In nuclear weapon design, the pit is the core of an implosion nuclear weapon, consisting of fissile material and any neutron reflector or tamper bonded to it. Early pits were spherical, while most modern pits are prolate spheroidal. Some weapons tested during the 1950s used pits made with uranium-235 alone, or as a composite with plutonium. All-plutonium pits are the smallest in diameter and have been the standard since the early 1960s. The pit is named after the hard core found in stonefruit such as peaches and apricots.
Designs
The pits of the first nuclear weapons were solid, with an urchin neutron initiator in their center. The Gadget and Fat Man used pits made of 6.2 kg of solid hot pressed plutonium–gallium alloy (at 400 °C and 200 MPa in steel dies – and ) half-spheres of diameter, with a internal cavity for the initiator. The Gadget's pit was electroplated with 0.13 mm of silver because of plutonium's susceptibility to corrosion in air. This layer, however, developed blisters, which had to be ground off. These gaps were then patched with gold leaf before the test. The Fat Man pit, and those of subsequent models, were all plated with nickel. A hollow pit was considered and known to be more efficient but ultimately rejected due to higher requirements for implosion accuracy.
Later designs used TOM initiators of similar design but with diameters of only about . The internal neutron initiators were later phased out and replaced with pulsed neutron sources, and with boosted fission weapons.
The solid-cores were known as the "Christy" design, after Robert Christy who made the solid pit design a reality after it was initially proposed by Edward Teller. Along with the pit, the whole physics package was also informally nicknamed "Christy['s] Gadget".
Levitated pits Efficiency of the implosion can be increased by leaving an empty space between the tamper and the pit, causing a rapid acceleration of the shock wave before it impacts the pit. This method is known as levitated-pit implosion. Levitated pits were tested in 1948 with Fat Man style bombs (Mark IV). The early weapons with a levitated pit had a removable pit, called an open pit. It was stored separately, in a special capsule called a birdcage.
Hollow pits During implosion of a hollow pit, the plutonium layer accelerates inwards, colliding in the middle and forming a supercritical highly dense sphere. Due to the added momentum, the plutonium itself plays part of the role of the tamper, requiring a smaller amount of uranium in the tamper layer, reducing the warhead weight and size. Hollow pits are more efficient than solid ones but require more accurate implosion; solid "Christy" pits were therefore favored for the first weapon designs. Following the war's end in August 1945, the laboratory focused on the problem of the hollow pit again, and for the rest of the year they were headed by Hans Bethe, Christy's group leader and successor to the theoretical division, with the hollow composite core being of greatest interest, due to the cost of plutonium and trouble ramping up the Hanford reactors.
The efficiency of the hollow pits can be further increased by injecting a 50%/50% mixture of deuterium and tritium into the cavity immediately before the implosion, so called "fusion boosting"; this also lowers the minimum amount of plutonium for achieving a successful explosion. The higher degree of control of the initiation, both by the amount of deuterium-tritium mixture injection and by timing and intensity of the neutron pulse from the external generator, facilitated the design of variable yield weapons.
Composite cores and uranium pits
In the early period of nuclear weapons development, the plutonium-239 supply was scarce. To lower its amount needed for a pit, a composite core was developed, where a hollow shell of plutonium was surrounded with an outer shell of then more plentiful highly enriched uranium. The composite cores were available for Mark 3 nuclear bombs by the end of 1947. For example, a composite core for a US Mark 4 bomb, the 49-LCC-C core was made of 2.5 kg of plutonium and 5 kg of uranium. Its explosion releases only 35% of energy of the plutonium and 25% of the uranium, so it is not highly efficient, but the weight saving of plutonium is significant.
Another factor for considering different pit materials is the different behavior of plutonium and uranium. Plutonium fissions faster and produces more neutrons, but it was then more expensive to produce, and scarce due to limitations of the available reactors. Uranium is slower to fission, so it can be assembled into a more supercritical mass, allowing higher yield of the weapon. A composite core was considered as early as of July 1945, and composite cores became available in 1946. The priority for Los Alamos then was the design of an all-uranium pit. The new pit designs were tested by the Operation Sandstone.
The plutonium-only core, with its high background neutron rate, had a high probability of predetonation, with reduced yield. Minimizing this probability required a smaller mass of plutonium, which limited the achievable yield to about 10 kt, or using highly pure plutonium-239 with impractically low level of plutonium-240 contamination. The advantage of the composite core was the possibility to maintain higher yields while keeping predetonation risk low, and to utilize both available fissile materials. The yield limitation was rendered irrelevant in the mid-1950s with the advent of fusion boosting, and later with using of fusion weapons.
The yield of a weapon can also be controlled by selecting among a choice of pits. For example, the Mark 4 nuclear bomb could be equipped with three different pits: 49-LTC-C (levitated uranium-235, tested in the Zebra test on 14 May 1948), 49-LCC-C (levitated composite uranium-plutonium), and 50-LCC-C (levitated composite). This approach is not suitable for field selectability of the yield of the more modern weapons with nonremovable pits, but allows production of multiple weapon subtypes with different yields for different tactical uses. The early US designs were based on standardized Type C and Type D pit assemblies. The Mark 4 bomb used the Type C and Type D pits, which were insertable manually in flight. The Mark 5 bomb used Type D pits, with automated in-flight insertion; the W-5 warhead used the same. Its successor, the Mark 6 bomb, was compatible with all but one of its pits.A later history of the stockpile lists the pit systems between warheads using three-digit "capsules"; the Mark 4 is listed, for example, as being compatible with 110, 130, and 140 capsules, while the Mark 5 was compatible with 8 capsules grouped into four pit-types (130/240; 110/170/260; 150/210; 190). Marks 4, 5, 6, 7, and the 12 all had a high degree of capsule compatibility. The thermonuclear Marks 14, 15, 17, 18, 21, and 24 all could use the same capsule type for their primary (150, which was also compatible with Marks 5, 6, and 7). The gun-type Marks 8 and 11 used mutually compatible capsules. For all of these early weapons, the capsules could be (and were) stored separately from the non-nuclear components of the weapons, as a political choice and a safety measure.
The pit can be composed of plutonium-239, plutonium-239/uranium-235 composite, or uranium-235 only. Plutonium is the most common choice, but e.g. the Violet Club bomb and Orange Herald warhead used massive hollow pits, consisting of 87 and 117 kg (98 and 125 kg according to other sources) of highly enriched uranium. The Green Grass fission core consisted of a sphere of highly enriched uranium, with an inner diameter of 560 mm, a wall thickness of 3.6 mm and a mass of 70–86 kg; the pit was completely supported by the surrounding natural uranium tamper. Such massive pits, consisting of more than one critical mass of fissile material, present a significant safety risk, as even an asymmetrical detonation of the implosion shell may cause a kiloton-range explosion. The largest-yield pure-fission weapon, the 500-kiloton Mark 18 nuclear bomb, used a hollow pit composed of more than 60 kg of highly enriched uranium, about four critical masses; the safing was done with an aluminium–boron chain inserted in the pit.
A composite pit of plutonium and uranium-233, based on the plutonium-U235 core from TX-7E Mark 7 nuclear bomb, was tested in 1955 during the Operation Teapot in the MET test. The yield was 22 kilotons instead of the expected 33 kilotons.
Sealed pits A sealed pit means that a solid metal barrier is formed around the pit inside a nuclear weapon, with no openings. This protects the nuclear materials from environmental degradation and helps reduce the chances of their release in case of an accidental fire or minor explosion. The first US weapon employing a sealed pit was the W25 warhead. The metal is often stainless steel, but beryllium, aluminium, and possibly vanadium are also used. Beryllium is brittle, toxic, and expensive, but is an attractive choice due to its role as a neutron reflector, lowering the needed critical mass of the pit. There is probably a layer of interface metal between plutonium and beryllium, capturing the alpha particles from decay of plutonium (and americium and other contaminants) which would otherwise react with the beryllium and produce neutrons. Beryllium tampers/reflectors came into use in the mid-1950s; the parts were machined from pressed-powder beryllium blanks in the Rocky Flats Plant.
More modern plutonium pits are hollow. An often-cited specification applicable to some modern pits describes a hollow sphere of a suitable structural metal, of the approximate size and weight of a bowling ball, with a channel for injection of tritium (in the case of boosted fission weapons), with the internal surface lined with plutonium. The principal factors influencing the weapon properties are the pit's size (usually between a bowling ball and a tennis ball), accuracy of sphericity, weight and isotopic composition of the fissile material, and are often classified. The hollow pits can be made of half shells with three joint welds around the equator, and a tube brazed (to beryllium or aluminium shell) or electron beam or TIG-welded (to stainless steel shell) for injection of the boost gas. Beryllium-clad pits are more vulnerable to fracture, more sensitive to temperature fluctuations, more likely to require cleaning, susceptible to corrosion with chlorides and moisture, and can expose workers to toxic beryllium.
Newer pits contain about 3 kilograms of plutonium. Older pits used about 4–5 kilograms.
Linear implosion pits Further miniaturization was achieved by linear implosion. An elongated subcritical solid pit, reshaped into a supercritical spherical shape by two opposite shock waves, and later a hollow pit with more precisely shaped shock waves, allowed construction of relatively very small nuclear warheads. The configuration was, however, considered prone to accidental high-yield detonation should the explosive get accidentally initiated, unlike a spherical implosion assembly where asymmetric implosion destroys the weapon without triggering a nuclear detonation. This necessitated special design precautions, and a series of safety tests, including one-point safety.
Pit sharing between weapons Pits can be shared between weapon designs. For example, the W89 warhead is said to reuse pits from the W68s. Many pit designs are standardized and shared between different physics packages; the same physics packages are often used in different warheads. Pits can be also reused; the sealed pits extracted from disassembled weapons are commonly stockpiled for direct reuse. Due to low aging rates of the plutonium-gallium alloy, the shelf life of pits is estimated to be a century or more. The oldest pits in the US arsenal are still less than 50 years old.