...

Boosting reduces diameter in three ways, all the result of faster fission:

• Since the compressed pit does not need to be held together as long, the massive U-238 tamper can be replaced by a light-weight beryllium shell (to reflect escaping neutrons back into the pit). The diameter is reduced.
• The mass of the pit can be reduced by half, without reducing yield. Diameter is reduced again.
• Since the mass of the metal being imploded (tamper plus pit) is reduced, a smaller charge of high explosive is needed, reducing diameter even further.

Since boosting is required to attain full design yield, any reduction in boosting reduces yield. Boosted weapons are thus variable-yield weapons. Yield can be reduced any time before detonation, simply by putting less than the full amount of tritium into the pit during the arming procedure. Variable yield, or Dial-a-yield, an option available on most modern nuclear bombs, allows the operator to specify a bombs yield, or explosive power, allowing a single design to be used in different situations. ...

The first device whose dimensions suggest employment of all these features (two-point, hollow-pit, fusion-boosted implosion) was the Swan device, tested June 22, 1956, as the Inca shot of Operation Redwing, at Eniwetok. Its yield was 15 kilotons, about the same as Little Boy, the Hiroshima bomb. It weighed 105 lb (47.6 kg) and was cylindrical in shape, 11.6 inches (29.5 cm) in diameter and 22.9 inches (58 cm) long. The above schematic illustrates what were probably its essential features. Operation Redwing was a United States series of 17 nuclear test detonations from May to July 1956. ...

Eleven days later, July 3, 1956, the Swan was test-fired again at Eniwetok, as the Mohawk shot of Redwing. This time it served as the primary, or first stage, of a two-stage thermonuclear device, a role it played in a dozen such tests during the 1950s. Swan was the first off-the-shelf, multi-use primary, and the prototype for all that followed.

After the success of Swan, 11 or 12 inches seemed to become the standard diameter of boosted single-stage devices tested during the 1950s. Length was usually twice the diameter, but one such device, which became the W54 warhead, was closer to a sphere, only 15 inches long. It was tested two dozen times in the 1957-62 period before being deployed. No other design had such a long string of test failures. Since the longer devices tended to work correctly on the first try, there must have been some difficulty in flattening the two high explosive lenses enough to achieve the desired length-to-width ratio. Image File history File links Download high resolution version (1924x860, 1226 KB) Summary A same-scale visual comparison of nuclear weapons miniaturization: The Mark-III (Fat Man) 1945 implosion bomb (21 kt yield) and the W54 1961 implosion bomb (up to 6 kt yield during tests), to the same scale... The W54 nuclear warhead was used in the man-portable M-388 Davy Crockett projectile. ...

One of the applications of the W54 was the Davy Crockett XM-388 recoilless rifle projectile, shown here in comparison to its Fat Man predecessor, dimensions in inches. Davy Crockett mounted to a recoilless rifle on a tripod The M-388 Davy Crockett was a tactical nuclear recoilless rifle projectile that was deployed by the United States during the Cold War. ...

Another benefit of boosting, in addition to making weapons smaller, lighter, and with less fissile material for a given yield, is that it renders weapons immune to radiation interference (RI). It was discovered in the mid-1950s that plutonium pits would be particularly susceptible to partial pre-detonation if exposed to the intense radiation of a nearby nuclear explosion (electronics might also be damaged, but this was a separate issue). RI was a particular problem before effective early warning radar systems because a first strike attack might make retaliatory weapons useless. Boosting reduces the amount of plutonium needed in a weapon to below the quantity which would be vulnerable to this effect. An early warning radar is any radar system used primarily for the long-range detection of its selected targets. ...

## Two-stage thermonuclear weapons

Main article: Teller-Ulam design

Pure fission or fusion-boosted fission weapons can be made to yield hundreds of kilotons, at great expense in fissile material and tritium, but by far the most efficient way to increase nuclear weapon yield beyond ten or so kilotons is to tack on a second independent stage, called a secondary. The basics of the Tellerâ€“Ulam configuration: a fission bomb uses radiation to compress and heat a separate section of fusion fuel. ...

Ivy Mike, the first two-stage thermonuclear detonation, 10.4 megatons, November 1, 1952.

In the 1940s, bomb designers at Los Alamos thought the secondary would be a canister of deuterium in liquified or hydride form. The fusion reaction would be D-D, harder to achieve than D-T, but more affordable. A fission bomb at one end would shock-compress and heat the near end, and fusion would propagate through the canister to the far end. Mathematical simulations showed it wouldn't work, even with large amounts of prohibitively expensive tritium added in. Image File history File links Download high resolution version (800x637, 55 KB) XX-11 IVY MIKE, was fired on Enewetak by the United States on October 31, 1952. ... Image File history File links Download high resolution version (800x637, 55 KB) XX-11 IVY MIKE, was fired on Enewetak by the United States on October 31, 1952. ... The mushroom cloud from the Mike shot. ...

The entire fusion fuel canister would need to be enveloped by fission energy, to both compress and heat it, as with the booster charge in a boosted primary. The design breakthrough came in January of 1951, when Edward Teller and Stanislav Ulam invented radiation implosion - for nearly three decades known publicly only as the Teller-Ulam H-bomb secret. Edward Teller (original Hungarian name Teller Ede) (January 15, 1908 â€“ September 9, 2003) was a Hungarian-born American theoretical physicist, known colloquially as the father of the hydrogen bomb, even though he did not care for the title. ... StanisÅ‚aw Ulam in the 1950s. ... The basics of the Teller-Ulam configuration: a fission bomb suspended above fusion fuel. ...

The concept of radiation implosion was first tested on May 9, 1951, in the George shot of Operation Greenhouse, Eniwetok, yield 225 kilotons. The first full test was on November 1, 1952, the Mike shot of Operation Ivy, Eniwetok, yield 10.4 megatons. Operation Greenhouse was the fifth American nuclear test series, the second conducted in 1951 and the first to test thermonuclear weapons (hydrogen bombs). ... The mushroom cloud from the Mike shot. ... This article is about the nuclear test. ...

In radiation implosion, the burst of x-ray energy coming from an exploding primary is captured and contained within an opaque-walled radiation channel which surrounds the nuclear energy components of the secondary. For a millionth of a second, most of the energy of several kilotons of TNT is absorbed by a plasma (superheated gas) generated from plastic foam in the radiation channel. With energy going in and not coming out, the plasma rises to solar core temperatures and expands with solar core pressures. Nearby objects which are still cool are crushed by the temperature difference.

The cool nuclear materials surrounded by the radiation channel are imploded much like the pit of the primary, except with vastly more force. This greater pressure enables the secondary to be significantly more powerful than the primary, without being much larger.

A. Warhead before firing; primary (fission bomb) at top, secondary (fusion fuel) at bottom, all suspended in polystyrene foam. B. High-explosive fires in primary, compressing plutonium core into supercriticality and beginning a fission reaction. C. Fission primary emits X-rays which reflects along the inside of the casing, irradiating the polystyrene foam. D. Polystyrene foam becomes plasma, compressing secondary, and fissile uranium (U-235) sparkplug begins to fission. E. Compressed and heated, lithium-6 deuteride fuel begins fusion reaction, neutron flux causes tamper to fission. A fireball is starting to form...

For example, for the Redwing Mohawk test on July 3, 1956, a secondary called the Flute was attached to the Swan primary. The Flute was 15 inches (38 cm) in diameter and 23.4 inches (59 cm) long, about the size of the Swan. But it weighed ten times as much and yielded 24 times as much energy (355 kilotons, vs 15 kilotons).

Equally important, the active ingredients in the Flute probably cost no more than those in the Swan. Most of the fission came from cheap U-238, and the tritium was manufactured in place during the explosion. Only the spark plug at the axis of the secondary needed to be fissile.

A spherical secondary can achieve higher implosion densities than a cylindrical secondary, because spherical implosion pushes in from all directions toward the same spot. However, in warheads yielding more than one megaton, the diameter of a spherical secondary would be too large for most applications. A cylindrical secondary is necessary in such cases. The small, cone-shaped re-entry vehicles in multiple-warhead ballistic missiles after 1970 tended to have warheads with spherical secondaries, and yields of a few hundred kilotons.

As with boosting, the advantages of the two-stage thermonuclear design are so great that there is little incentive not to use it, once a nation has mastered the technology.

In engineering terms, radiation implosion allows for the exploitation of several known features of nuclear bomb materials which heretofore had eluded practical application. For example:

• The best way to store deuterium in a reasonably dense state is to chemically bond it with lithium, as lithium deuteride. But the lithium-6 isotope is also the raw material for tritium production, and an exploding bomb is a nuclear reactor. Radiation implosion will hold everything together long enough to permit the complete conversion of lithium-6 into tritium, while the bomb explodes. So the bonding agent for deuterium permits use of the D-T fusion reaction without any pre-manufactured tritium being stored in the secondary. The tritium production constraint disappears.
The W87 warhead for the Minuteman III missile.
• For the secondary to be imploded by the hot, radiation-induced plasma surrounding it, it must remain cool for the first microsecond, i.e., it must be encased in a massive radiation (heat) shield. The shield's massiveness allows it to double as a tamper, adding momentum and duration to the implosion. No material is better suited for both of these jobs than ordinary, cheap uranium-238, which happens, also, to undergo fission when struck by the neutrons produced by D-T fusion. This casing, called the pusher, thus has three jobs: to keep the secondary cool, to hold it, inertially, in a highly compressed state, and, finally, to serve as the chief energy source for the entire bomb. The consumable pusher makes the bomb more a uranium fission bomb than a hydrogen fusion bomb. It is noteworthy that insiders never used the term hydrogen bomb.
• Finally, the heat for fusion ignition comes not from the primary but from a second fission bomb called the spark plug, imbedded in the heart of the secondary. The implosion of the secondary implodes this spark plug, detonating it and igniting fusion in the material around it, but the spark plug then continues to fission in the neutron-rich environment until it is fully consumed, adding significantly to the yield.[15]

The initial impetus behind the two-stage weapon was President Truman's 1950 promise to build a 10-megaton hydrogen superbomb as America's response to the 1949 test of the first Soviet fission bomb. But the resulting invention turned out to be the cheapest and most compact way to build small nuclear bombs as well as large ones, erasing any meaningful distinction between A-bombs and H-bombs, and between boosters and supers. All the best techniques for fission and fusion explosions are incorporated into one all-encompassing, fully-scalable design principle. Even six-inch diameter nuclear artillery shells can be two-stage thermonuclears. Image File history File linksMetadata Download high resolution version (972x764, 142 KB) // Summary Diagram of the W87, a modern thermonuclear weapon. ... Image File history File linksMetadata Download high resolution version (972x764, 142 KB) // Summary Diagram of the W87, a modern thermonuclear weapon. ... The Mk21 Re-entry Vehicles shown here for the LGM-118A Peacekeeper contain W87 warheads. ... The LGM-30 Minuteman is a United States nuclear missile, a land-based intercontinental ballistic missile (ICBM). ...

In the ensuing fifty years, nobody has come up with a better way to build a nuclear bomb. It is the design of choice for the U.S., Russia, Britain, France, and China, the five thermonuclear powers. The other nuclear-armed nations, Israel, India, Pakistan, and North Korea, probably have single-stage weapons, possibly boosted.

### Interstage

In a two-stage thermonuclear weapon, three types of energy emerge from the primary to impact the secondary: the expanding hot gases from high explosive charges which implode the primary, plus the electromagnetic radiation and the neutrons from the primary's nuclear detonation. An essential energy transfer modulator called the interstage, between the primary and the secondary, protects the secondary from the hot gases and channels the electromagnetic radiation and neutrons toward the right place at the right time.

There is very little information in the open literature about the mechanism of the interstage. Its first mention in a U.S. government document formally released to the public appears to be a caption in a recent graphic promoting the Reliable Replacement Warhead Program. If built, this new design would replace "toxic, brittle material" and "expensive 'special' material" in the interstage.[16] This statement suggests the interstage may contain beryllium to moderate the flux of neutrons from the primary, and perhaps something to absorb and re-radiate the x-rays in a particular manner.[17]

The interstage and the secondary are encased together inside a stainless steel membrane to form the canned subassembly (CSA), an arrangement which has never been depicted in any open-source drawing.[18] The most detailed illustration of an interstage shows a British thermonuclear weapon with a cluster of items between its primary and a cylindrical secondary. They are labeled "end-cap and neutron focus lens," "reflector/neutron gun carriage," and "reflector wrap." The origin of the drawing, posted on the internet by Greenpeace, is uncertain, and there is no accompanying explanation.[19]

## Specific designs

While every nuclear weapon design falls into one of the above categories, specific designs have occasionally become the subject of news accounts and public discussion, often with incorrect descriptions about how they work and what they do. Examples:

### Hydrogen bombs

All modern nuclear weapons make some use of D-T fusion. Even pure fission weapons include neutron generators which are high-voltage vacuum tubes containing trace amounts of tritium and deuterium.

However, in the public perception, hydrogen bombs, or H-bombs, are multi-megaton devices a thousand times more powerful than Hiroshima's Little Boy. Such high-yield bombs are actually two-stage thermonuclears, scaled up to the desired yield, with uranium fission, as usual, providing most of their destructive energy.

The idea of the hydrogen bomb first came to public attention in 1949, when prominent scientists openly recommended against building nuclear bombs more powerful than the standard pure-fission model, on both moral and practical grounds. Their assumption was that critical mass considerations would limit the potential size of fission explosions, but that a fusion explosion could be as large as its supply of fuel, which has no critical mass limit. In 1949, the Russians exploded their first fission bomb, and in 1950 President Truman ended the H-bomb debate by ordering the Los Alamos designers to build one.

In 1952, the 10.4-megaton Ivy Mike explosion was announced as the first hydrogen bomb test, reinforcing the idea that hydrogen bombs are a thousand times more powerful than fission bombs.

In 1954, J. Robert Oppenheimer was labeled a hydrogen bomb opponent. The public did not know there were two kinds of hydrogen bomb (neither of which is accurately described as a hydrogen bomb). On May 23, when his security clearance was revoked, item three of the four public findings against him was "his conduct in the hydrogen bomb program." In 1949, Oppenheimer had supported single-stage fusion-boosted fission bombs, to maximize the explosive power of the arsenal given the trade-off between plutonium and tritium production. He opposed two-stage thermonuclear bombs until 1951, when radiation implosion, which he called "technically sweet," first made them practical. He no longer objected. The complexity of his position was not revealed to the public until 1976, thirteen years after his death.[20] J. Robert Oppenheimer[1] (April 22, 1904 â€“ February 18, 1967) was an American theoretical physicist, best known for his role as the director of the Manhattan Project, the World War II effort to develop the first nuclear weapons, at the secret Los Alamos laboratory in New Mexico. ...

When ballistic missiles replaced bombers in the 1960s, most multi-megaton bombs were replaced by missile warheads (also two-stage thermonuclears) scaled down to one megaton or less.

### Alarm Clock/Sloika

The first effort to exploit the symbiotic relationship between fission and fusion was a 1940s design that mixed fission and fusion fuel in alternating thin layers. As a single-stage device, it would have been a cumbersome application of boosted fission. It first became practical when incorporated into the secondary of a two-stage thermonuclear weapon.[21]

The U.S. name, Alarm Clock, was a nonsense code name, supposedly coined by Oppenheimer to alert the world to the possibility of thermonuclear weapons. The Russian name for the same design was more descriptive: Sloika, a layered pastry cake. A single-stage Russian Sloika was tested on August 12, 1953. No single-stage U.S. version was tested, but the Union shot of Operation Castle, April 26, 1954, was a two-stage thermonuclear code-named Alarm Clock. Its yield, at Bikini, was 6.9 megatons.

Because the Russian Sloika test used dry lithium-6 deuteride eight months before the first U.S. test to use it (Castle Bravo, March 1, 1954), it was sometimes claimed that Russia won the H-bomb race. (The 1952 U.S. Ivy Mike test used cryogenically-cooled liquid deuterium as the fusion fuel in the secondary, and employed the D-D fusion reaction.) However, the first Russian test to use a radiation-imploded secondary, the essential feature of a true H-bomb, was on November 23, 1955, three years after Ivy Mike.

### Clean bombs

Bassoon, the prototype for a 3.5-megaton clean bomb or a 25-megaton dirty bomb. Dirty version shown here, before its 1956 test.

On March 1, 1954, America's largest-ever nuclear test explosion, the 15-megaton Bravo shot of Operation Castle at Bikini, delivered a promptly lethal dose of fission-product fallout to more than 6,000 square miles of Pacific Ocean surface.[22] Radiation injuries to Marshall Islanders and Japanese fishermen made that fact public and revealed the role of fission in hydrogen bombs. A black-and-white photograph of the Castle Bravo mushroom cloud. ...

In response to the public alarm over fallout, an effort was made to design a clean multi-megaton weapon, relying almost entirely on fusion. Since it takes roughly five megatons of fusion to produce the same blast and fire effect as one megaton of fission, the clean bomb needed to be very large. For the first and only time, a third stage, called the tertiary, was added, using the secondary as its primary. The device was called Bassoon. It was tested as the Zuni shot of Operation Redwing, at Bikini on May 28, 1956. With all the uranium in Bassoon replaced with a substitute material such as lead, its yield was 3.5 megatons, 85% fusion and only 15% fission.

On July 19, AEC Chairman Lewis Strauss said the clean bomb test "produced much of importance . . . from a humanitarian aspect." However, two days later the dirty version of Bassoon, with the uranium parts restored, was tested as the Tewa shot of Redwing. Its 5-megaton yield, 87% fission, was deliberately suppressed to keep fallout within a smaller area. This dirty version was later deployed as the three-stage, 25-megaton Mark-41 bomb, which was carried by U.S. Air Force bombers, but never tested at full yield.

As such, high-yield clean bombs were a public relations exercise. The actual deployed weapons were the dirty version, which maximized yield for the same size device.

### Cobalt bombs

Main article: Cobalt bomb

A fictional doomsday bomb, made popular by the 1957 Neville Shute novel, and subsequent 1959 movie, On the Beach, the cobalt bomb was a hydrogen bomb with a jacket of cobalt metal. The neutron-activated cobalt would supposedly have maximized the environmental damage from radioactive fallout. This bomb was popularized as the 'Doomsday Device' in the 1964 film 'Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb' in the film the bomb brings about the end of mankind by covering the planet in a radioactive shroud for 93 years. The element added to the bombs is referred to in the film as 'cobalt-chlorium G' This article or section does not cite its references or sources. ... Nevil Shute (London, January 17, 1899 â€“ Melbourne, January 12, 1960) (full name Nevil Shute Norway) was one of the most popular novelists of the mid-20th century. ... On the Beach is a post-apocalyptic end-of-the-world novel written by British author Nevil Shute after he had emigrated to Australia. ...

Such "salted" weapons were requested by the U.S. Air Force and seriously investigated, possibly built and tested, but not deployed. In the 1964 edition of the DOD/AEC book The Effects of Nuclear Weapons, a new section titled Radiological Warfare clarified the issue.[23] Fission products are as deadly as neutron-activated cobalt. The standard high-fission thermonuclear weapon is automatically a weapon of radiological warfare, as dirty as a cobalt bomb.

Initially, gamma radiation from the fission products from an equivalent size fission-fusion-fission bomb are much more intense than Co-60: 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long lived isotopes produced by fission would overtake the 60Co again after about 75 years. [24]

### Fission-fusion-fission bombs

In 1954, to explain the surprising amount of fission-product fallout produced by hydrogen bombs, Ralph Lapp coined the term fission-fusion-fission to describe a process inside what he called a three-stage thermonuclear weapon. His process explanation was correct, but his choice of terms caused confusion in the open literature. The stages of a nuclear weapon are not fission, fusion, and fission. They are the primary, the secondary, and, in one exceptionally powerful weapon, the tertiary. Each of these stages employs fission, fusion, and fission.

### Neutron bombs

Main article: Neutron bomb

While high-yield clean bombs were never deployed, some low-yield clean bombs were. Officially known as enhanced radiation weapons, ERWs, they are more accurately described as suppressed yield weapons. When the yield of a nuclear weapon is less than one kiloton, its lethal radius from blast, 700 m (2300 ft), is less than that from its neutron radiation. If a one-kiloton ERW is exploded 800 m above ground, buildings at ground zero will survive but people in them will die of radiation illness caused by neutrons and other fireball radiation. A neutron bomb is a type of tactical nuclear weapon developed specifically to release a relatively large portion of its energy as energetic neutron radiation. ...

Although the buildings would survive the blast, neutron activation would make them radioactive. If detonation occurred at a lower altitude, the full force of one kiloton (i.e., four thousand 500 lb bombs) would flatten them.

ERWs were two-stage thermonuclears with all non-essential uranium removed to minimize fission yield. Fusion provided the neutrons. Developed in the 1950s, they were first deployed in the 1970s, by U.S. forces in Europe. The last ones were retired in the 1990s.

Energy distribution of weapon
Standard Enhanced
Blast 50% 40%
Thermal energy 35% 25%

Samuel Cohen in 1958 investigated a low-yield 'clean' nuclear weapon and discovered that the 'clean' bomb case thickness scales as the cube-root of yield. So a larger percentage of neutrons escapes from a small detonation, due to the thinner case required to reflect back X-rays during the secondary stage ignition. For example, a 1-kiloton bomb only needs a case 1/10th the thickness of that required for 1-megaton. Sam Cohen, neutron bomb inventor and author of Shame For the composer, see Samuel Cohen (composer). ...

So although most neutrons are absorbed by the casing in a 1-megaton bomb, in a 1-kiloton bomb they would mostly escape. A neutron bomb is only feasible if the yield is sufficiently high that efficient fusion stage ignition is possible, and if the yield is low enough that the case thickness will not absorb too many neutrons. This means that neutron bombs have a yield range of 1-10 kilotons, with fission proportion varying from 50% at 1-kiloton to 25% at 10-kilotons (all of which comes from the primary stage). The neutron output per kiloton is then 10-15 times greater than for a pure fission implosion weapon or for a strategic warhead like a W87 or W88. [25] The Mk21 Re-entry Vehicles shown here for the LGM-118A Peacekeeper contain W87 warheads. ... In 1999, information came out implying that in some U.S. designs, the primary (top) is prolate, while the secondary (bottom) is spherical. ...

In 1999, nuclear weapon design was in the news again, for the first time in decades. In January, the U.S. House of Representatives released the Cox Report (Christopher Cox R-CA) which alleged that China had somehow acquired classified information about the U.S. W88 warhead. Nine months later, Wen Ho Lee, a Taiwanese immigrant working at Los Alamos, was publicly accused of spying, arrested, and served nine months in pre-trial detention, before the case against him was dismissed. It is not clear that there was, in fact, any espionage. U.S. Representative Chris Cox (Republican-California) chaired the Committee that produced the report. ... Chris Cox For other people named Chris Cox, see Chris Cox (disambiguation). ... In 1999, information came out implying that in some U.S. designs, the primary (top) is prolate, while the secondary (bottom) is spherical. ... Wen Ho Lee (Chinese: æŽæ–‡å’Œ; Pinyin: LÇ WÃ©nhÃ©; born December 21, 1939) is a Taiwanese American scientist who worked for the University of California operated Los Alamos National Laboratory and was accused of stealing secrets about the U.S.s nuclear arsenal for China. ... Los Alamos National Laboratory, aerial view from 1995. ... Spy and Secret agent redirect here. ...

In the course of eighteen months of news coverage, the W88 warhead was described in unusual detail. The New York Times printed a schematic diagram on its front page.[26] The most detailed drawing appeared in A Convenient Spy, the 2001 book on the Wen Ho Lee case by Dan Stober and Ian Hoffman, adapted and shown here with permission. The New York Times is an internationally known daily newspaper published in New York City and distributed in the United States and many other nations worldwide. ...

Designed for use on Trident II (D-5) submarine-launched ballistic missiles, the W88 entered service in 1990 and was the last warhead designed for the U.S. arsenal. It has been described as the most advanced, although open literature accounts do not indicate any major design features that were not available to U.S. designers in 1958. The Trident missile is an intercontinental ballistic missile (ICBM) which is armed with nuclear warheads and is launched from submarines (SSBNs), making it an SLBM. There are 14 active US Ohio class submarines and 4 UK Vanguard class submarines equipped with the two variants of Trident: the initial Trident-I... French M45 SLBM and M51 SLBM Submarine-launched ballistic missiles or SLBMs are ballistic missiles delivering nuclear weapons that are launched from submarines. ...

The above diagram shows all the standard features of ballistic missile warheads since the 1960s, with two exceptions that give it a higher yield for its size.

• The outer layer of the secondary, called the "pusher," which serves three functions: heat shield, tamper, and fission fuel, is made of U-235 instead of U-238, hence the name Oralloy (U-235) Thermonuclear. Being fissile, rather than merely fissionable, allows the pusher to fission faster and more completely, increasing yield. This feature is available only to nations with a great wealth of fissile uranium. The U.S. is estimated to have 500 tons.
• The secondary is located in the wide end of the re-entry cone, where it can be larger, and thus more powerful. The usual arrangement is to put the heavier, denser secondary in the narrow end for greater aerodynamic stability during re-entry from outer space, and to allow more room for a bulky primary in the wider part of the cone. (The W87 warhead drawing in the previous section shows the usual arrangement.) Because of this new geometry, the W88 primary uses compact conventional high explosives (CHE) to save space,[27] rather than the more usual, and bulky but safer, insensitive high explosives (IHE). The re-entry cone probably has ballast in the nose for aerodynamic stability.[28]

Notice that the alternating layers of fission and fusion material in the secondary are an application of the Alarm Clock/Sloika principle. In aeronautics, a heat shield is a protective layer on a spacecraft or ballistic missile that is designed to protect it from high temperatures, usually those that result from aerobraking during entry into a planets atmosphere. ... Nuclear Fuel Process A graph comparing nucleon number against binding energy Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. ... Enriched uranium is uranium whose uranium-235 content has been increased through the process of isotope separation. ...

The United States has not produced any nuclear warheads since 1989, when the Rocky Flats pit production plant, near Boulder, Colorado, was shut down for environmental reasons. With the end of the Cold War coming two years later, the production line has remained idle except for inspection and maintenance functions. The Reliable Replacement Warhead also known as RRW is a controversial new design American nuclear warhead and bomb family that its supporters claim will be simple and reliable and provide a long lasting, low maintenance future nuclear force for the United States. ... The Rocky Flats Plant was a weapons production facility of the Atomic Energy Commission (AEC) that operated from 1952 to 1988. ... Boulder is a Home Rule Municipality that is the county seat and most populous city of Boulder County, Colorado, in the United States. ...

The National Nuclear Security Administration, the latest successor for nuclear weapons to the Atomic Energy Commission and the Department of Energy, has proposed building a new pit facility and starting the production line for a new warhead called the Reliable Replacement Warhead (RRW).[29] Two advertised safety improvements of the RRW would be a return to the use of "insensitive high explosives which are far less susceptible to accidental detonation," and the elimination of "certain hazardous materials, such as beryllium, that are harmful to people and the environment."[30] Since the new warhead would not require any nuclear testing, it could not use a new design with untested concepts. The United States National Nuclear Security Administration (NNSA) is part of the United States Department of Energy. ... Shield of the U.S. Atomic Energy Commission. ... The United States Department of Energy (DOE) is a Cabinet-level department of the United States government responsible for energy policy and nuclear safety. ... The Reliable Replacement Warhead also known as RRW is a controversial new design American nuclear warhead and bomb family that its supporters claim will be simple and reliable and provide a long lasting, low maintenance future nuclear force for the United States. ...

## The Weapon Design Laboratories

### Berkeley

The first systematic exploration of nuclear weapon design concepts took place in the summer of 1942 at the University of California, Berkeley. Important early discoveries had been made at the adjacent Lawrence Berkeley Laboratory, such as the 1940 production and isolation of plutonium. A Berkeley professor, J. Robert Oppenheimer, had just been hired to run the nation's secret bomb design effort. His first act was to convene the 1942 summer conference. The Ernest Orlando Lawrence Berkeley National Laboratory (LBNL), formerly the Berkeley Radiation Laboratory and usually shortened to Berkeley Lab or LBL, is a U.S. Department of Energy (DOE) national laboratory conducting unclassified scientific research. ... Sather tower (the Campanile) looking out over the San Francisco Bay and Mount Tamalpais. ... The Berkeley Lab is perched on a hill overlooking the Berkeley central campus and San Francisco Bay. ... J. Robert Oppenheimer, father of the atomic bomb, served as the first director of Los Alamos National Laboratory, beginning in 1943. ...

By the time he moved his operation to the new secret town of Los Alamos, New Mexico, in the spring of 1943, the accumulated wisdom on nuclear weapon design consisted of five lectures by Berkeley professor Robert Serber, transcribed and distributed as the Los Alamos Primer. The Primer addressed fission energy, neutron production and capture, nuclear chain reactions, critical mass, tampers, predetonation, and three methods of assembling a bomb: gun assembly, implosion, and "autocatalytic methods," the one approach that turned out to be a dead end. Robert Serber (1909 - June 1, 1997) was a physicist who participated in the Manhattan Project. ... This article or section does not adequately cite its references or sources. ... The process of neutron capture can proceed in two ways - as a rapid process (an r-process) or a slow process (an s-process). ... A schematic nuclear fission chain reaction. ... For other uses of critical mass, see critical mass (disambiguation). ...

### Los Alamos

At Los Alamos, it was found in April 1944 by Emilio G. Segrè that the proposed Thin Man Gun assembly type bomb would not work for plutonium because of predetonation problems caused by Pu-240 impurities. So Fat Man the Implosion type bomb was given high priority as the only option for plutonium. The Berkeley discussions had generated theoretical estimates of critical mass, but nothing precise. The main wartime job at Los Alamos was the experimental determination of critical mass, which had to wait until sufficient amounts of fissile material arrived from the production plants: uranium from Oak Ridge, Tennessee, and plutonium from the Hanford site in Washington. Los Alamos National Laboratory, aerial view from 1995. ... Los Alamos usually refers to the United States national laboratory in Los Alamos, New Mexico which was founded during the World War II effort to develop the atomic bomb (the Manhattan Project), was one of the two laboratories developing the USAs nuclear weapons during the Cold War, and is... Portrait of Emilio SegrÃ¨. Emilio Gino SegrÃ¨ (February 1, 1905 â€“ April 22, 1989) was an Italian American physicist who, with Owen Chamberlain, won the 1959 Nobel Prize in Physics for their discovery of the antiproton. ... DVD cover The Thin Man is the title of the first of six comic detective films starring William Powell and Myrna Loy as Nick and Nora Charles, a hard-drinking and flirtatious married couple who banter wittily as they easily solve crimes. ... This article is about the nuclear weapon used in World War II. For other uses, see Fat Man (disambiguation). ... Oak Ridge is an incorporated city in Anderson and Roane Counties in East Tennessee, about 25 miles northwest of Knoxville. ... Hanford Site plutonium production reactors along the Columbia River during the Manhattan Project. ...

All of the essential ideas for incorporating fusion into nuclear weapons originated at Los Alamos between 1946 and 1952. After the Teller-Ulam radiation implosion breakthrough of 1951, the technical implications and possibilities were fully explored, but ideas not directly relevant to making the largest possible bombs for long-range Air Force bombers were shelved. The basics of the Tellerâ€“Ulam configuration: a fission bomb uses radiation to compress and heat a separate section of fusion fuel. ...

Because of Oppenheimer's initial position in the H-bomb debate, in opposition to large thermonuclear weapons, and the assumption that he still had influence over Los Alamos despite his departure, political allies of Edward Teller decided he needed his own laboratory in order to pursue H-bombs. By the time it was opened in 1952, in Livermore, California, Los Alamos had finished the job Livermore was designed to do. Edward Teller (original Hungarian name Teller Ede) (January 15, 1908 â€“ September 9, 2003) was a Hungarian-born American theoretical physicist, known colloquially as the father of the hydrogen bomb, even though he did not care for the title. ... Aerial view of the lab and surrounding area, facing NW. The Lawrence Livermore National Laboratory (LLNL) in Livermore, California is a United States Department of Energy (DOE) national laboratory, managed and operated by Lawrence Livermore National Security, LLC (LLNS), a limited liability consortium comprised of Bechtel National, the University of...

### Livermore

With its original mission no longer available, the Livermore lab tried radical new designs, that failed. Its first three nuclear tests were fizzles: in 1953, two single-stage fission devices with uranium hydride pits, and in 1954, a two-stage thermonuclear device in which the secondary heated up prematurely, too fast for radiation implosion to work properly. Aerial view of the lab and surrounding area, facing NW. The Lawrence Livermore National Laboratory (LLNL) in Livermore, California is a United States Department of Energy (DOE) national laboratory, managed and operated by Lawrence Livermore National Security, LLC (LLNS), a limited liability consortium comprised of Bechtel National, the University of...

Shifting gears, Livermore settled for taking ideas Los Alamos had shelved and developing them for the Army and Navy. This led Livermore to specialize in small-diameter tactical weapons, particularly ones using two-point implosion systems, such as the Swan. Small-diameter tactical weapons became primaries for small-diameter secondaries. Around 1960, when the superpower arms race became a ballistic missile race, Livermore warheads were more useful than the large, heavy Los Alamos warheads. Los Alamos warheads were used on the first intermediate-range ballistic missiles, IRBMs, but smaller Livermore warheads were used on the first intercontinental ballistic missiles, ICBMs, and submarine-launched ballistic missiles, SLBMs, as well as on the first multiple warhead systems on such missiles.[31] An intermediate-range ballistic missile (IRBM) is a ballistic missile with a range of 3,000-5,500 km. ... ICBM redirects here. ... French M45 SLBM and M51 SLBM Submarine-launched ballistic missiles or SLBMs are ballistic missiles delivering nuclear weapons that are launched from submarines. ... For the article about the band, see M.I.R.V. The MIRVed U.S. Peacekeeper missile, with the re-entry vehicles highlighted in red. ...

The period of real innovation was ending by then, anyway. Warhead designs after 1960 took on the character of model changes, with every new missile getting a new warhead for marketing reasons. The chief substantive change involved packing more fissile uranium into the secondary, as it became available with continued uranium enrichment and the dismantlement of the large high-yield bombs. Enriched uranium is uranium whose uranium-235 content has been increased through the process of isotope separation. ...

## Explosive testing

Nuclear weapons are designed by trial and error. The trial often involves exploding a prototype.

In a nuclear explosion, a large number of discrete events, with various probabilities, aggregate into short-lived, chaotic energy flows inside the device casing. Complex mathematical models are required to approximate the processes, and in the 1950s there were no computers powerful enough to run them properly. Even today's computers and their codes are not fully adequate.[32]

It was easy enough to design reliable weapons for the stockpile. If the prototype worked, it could be weaponized and mass produced.

It was much more difficult to understand how it worked or why it failed. Designers gathered as much data as possible during the explosion, before the device destroyed itself, and used the data to calibrate their models, often by inserting fudge factors into equations to make the simulations match experimental results. They also analyzed the weapon debris in fallout to see how much of a potential nuclear reaction had taken place.

### Light pipes

An important tool for test analysis was the diagnostic light pipe. A probe inside a test device could transmit information by heating a plate of metal to incandescence, an event that could be recorded at the far end of a long, very straight pipe.

The picture below shows the Shrimp device, detonated on March 1, 1954 at Bikini, as the Castle Bravo test. Its 15-megaton explosion was the largest ever by the United States. The silhouette of a man is shown for scale. The device is supported from below, at the ends. The pipes going into the shot cab ceiling, which appear to be supports, are diagnostic light pipes. The eight pipes at the right end (1) sent information about the detonation of the primary. Two in the middle (2) marked the time when x-radiation from the primary reached the radiation channel around the secondary. The last two pipes (3) noted the time radiation reached the far end of the radiation channel, the difference between (2) and (3) being the radiation transit time for the channel.[33] A black-and-white photograph of the Castle Bravo mushroom cloud. ...

From the shot cab, the pipes turned horizontal and traveled 7500 ft (2.3 km), along a causeway built on the Bikini reef, to a remote-controlled data collection bunker on Namu Island.

While x-rays would normally travel at the speed of light through a low density material like the plastic foam channel filler between (2) and (3), the intensity of radiation from the exploding primary created a relatively opaque radiation front in the channel filler which acted like a slow-moving logjam to retard the passage of radiant energy. Behind this moving front was a fully-ionized, low-z (low atomic number) plasma heated to 20,000 degrees Celsius, soaking up energy like a black box, and eventually driving the implosion of the secondary.[34]

The radiation transit time, on the order of half a microsecond, is the time it takes the entire radiation channel to reach thermal equilibrium as the radiation front moves down its length. The implosion of the secondary is based on the temperature difference between the hot channel and the cool interior of the secondary. Its timing is important because the interior of the secondary is subject to neutron preheat.

While the radiation channel is heating and starting the implosion, neutrons from the primary catch up with the x-rays, penetrate into the secondary and start breeding tritium with the third reaction noted in the first section above. This Li-6 + n reaction is exothermic, producing 5 Mev per event. The spark plug is not yet compressed and thus is not critical, so there won't be significant fission or fusion. But if enough neutrons arrive before implosion of the secondary is complete, the crucial temperature difference will be degraded. This is the reported cause of failure for Livermore's first thermonuclear design, the Morgenstern device, tested as Castle Koon, April 7, 1954. The Koon shot of Operation Castle was a test of a University of California Radiation Laboratory designed thermonuclear device. ...

These timing issues are measured by light-pipe data. The mathematical simulations which they calibrate are called radiation flow hydrodynamics codes, or channel codes. They are used to predict the effect of future design modifications.

It is not clear from the public record how successful the Shrimp light pipes were. The data bunker was far enough back to remain outside the mile-wide crater, but the 15-megaton blast, two and a half times greater than expected, breached the bunker by blowing its 20-ton door off the hinges and across the inside of the bunker. (The nearest people were twenty miles farther away, in a bunker that survived intact.)[35]

### Fallout analysis

The most interesting data from Castle Bravo came from radio-chemical analysis of weapon debris in fallout. Because of a shortage of enriched lithium-6, 60% of the lithium in the Shrimp secondary was ordinary lithium-7, which doesn't breed tritium as easily as lithium-6 does. But it does breed lithium-6 as the product of an "n, 2n" reaction (one neutron in, two neutrons out), a known fact, but with unknown probability. The probability turned out to be high.

Fallout analysis revealed to designers that, with the n, 2n reaction, the Shrimp secondary effectively had two and half times as much lithium-6 as expected. The tritium, the fusion yield, the neutrons, and the fission yield were all increased accordingly.[36]

As noted above, Bravo's fallout analysis also told the outside world, for the first time, that thermonuclear bombs are more fission devices than fusion devices. A Japanese fishing boat named the Lucky Dragon sailed home with enough fallout on its decks to allow scientists in Japan and elsewhere to determine, and announce, that most of the fallout had come from the fission of U-238 by fusion-produced 14 MeV neutrons. Daigo Fukuryu Maru (第五福龍丸, Daigo Fukuryū Maru) was a Japanese tuna fishing boat, which was exposed to and contaminated by radiation caused by the United States hydrogen bomb experiment in Bikini Atoll on March 1, 1954. ...

### Underground testing

Subsidence Craters at Yucca Flat, Nevada Test Site.

The global alarm over radioactive fallout, which began with the Castle Bravo event, eventually drove nuclear testing underground. The last U.S. above-ground test took place at Johnston Island on November 4, 1962. During the next three decades, until September 23, 1992, the U.S. conducted an average of 2.4 underground nuclear explosions per month, all but a few at the Nevada Test Site (NTS) northwest of Las Vegas. The crater-scarred landscape of the Nevada Test Site. ... The crater-scarred landscape of the Nevada Test Site. ... The Nevada Test Site is a United States Department of Energy reservation located in Nye County, Nevada, about 65 miles (105 km) northwest of the City of Las Vegas, near . ...

The Yucca Flat section of the NTS is covered with subsidence craters resulting from the collapse of terrain over intensely radioactive underground caverns created by nuclear explosions (see photo).

After the 1974 Threshold Test Ban Treaty (TTBT), which limited underground explosions to 150 kilotons or less, warheads like the half-megaton W88 had to be tested at less than full yield. Since the primary must be detonated at full yield in order to generate data about the implosion of the secondary, the reduction in yield had to come from the secondary. Replacing much of the lithium-6 deuteride fusion fuel with lithium-7 hydride limited the deuterium available for fusion, and thus the overall yield, without changing the dynamics of the implosion. The functioning of the device could be evaluated using light pipes, other sensing devices, and analysis of trapped weapon debris. The full yield of the stockpiled weapon could be calculated by extrapolation. The Treaty on the Limitation of Underground Nuclear Weapon Tests, also known as the Threshold Test Ban Treaty (TTBT), was signed in July 1974. ...

## Production facilities

When two-stage weapons became standard in the early 1950s, weapon design determined the layout of America's new, widely dispersed production facilities, and vice versa.

Because primaries tend to be bulky, especially in diameter, plutonium is the fissile material of choice for pits, with beryllium reflectors. It has a smaller critical mass than uranium. The Rocky Flats plant in Boulder, Colorado, was built in 1952 for pit production and consequently became the plutonium and beryllium fabrication facility. The Rocky Flats Plant was a weapons production facility of the Atomic Energy Commission (AEC) that operated from 1952 to 1988. ...

Y-12 made lithium-6 deuteride fusion fuel and U-238 parts, the other two ingredients of secondaries.

The Savannah River plant in Aiken, South Carolina, also built in 1952, operated nuclear reactors which converted U-238 into Pu-239 for pits, and lithium-6 (produced at Y-12) into tritium for booster gas. Since its reactors were moderated with heavy water, deuterium oxide, it also made deuterium for booster gas and for Y-12 to use in making lithium-6 deuteride. For the Department of Energy facility, see Savannah River Site The Savannah River is a major river in the southeastern United States, forming most of the border between the states of South Carolina and Georgia. ... Aiken can refer to: Aiken County, South Carolina Aiken, South Carolina, Aiken Countys county seat The University of South Carolina Aiken See also Aiken (surname) This is a disambiguation page: a list of articles associated with the same title. ... Official language(s) English Capital Columbia Largest city Columbia Largest metro area Columbia Area  Ranked 40th  - Total 34,726 sq mi (82,965 kmÂ²)  - Width 200 miles (320 km)  - Length 260 miles (420 km)  - % water 6  - Latitude 32Â° 2â€² N to 35Â° 13â€² N  - Longitude 78Â° 32â€² W to 83... Nuclear power station at Leibstadt, Switzerland. ...

• Gun-type weapons

It is inherently dangerous to have a weapon containing a quantity and shape of fissile material which can form a critical mass through a relatively simple accident. Because of this danger, the high explosives in Little Boy (four bags of Cordite powder) were inserted into the bomb in flight, shortly after takeoff on August 6, 1945. It was the first time a gun-type nuclear weapon had ever been fully assembled.

Also, if the weapon falls into water, the moderating effect of the water can also cause a criticality accident, even without the weapon being physically damaged. This does not cite any references or sources. ... A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. ... A criticality accident (also sometimes referred to as an excursion or power excursion) occurs when a nuclear chain reaction is accidentally allowed to occur in fissile material, such as enriched uranium or plutonium. ...

Gun-type weapons have always been inherently unsafe.

• In-flight pit insertion

Neither of these effects is likely with implosion weapons since there is normally insufficient fissile material to form a critical mass without the correct detonation of the lenses. However, the earliest implosion weapons had pits so close to criticality that accidental detonation with some nuclear yield was a concern.

On August 9, 1945, Fat Man was loaded onto its airplane fully assembled, but later, when levitated pits made a space between the pit and the tamper, it was feasible to utilize in-flight pit insertion. The bomber would take off with no fissile material in the bomb. Some older implosion-type weapons, such as the US Mark 4 and Mark 5, used this system. The Mark 4 nuclear bomb was an American nuclear bomb design produced starting in 1949 and in use until 1953. ... The Mark 5 nuclear bomb (open doors at front are for insertion of nuclear core) The Mark 5 nuclear bomb and W5 nuclear warhead were a common core nuclear weapon design, designed in the early 1950s and which saw service from 1952 to 1963. ...

In-flight pit insertion will not work with a hollow pit in contact with its tamper.

• Steel ball safety method
A diagram of the Green Grass warhead's steel ball-bearing safety device, shown left, filled (safe) and right, empty (live). The steel balls were emptied into a hopper underneath the aircraft before flight, the steel balls could be re-inserted using a funnel by rotating the bomb on its trolley and raising the hopper.

As shown in the diagram, one method used to decrease the likelihood of accidental detonation used metal balls. The balls were emptied into the pit; this would prevent detonation by increasing density of the hollowed pit. This design was used in the Green Grass weapon, also known as the Interim Megaton Weapon and was also used in Violet Club and the Yellow Sun Mk.1 bombs. Image File history File links Steel_balls_png. ... Image File history File links Steel_balls_png. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequenly refined in terms of other characteristics. ... Violet Club was a nuclear weapon deployed by the United Kingdom during the cold war. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequenly refined in terms of other characteristics. ...

• Chain safety method

Alternatively, the pit can be "safed" by having its normally-hollow core filled with an inert material such as a fine metal chain, possibly made of cadmium to absorb neutrons. While the chain is in the center of the pit, the pit can't be compressed into an appropriate shape to fission; when the weapon is to be armed, the chain is removed. Similarly, although a serious fire could detonate the explosives, destroying the pit and spreading plutonium to contaminate the surroundings as has happened in several weapons accidents, it could not however, cause a nuclear explosion. Pathways from airborne radioactive contamination to man This article covers notable accidents involving nuclear material. ...

• Wire safety method

The US W47 warhead used in Polaris A1 and Polaris A2 had a safety device consisting of a boron-coated-wire inserted into the hollow pit at manufacture. The warhead was armed by withdrawing the wire onto a spool driven by an electric motor. However, once withdrawn the wire could not be re-inserted.[37] The W47 was an American thermonuclear warhead used on thePolaris A-1 sub-launched ballistic missile system. ... The Polaris Missile was a submarine-launched ballistic missile (SLBM) carrying a nuclear warhead developed during the Cold War for the United States Navy. ... The Polaris Missile was a submarine-launched ballistic missile (SLBM) carrying a nuclear warhead developed during the Cold War for the United States Navy. ...

• One-point safety

While the firing of one detonator out of many will not cause a hollow pit to go critical, especially a low-mass hollow pit that requires boosting, the introduction of two-point implosion systems made that possibility a real concern.

In a two-point system, if one detonator fires, one entire hemisphere of the pit will implode as designed. The high-explosive charge surrounding the other hemisphere will explode progressively, from the equator toward the opposite pole. Ideally, this will pinch the equator and squeeze the second hemisphere away from the first, like toothpaste in a tube. By the time the explosion envelops it, its implosion will be separated both in time and space from the implosion of the first hemisphere. The resulting dumbbell shape, with each end reaching maximum density at a different time, may not become critical.

Unfortunately, it is not possible to tell on the drawing board how this will play out. Nor is it possible using a dummy pit of U-238 and high-speed x-ray cameras, although such tests are helpful. For final determination, a test needs to be made with real fissile material. Consequently, starting in 1957, a year after Swan, both labs began one-point safety tests.

Out of 25 one-point safety tests conducted in 1957 and 1958, seven had zero or slight nuclear yield (success), three had high yields of 300 kt to 500 kt (severe failure), and the rest had unacceptable yields between those extremes.

Of particular concern was Livermore's W47 warhead for the Polaris submarine missile. The last test before the 1958 moratorium was a one-point test of the W47 primary, which had an unacceptably high nuclear yield of 400 lb of TNT equivalent (Hardtack II Titania). With the test moratorium in force, there was no way to refine the design and make it inherently one-point safe. Los Alamos had a suitable primary that was one-point safe, but rather than share with Los Alamos the credit for designing the first SLBM warhead, Livermore chose to use mechanical safing on its own inherently unsafe primary. The wire safety scheme described above was the result.[38]

It turns out that the W47 may have been safer than anticipated. The wire-safety system may have rendered most of the warheads "duds," unable to fire when detonated. The W47 was an American thermonuclear warhead used on thePolaris A-1 sub-launched ballistic missile system. ...

When testing resumed in 1961, and continued for three decades, there was sufficient time to make all warhead designs inherently one-point safe, without need for mechanical safing.

In addition to the above steps to reduce the probability of a nuclear detonation arrising from a single fault, locking mechanisms referred to by NATO states as Permissive Action Links are sometimes attached to the control mechanisms for nuclear warheads. Permissive Action Links act solely to prevent an unauthorised use of a nuclear weapon. A Permissive Action Link is a security device for nuclear weapons. ...

## References

### Specific

1. ^ The physics package is the nuclear explosive module inside the bomb casing, missile warhead, or artillery shell, etc., which delivers the weapon to its target. While photographs of weapon casings are common, photographs of the physics package are quite rare, even for the oldest and crudest nuclear weapons. For a photograph of a modern physics package see W80.
2. ^ Carson Mark, Theodore Taylor, Eugene Eyster, William Maraman, and Jacob Wechsler, "Can Terrorists Build Nuclear Weapons?" Nuclear Control Institute, undated (the first author died in 1997).
3. ^ The United States and the Soviet Union were the only nations to build large nuclear arsenals with every possible type of nuclear weapon. The U.S. had a four-year head start and was the first to produce fissile material and fission weapons, all in 1945. The only Soviet claim for a design first was the Joe 4 detonation on August 12, 1953, said to be the first deliverable hydrogen bomb. However, as Herbert York first revealed in The Advisors: Oppenheimer, Teller and the Superbomb (W.H. Freeman, 1976), it was not a true hydrogen bomb (it was a boosted fission weapon of the Sloika/Alarm Clock type, not a two-stage thermonuclear). Soviet dates for the essential elements of warhead miniaturization – boosted, hollow-pit, two-point, air lens primaries – are not available in the open literature, but the larger size of Soviet ballistic missiles is often explained as evidence of an initial Soviet difficulty in miniaturizing warheads.
4. ^ The main source for this section is Samuel Glasstone and Philip Dolan, The Effects of Nuclear Weapons, Third Edition, 1977, U.S. Dept of Defense and U.S. Dept of Energy (see links in General References, below), with the same information in more detail in Samuel Glasstone, Sourcebook on Atomic Energy, Third Edition, 1979, U.S. Atomic Energy Commission, Krieger Publishing.
5. ^ Glasstone and Dolan, Effects, p. 12.
6. ^ Glasstone, Sourcebook, p. 503.
7. ^ a b Glasstone and Dolan, Effects, p. 21.
8. ^ "Restricted Data Declassification Decisions from 1946 until Present" - "Fact that plutonium and uranium may be bonded to each other in unspecified pits or weapons."
9. ^ "Restricted Data Declassification Decisions from 1946 until Present"
10. ^ Fissionable Materials section of the Nuclear Weapons FAQ, Carey Sublette, accessed Sept 23, 2006
11. ^ All information on nuclear weapon tests comes from Chuck Hansen, The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945, October 1995, Chucklea Productions, Volume VIII, p. 154, Table A-1, "U.S. Nuclear Detonations and Tests, 1945-1962."
12. ^ Nuclear Weapons FAQ: 4.1.6.3 Hybrid Assembly Techniques, accessed December 1, 2007. Drawing adapted from the same source.
13. ^ Nuclear Weapons FAQ: 4.1.6.2.2.4 Cylindrical and Planar Shock Techniques, accessed December 1, 2007.
14. ^ "Restricted Data Declassification Decisions from 1946 until Present", Section V.B.2.k "The fact of use in high explosive assembled (HEA) weapons of spherical shells of fissile materials, sealed pits; air and ring HE lenses," declassified November 1972.
15. ^ Howard Morland, "Born Secret," Cardozo Law Review, March 2005, pp. 1401-1408.
16. ^ "Improved Security, Safety & Manufacturability of the Reliable Replacement Warhead," NNSA March 2007.
17. ^ A 1976 drawing which depicts an interstage that absorbs and re-radiates x-rays. From Howard Morland, "The Article," Cardozo Law Review, March 2005, p 1374.
18. ^ "SAND8.8 - 1151 Nuclear Weapon Data -- Sigma I," Sandia Laboratories, September 1988.
19. ^ The Greenpeace drawing. From Morland, Cardozo Law Review, March 2005, p 1378.
20. ^ Herbert York, The Advisors: Oppenheimer, Teller and the Superbomb (1976).
21. ^ "The ‘Alarm Clock' . . . became practical only by the inclusion of Li6 (in 1950) and its combination with the radiation implosion." Hans A. Bethe, Memorandum on the History of Thermonuclear Program, May 28, 1952.
22. ^ See map.
23. ^ Samuel Glasstone, The Effects of Nuclear Weapons, 1962, Revised 1964, U.S. Dept of Defense and U.S. Dept of Energy, pp.464-5. This section was removed from later editions, but, according to Glasstone in 1978, not because it was inaccurate or because the weapons had changed.
24. ^ Nuclear Weapons FAQ: 1.6.
25. ^ Neutron bomb: Why 'clean' is deadly.
26. ^ Broad, William J. (7 September 1999), "Spies versus sweat, the debate over China's nuclear advance," New York Times, p 1. The front page drawing was similar to one that appeared four months earlier in the the San Jose Mercury News.
27. ^ Jonathan Medalia, "The Reliable Replacement Warhead Program: Background and Current Developments," CRS Report RL32929, Dec 18, 2007, p CRS-11.
28. ^ Richard Garwin, "Why China Won't Build U.S. Warheads", Arms Control Today, April-May 1999.
29. ^ Home - NNSA
30. ^ DoE Fact Sheet: Reliable Replacement Warhead Program
31. ^ Sybil Francis, Warhead Politics: Livermore and the Competitive System of Nuclear Warhead Design, UCRL-LR-124754, June 1995, Ph.D. Dissertation, Massachusetts Institute of Technology, available from National Technical Information Service. This 233-page thesis was written by a weapons-lab outsider for public distribution. The author had access to all the classified information at Livermore that was relevant to her research on warhead design; consequently, she was required to use non-descriptive code words for certain innovations.
32. ^ Walter Goad, Declaration for the Wen Ho Lee case, May 17, 2000. Goad began thermonuclear weapon design work at Los Alamos in 1950. In his Declaration, he mentions "basic scientific problems of computability which cannot be solved by more computing power alone. These are typified by the problem of long range predictions of weather and climate, and extend to predictions of nuclear weapons behavior. This accounts for the fact that, after the enormous investment of effort over many years, weapons codes can still not be relied on for significantly new designs."
33. ^ Chuck Hansen, The Swords of Armageddon, Volume IV, pp. 211-212, 284.
34. ^ The public literature mentions three different force mechanism for this implosion: radiation pressure, plasma pressure, and explosive ablation of the outer surface of the secondary pusher. All three forces are present; and the relative contribution of each is one of the things the computer simulations try to explain. See Teller-Ulam design.
35. ^ Dr. John C. Clark, as told to Robert Cahn, "We Were Trapped by Radioactive Fallout," The Saturday Evening Post, July 20, 1957, pp. 17-19, 69-71.[1]
36. ^ Richard Rhodes, Dark Sun; the Making of the Hydrogen Bomb, Simon and Schuster, 1995, p. 541.
37. ^ Chuck Hansen, The Swords of Armageddon, Volume VII, pp. 396-397.
38. ^ Sybil Francis, Warhead Politics, pp. 141, 160.

A W80 nuclear warhead. ... The first (not true) Soviet Hydrogen (Super) Test, dubbed Joe 4 Joe 4 was an American nickname for the first Soviet test of a hydrogen bomb and was on August 12, 1953. ... The basics of the Tellerâ€“Ulam configuration: a fission bomb uses radiation to compress and heat a separate section of fusion fuel. ...

### General

Wikimedia Commons has media related to:
• Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons (third edition) (hosted at the Trinity Atomic Web Site), U.S. Government Printing Office, 1977. PDF Version
• Cohen, Sam, The Truth About the Neutron Bomb: The Inventor of the Bomb Speaks Out, William Morrow & Co., 1983
• Grace, S. Charles, Nuclear Weapons: Principles, Effects and Survivability (Land Warfare: Brassey's New Battlefield Weapons Systems and Technology, vol 10)
• Hansen, Chuck, The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945, October 1995, Chucklea Productions, eight volumes (CD-ROM), two thousand pages.
• Smyth, Henry DeWolf, Atomic Energy for Military Purposes, Princeton University Press, 1945. (see: Smyth Report)
• The Effects of Nuclear War, Office of Technology Assessment (May 1979).
• Rhodes, Richard. Dark Sun: The Making of the Hydrogen Bomb. Simon and Schuster, New York, (1995 ISBN 0-684-82414-0)
• Rhodes, Richard. The Making of the Atomic Bomb. Simon and Schuster, New York, (1986 ISBN 0-684-81378-5)

Image File history File links Commons-logo. ... Samuel T. Cohen is a physicist who is known for inventing the neutron bomb. ... Henry DeWolf Smyth (May 1, 1898 – September 11, 1986) was an American physicist, diplomat, and a bureaucrat who played a number of key roles in the early development of nuclear energy. ... The Smyth Report was the common name given to an administrative history written by physicist Henry DeWolf Smyth about the Allied World War II effort to develop the atomic bomb, the Manhattan Project. ... Richard Rhodes (born July 4, 1937) is an American author of fiction and verity, including the Pulitzer Prize-winning The Making of the Atomic Bomb in 1986, and most recently, John James Audubon: the Making of an American in 2004. ...

Results from FactBites:

 Nuclear Weapon Design (3646 words) However, after the introduction of the principal families of weapons in the modern stockpile (approximately the mid 1970s), the rate of design innovations and truly new concepts slowed as nuclear weapon technology became a mature science. The design of the nuclear device for a specific nuclear weapon is constrained by several factors. Nuclear weapons are particularly destructive, with immediate effects including blast and thermal radiation and delayed effects produced by ionizing radiation, neutrons, and radioactive fallout.
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