Gun-type fission weapons are
fission-based
nuclear weapons
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions (thermonuclear bomb), producing a nuclear explosion. Both bomb ...
whose
design
A design is a plan or specification for the construction of an object or system or for the implementation of an activity or process or the result of that plan or specification in the form of a prototype, product, or process. The verb ''to design'' ...
assembles their
fissile material
In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be ty ...
into a
supercritical mass
In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fissio ...
by the use of the "gun" method: shooting one piece of sub-critical material into another. Although this is sometimes pictured as two sub-critical hemispheres driven together to make a supercritical sphere, typically a hollow projectile is shot onto a spike which fills the hole in its center. Its name is a reference to the fact that it is shooting the material through an
artillery
Artillery is a class of heavy military ranged weapons that launch munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during siege ...
barrel as if it were a projectile.
Since it is a relatively slow method of assembly,
plutonium
Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
cannot be used unless it is purely the 239 isotope. Production of impurity-free plutonium is very difficult and is impractical. The required amount of
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
is relatively large, and thus the overall
efficiency
Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
is relatively low. The main reason for this is the uranium metal does not undergo compression (and resulting density increase) as does the implosion design. Instead, gun type bombs assemble the supercritical mass by amassing such a large quantity of uranium that the overall distance through which daughter neutrons must travel has so many
mean free path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
s it becomes very probable most neutrons will find uranium nuclei to collide with, before escaping the supercritical mass.
The method was applied in four known programs. First, the "
Little Boy
"Little Boy" was the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II, making it the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress ''Enola Gay'' p ...
" weapon which was detonated over
Hiroshima
is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui h ...
and several additional units of the same design prepared after World War II, in 40
Mark 8
Mark 8 is the eighth chapter of the Gospel of Mark in the New Testament of the Christian Bible. It contains two miracles of Jesus, Peter's confession that he believes Jesus is the Messiah, and Jesus' first prediction of his own death and resurr ...
bombs, and their replacement, 40
Mark 11
Mark 11 is the eleventh chapter of the Gospel of Mark in the New Testament of the Christian Bible, beginning Jesus' final week before His death as He arrives in Jerusalem for the coming Passover. It contains the stories of Jesus' entry into Je ...
bombs. Both the Mark 8 and Mark 11 designs were intended for use as earth-penetrating bombs (see
nuclear bunker buster
A nuclear bunker buster, also known as an earth-penetrating weapon (EPW), is the nuclear equivalent of the conventional bunker buster. The non-nuclear component of the weapon is designed to penetrate soil, rock, or concrete to deliver a nuclear ...
), for which the gun-type method was preferred for a time by designers who were less than certain that early
implosion-type weapons would successfully detonate following an impact. The second program was a family of 11-inch (280 mm)
nuclear artillery
Nuclear artillery is a subset of limited- yield tactical nuclear weapons, in particular those weapons that are launched from the ground at battlefield targets. Nuclear artillery is commonly associated with shells delivered by a cannon, but in ...
shells, the
W9 and its derivative
W19, plus a repackaged W19 in a 16-inch (406 mm) shell for US Navy battleships, the
W23. The third family was an 8-inch (203 mm) artillery shell, the
W33. Later, South Africa also developed six nuclear bombs based on the gun-type principle, and was working on missile warheads using the same basic design – See
South Africa and weapons of mass destruction
From the 1960s to the 1990s, South Africa pursued research into weapons of mass destruction, including nuclear, biological, and chemical weapons under the apartheid government. Six nuclear weapons were assembled. South African strategy was, if ...
.
There are currently no known gun-type weapons in service: advanced nuclear weapon states tended to abandon the design in favor of the
implosion-type weapons,
boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released du ...
s, and
thermonuclear weapon
A thermonuclear weapon, fusion weapon or hydrogen bomb (H bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a lowe ...
s. New nuclear weapon states tend to develop boosted fission and thermonuclear weapons only. All known gun-type nuclear weapons previously built worldwide have been dismantled.
Little Boy
The "gun" method is roughly how the Little Boy weapon, which was detonated over
Hiroshima
is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui h ...
, worked, using
uranium-235
Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
as its fissile material. In the Little Boy design, the U-235 "bullet" had a mass of around , and it was long, with a diameter of . The hollow cylindrical shape made it subcritical. It was powered by a
cordite
Cordite is a family of smokeless propellants developed and produced in the United Kingdom since 1889 to replace black powder as a military propellant. Like modern gunpowder, cordite is classified as a low explosive because of its slow burni ...
charge. The uranium target spike was about . Both the bullet and the target consisted of multiple rings stacked together.
The use of "rings" had two advantages: it allowed the larger bullet to confidently remain subcritical (the hollow column served to keep the material from having too much contact with other material), and it allowed sub-critical assemblies to be tested using the same bullet but with just one ring.
The barrel had an inside diameter of . Its length was , which allowed the bullet to accelerate to its final speed of about
before coming into contact with the target.
When the bullet is at a distance of , the combination becomes critical. This means that some free neutrons may cause the chain reaction to take place before the material could be fully joined (see
nuclear chain reaction
In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...
).
Typically the chain reaction takes less than 1 μs (100
shakes), during which time the bullet travels only 0.3 mm. Although the chain reaction is slower when the supercriticality is low, it still happens in a time so short that the bullet hardly moves in that time.
This could cause a
fizzle, a
predetonation which would blow the material apart before creating much of an explosion. Thus, it is important that the frequency at which free neutrons occur is kept low, compared with the assembly time from this point. This also means that the speed of the projectile must be sufficiently high; its speed can be increased but this requires a longer and heavier barrel, or a higher pressure of the propellant gas for greater acceleration of the bullet subcritical mass.
In the case of Little Boy, the 20%
U-238 in the uranium had 70
spontaneous fission
Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...
s per second. With the fissionable material in a supercritical state, each gave a large probability of detonation: each fission creates on average 2.52 neutrons, which each have a probability of more than 1:2.52 of creating another fission. During the 1.35 ms of supercriticality prior to full assembly, there was a 10% probability of a fission, with somewhat less probability of pre-detonation.
![AtomicEffects-Hiroshima](https://upload.wikimedia.org/wikipedia/commons/2/2c/AtomicEffects-Hiroshima.jpg)
Initially the Manhattan Project gun-type effort was directed at making a gun weapon that used
plutonium
Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
as its source of fissile material, known as the "
Thin Man" because of its extreme length. It was thought that if a plutonium gun-type bomb could be created, then the uranium gun-type bomb would be very easy to make by comparison. However, it was discovered in April 1944 that
reactor-bred plutonium (
Pu-239
Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main ...
) is contaminated with another isotope of plutonium,
Pu-240
Plutonium-240 ( or Pu-240) is an isotope of plutonium formed when plutonium-239 captures a neutron. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.
240 ...
, which increases the material's spontaneous neutron-release rate, making pre-detonation inevitable. For this reason, a gun-type bomb is thought to only be usable with enriched-uranium fuel. It is unknown though possible to make a composite design using high grade plutonium in the bullet only.
After it was discovered that the "Thin Man" program would not be successful, Los Alamos redirected its efforts into creating the implosion-type plutonium weapon: "
Fat Man
"Fat Man" (also known as Mark III) is the codename for the type of nuclear bomb the United States detonated over the Japanese city of Nagasaki on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the fir ...
". The gun program switched completely over to developing a uranium bomb.
Although in Little Boy of 80%-grade U-235 was used (hence ), the minimum is about 44 to 55 pounds (20 to 25 kg), versus for the implosion method.
Little Boy's target subcritical mass was enclosed in a neutron reflector made of tungsten carbide (WC). The presence of a neutron reflector reduced neutron losses during the chain reaction, and so reduced the quantity of uranium fuel needed. A more effective reflector material would be metallic beryllium, but this was not known until the postwar years when
Ted Taylor developed an implosion design known as "Scorpion".
The scientists who designed the "Little Boy" weapon were confident enough of its success that they did not field-test a design before using it in war (though scientists such as
Louis Slotin
Louis Alexander Slotin (1 December 1910 – 30 May 1946) was a Canadian physicist and chemist who took part in the Manhattan Project. Born and raised in the North End of Winnipeg, Manitoba, Slotin earned both his Bachelor of Science and M ...
did perform non-destructive tests with sub-critical assemblies, dangerous experiments nicknamed
tickling the dragon's tail). In any event, it could not be tested before being deployed, as there was only sufficient U-235 available for one device. Even though the design was never proof-tested, there was thought to be no risk of the device being captured by an enemy if it malfunctioned.
Even a "fizzle" would have completely disintegrated the device, while the multiple redundancies built into the "Little Boy" design meant there was negligible if any potential for the device to strike the ground without detonating at all.
For a quick start of the chain reaction at the right moment a neutron trigger/initiator is used. An initiator is not strictly necessary for an effective gun design,
as long as the design uses "target capture" (in essence, ensuring that the two subcritical masses, once fired together, cannot come apart until they explode). Considering the 70 spontaneous fissions per second, this only causes a delay of a few times 1/70 second, which in this case does not matter. Initiators were only added to
Little Boy
"Little Boy" was the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II, making it the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress ''Enola Gay'' p ...
late in its design.
Proliferation and terrorism
With regard to the risk of
proliferation and
use by terrorists, the relatively simple design is a concern, as it does not require as much fine engineering or manufacturing as other methods. With enough highly enriched uranium, nations or groups with relatively low levels of technological sophistication could create an inefficient—though still quite powerful—gun-type nuclear weapon.
Comparison with the implosion method
For technologically advanced states the gun-type method is now essentially obsolete, for reasons of efficiency and safety (discussed above).
The gun type method was largely abandoned by the United States as soon as the implosion technique was perfected, though it was retained in the specialised role of
nuclear artillery
Nuclear artillery is a subset of limited- yield tactical nuclear weapons, in particular those weapons that are launched from the ground at battlefield targets. Nuclear artillery is commonly associated with shells delivered by a cannon, but in ...
for a time. Other nuclear powers, such as the United Kingdom, and the
Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...
never built an example of this type of weapon. Besides requiring the use of highly enriched U-235, the technique has other severe limitations. The implosion technique is much better suited to the various methods employed to reduce the weight of the weapon and increase the proportion of material which fissions. South Africa built around five gun-type weapons, and no implosion-type weapons. They later abandoned their
nuclear weapon program altogether. They were unique in their abandonment of nuclear weapons, and probably also by building gun-type weapons rather than implosion-type weapons.
There are also safety problems with gun-type weapons. For example, it is inherently dangerous to have a weapon containing a quantity and shape of fissile material that can form a critical mass through a relatively simple accident. Furthermore, if the weapon is dropped from an aircraft into the sea, then the
moderating effect of the
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
can also cause a
criticality accident
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, or divergent chain reaction. Any such event involves the unintended accumulation ...
without the weapon even being physically damaged. Neither can happen with an implosion-type weapon, since there is normally insufficient fissile material to form a critical mass without the correct detonation of the explosive lenses.
US nuclear artillery
![Nuclear artillery test Grable Event - Part of Operation Upshot-Knothole](https://upload.wikimedia.org/wikipedia/commons/f/fe/Nuclear_artillery_test_Grable_Event_-_Part_of_Operation_Upshot-Knothole.jpg)
The gun method has also been applied for
nuclear artillery
Nuclear artillery is a subset of limited- yield tactical nuclear weapons, in particular those weapons that are launched from the ground at battlefield targets. Nuclear artillery is commonly associated with shells delivered by a cannon, but in ...
shells, since the simpler design can be more easily engineered to withstand the rapid acceleration and g-forces imparted by an artillery gun, and since the smaller diameter of the gun-type design can be relatively easily fitted to projectiles that can be fired from existing artillery.
A US gun-type nuclear artillery weapon, the
W9, was tested on May 25, 1953 at the
Nevada Test Site
The Nevada National Security Site (N2S2 or NNSS), known as the Nevada Test Site (NTS) until 2010, is a United States Department of Energy (DOE) reservation located in southeastern Nye County, Nevada, about 65 miles (105 km) northwest of th ...
. Fired as part of
Operation Upshot–Knothole
Operation Upshot–Knothole was a series of eleven nuclear test shots conducted in 1953 at the Nevada Test Site. It followed ''Operation Ivy'' and preceded ''Operation Castle''.
Over 21,000 soldiers took part in the ground exercise Desert Ro ...
and codenamed ''Shot GRABLE'', a 280 mm shell was fired 10,000 m and detonated 160 m above the ground with an estimated yield of 15
kiloton
TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. The is a unit of energy defined by that convention to be , which is the approximate energy released in the detonation of a t ...
s. This is approximately the same
yield as
Little Boy
"Little Boy" was the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II, making it the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress ''Enola Gay'' p ...
, although the W9 had less than 1/10 of Little Boy's weight (365 kg vs. 4,000 kg, or 805 lbs vs. 8,819 lbs). The shell was 1,384 mm long.
This was the only nuclear artillery shell ever actually fired (from an artillery gun) in the US test program. It was fired from a specially built artillery piece, nicknamed
Atomic Annie. Eighty shells were produced from 1952–53. It was retired in 1957.
The
W19 was also a 280 mm gun-type nuclear shell, a longer version of the W-9. Eighty warheads were produced and the system was retired in 1963.
The
W33 was a smaller, 8 inch (203 mm) gun-type nuclear artillery shell, which was produced starting in 1957 and in service until 1992. Two were test fired (detonated, not fired from an artillery gun), one hung under a balloon in the open air, and one in a tunnel.
[Operation Nougat](_blank)
at nuclearweaponarchive.org website, Carey Sublette (2005).
Later versions were based on the implosion design.
List of US gun-type weapons
Bombs
*
Little Boy
"Little Boy" was the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II, making it the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress ''Enola Gay'' p ...
, 1945–1951
*
Thin Man, cancelled
*
Mark 8
Mark 8 is the eighth chapter of the Gospel of Mark in the New Testament of the Christian Bible. It contains two miracles of Jesus, Peter's confession that he believes Jesus is the Messiah, and Jesus' first prediction of his own death and resurr ...
(
bunker buster
A bunker buster is a type of munition that is designed to penetrate hardened targets or targets buried deep underground, such as military bunkers.
Armor piercing shells
Germany
Röchling shells were bunker-busting artillery shells, developed by ...
), 1952–1957
*
Mark 10
Mark 10 is the tenth chapter of the Gospel of Mark in the New Testament of the Christian Bible. It presents further teachings of Jesus as his journey progresses towards Jerusalem.
Text
The original text was written in Koine Greek. This chapter ...
, cancelled
*
Mark 11
Mark 11 is the eleventh chapter of the Gospel of Mark in the New Testament of the Christian Bible, beginning Jesus' final week before His death as He arrives in Jerusalem for the coming Passover. It contains the stories of Jesus' entry into Je ...
(
bunker buster
A bunker buster is a type of munition that is designed to penetrate hardened targets or targets buried deep underground, such as military bunkers.
Armor piercing shells
Germany
Röchling shells were bunker-busting artillery shells, developed by ...
), 1956–1960
Artillery
*
W9, 1952–1957
*
W19, 1955–1963
*
W23, 1956–1962
*
W33, 1957–1992
Others
*
T-4 Atomic Demolition Munition
The T4 Atomic Demolition Munition (ADM) was a nuclear weapon derived from the American W9 nuclear artillery shell.
History
The T4 was produced in 1957 from recycled W9 fissile components and was in service until 1963, when it was replaced with W3 ...
, 1957–1963
*
W8 for the
SSM-N-8 Regulus cruise missile, cancelled
References
{{reflist
External links
Simulation of "Little Boy"an interactive simulation of the gun-type design atomic bomb "Little Boy"
Nuclear weapon design
Gun-type nuclear bombs
Nuclear fission