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20:
342:"). 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.
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366:, 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.
391:, 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 mass of the weapon and increase the proportion of material which fissions.
382:
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
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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,
90:
nuclear bomb development program, the world's first nuclear bomb development program. The
British MAUD Report of 1941 laid out how "an effective uranium bomb which, containing some 25 lb of active material, would be equivalent as regards destructive effect to 1,800 tons of T.N.T". The bomb would use
402:
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,
237:
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
196:
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.
50:
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
345:
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
443:
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.
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as its fissile material. In the Little Boy design, the U-235 "bullet" had a mass of around 86 pounds (39 kg), and it was 7 inches (17.8 cm) long, with a diameter of 6.25 inches (15.9 cm). The hollow cylindrical shape made it subcritical. It was powered by a
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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.
327:(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
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The barrel had an inside diameter of 6.5 inches (16.5 cm). Its length was 70.8 inches (1.8 m), which allowed the bullet to accelerate to its final speed of about 1,000 feet per second (300 m/s) before coming into contact with the target.
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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.
78:
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
315:
Although in Little Boy 132 pounds (60 kg) of 80%-grade U was used (hence 106 pounds or 48 kilograms), the minimum is about 44 to 55 pounds (20 to 25 kg), versus 33 pounds (15 kg) for the implosion method.
204:
When the bullet is at a distance of 9.8 inches (25 cm), 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
289:" 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
346:
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
91:
the gun-type design "to bring the two halves together at high velocity and it is proposed to do this by firing them together with charges of ordinary explosive in a form of double gun".
334:
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
160:. 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.
488:
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
301:, 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
308:
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: "
226:
inch). Although the chain reaction is slower when the supercriticality is low, it still happens in a time so brief that the bullet hardly moves in that time.
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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.
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altogether. They were unique in their abandonment of nuclear weapons, and probably also by building gun-type weapons rather than implosion-type weapons.
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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
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charge. The uranium target spike was about 57.3 pounds (26 kg). Both the bullet and the target consisted of multiple rings stacked together.
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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.
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of Little Boy's weight (365 kg vs. 4,000 kg, or 805 lbs vs. 8,819 lbs). The shell was 1,384 mm (54.5 in) long.
629:"Rudolf Peierls's "Outline of the Development of the British Tube Alloy Project": A 1945 Account of the Earliest UK Work on Atomic Energy"
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There are currently no known gun-type weapons in service: advanced nuclear weapon states tended to abandon the design in favor of the
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it becomes very probable most neutrons will find uranium nuclei to collide with, before escaping the supercritical mass.
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fuel. It is unknown though possible to make a composite design using high grade plutonium in the bullet only.
114:), for which the gun-type method was preferred for a time by designers who were less than certain that early
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is relatively low. The main reason for this is the uranium metal does not undergo compression (and resulting
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would successfully detonate following an impact. The second program was a family of 11-inch (280 mm)
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did perform non-destructive tests with sub-critical assemblies, dangerous experiments nicknamed "
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bombs. Both the Mark 8 and Mark 11 designs were intended for use as earth-penetrating bombs (see
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there was a 10% probability of a fission, with somewhat less probability of pre-detonation.
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built around five gun-type weapons, and no implosion-type weapons. They later abandoned
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and several additional units of the same design prepared after World War II, in 40
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The first time gun-type fission weapons were discussed was as part of the
British
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The "gun" method is roughly how the Little Boy weapon, which was detonated over
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Schematic of the gun-type method (above) and the implosion-type method (below).
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Weapon effects – Hiroshima in ruins after the Little Boy atomic bomb exploded
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name is a reference to the fact that it is shooting the material through an
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an interactive simulation of the gun-type design atomic bomb "Little Boy"
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492:. Eighty shells were produced from 1952 to 1953. It was retired in 1957.
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In July 1944 the laboratory abandoned the plutonium gun-type bomb ("
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weapon used against
Hiroshima. The uranium-235 is indicated in red.
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for a time. Other nuclear powers, such as the United
Kingdom and
435:(photo depicts 280 mm gun and explosion), used a gun-type shell.
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gun-type effort was directed at making a gun weapon that used
265:) and focused almost entirely around the problem of implosion.
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The method was applied in four known US programs. First, the "
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at nuclearweaponarchive.org website, Carey
Sublette (2005).
216:), during which time the bullet travels only 0.3 mm (
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Little Boy's target subcritical mass was enclosed in a
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Typically the chain reaction takes less than 1 ÎĽs (100
297:) is contaminated with another isotope of plutonium,
510:Later versions were based on the implosion design.
431:, a 1953 test of a nuclear artillery projectile at
331:developed an implosion design known as "Scorpion".
58:Since it is a relatively slow method of assembly,
285:as its source of fissile material, known as the "
733:"The MAUD Report, 1941 | Historical Documents"
709:"The MAUD Report, 1941 | Historical Documents"
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447:A US gun-type nuclear artillery weapon, the
143:South Africa and weapons of mass destruction
765:Nuclear Weapons Frequently Asked Questions
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685:"Manhattan Project: The Maud Report, 1941"
70:is relatively large, and thus the overall
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439:The gun method has also been applied for
779:"The First Nuclear Weapons: Little Boy"
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62:cannot be used unless it is purely the
451:, was tested on May 25, 1953, at the
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370:Comparison with the implosion method
760:"Elements of Fission Weapon Design"
241:In the case of Little Boy, the 20%
55:barrel as if it were a projectile.
98:" weapon which was detonated over
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467:. This is approximately the same
106:bombs, and their replacement, 40
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609:cruise missile, cancelled 1955
597:T-4 Atomic Demolition Munition
1:
654:10.1080/00295450.2021.1910004
627:Moore, Richard (2021-12-03).
781:. 2015-03-15. Archived from
397:their nuclear weapon program
16:Fission-based nuclear weapon
514:List of US gun-type weapons
358:With regard to the risk of
354:Proliferation and terrorism
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819:Simulation of "Little Boy"
555:(bunker buster), 1956–1960
340:tickling the dragon's tail
457:Operation Upshot–Knothole
23:The "gun" assembly method
28:Gun-type fission weapons
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840:Gun-type nuclear bombs
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645:2021NucTe.207S.374M
639:(sup1): S374–S379.
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455:. Fired as part of
229:This could cause a
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48:supercritical mass
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785:on March 15, 2015
531:Mark 2 "Thin Man"
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568:, 1952–1957
527:, 1945–1951
461:Shot GRABLE
263:shown above
186:uranium-235
88:Tube Alloys
829:Categories
789:2015-12-30
742:2023-07-20
718:2023-07-20
694:2023-07-20
614:References
473:Little Boy
405:moderating
348:Little Boy
329:Ted Taylor
174:Little Boy
164:Little Boy
96:Little Boy
72:efficiency
64:Pu isotope
671:244134054
663:0029-5450
560:Artillery
403:then the
283:plutonium
182:Hiroshima
100:Hiroshima
60:plutonium
53:artillery
605:for the
465:kilotons
409:seawater
323:made of
287:Thin Man
259:Thin Man
641:Bibcode
553:Mark 11
547:Mark 10
480:⁄
310:Fat Man
291:reactor
221:⁄
191:cordite
108:Mark 11
76:density
68:uranium
46:into a
34:-based
32:fission
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591:Others
537:Mark 8
299:Pu-240
295:Pu-239
231:fizzle
214:shakes
156:, and
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40:design
38:whose
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519:Bombs
469:yield
659:ISSN
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