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Fission based nuclear weapon

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. 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 barrel as if it were a projectile. Since it is a relatively slow method of assembly, plutonium 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 is relatively large, and thus the overall efficiency 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 paths it becomes very probable most neutrons will find uranium nuclei to collide with, before escaping the supercritical mass. The first time gun type fission weapons were discussed was as part of the British Tube Alloys 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 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". The method was applied in four known US programs. First, the "Little Boy" weapon which was detonated over Hiroshima and several additional units of the same design prepared after World War II, in 40 Mark 8 bombs, and their replacement, 40 Mark 11 bombs. Both the Mark 8 and Mark 11 designs were intended for use as earth penetrating bombs (see nuclear bunker buster), 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 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. 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. 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 weapons, and thermonuclear weapons. 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.

The "gun" method is roughly how the Little Boy weapon, which was detonated over Hiroshima, worked, using uranium 235 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 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 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 late in its design.

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.

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 for a time. Other nuclear powers, such as the United Kingdom, and the Soviet Union 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 can also cause a criticality accident 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.

The gun method has also been applied for nuclear artillery 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. Fired as part of Operation Upshot–Knothole 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 kilotons. This is approximately the same yield as Little Boy, 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 to 1953. 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. Later versions were based on the implosion design.