Pit (nuclear weapon)

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Designs

The pits of the first nuclear weapons were solid, with an

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 back on to the problem of the hollow pit, and for the rest of the year they were headed by Hans Bethe, his group leader and successor to the theoretical division, with the hollow composite core being of greatest interest,^[9] 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.^{[citation needed]}

In the early period of nuclear weapons development, 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

Another factor for considering different pit materials is the different behavior of plutonium and uranium.[12] 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

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).^[15] 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, presumably used the same or similar pits.^{[citation needed]}

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

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.^{[citation needed]}

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.^[18]

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

Newer pits contain about 3 kilograms of plutonium. Older pits used about 4-5 kilograms.[20]

Further miniaturization was achieved by

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.^{[citation needed]}

The sealed pits can be classified as bonded or non-bonded. Non-bonded pits can be disassembled mechanically; a lathe is sufficient for separating the plutonium. Recycling of bonded pits requires chemical processing.^[19]

Pits of modern weapons are said to have radii of about 5 cm.

The first weapons had removable pits, which were installed into the bomb shortly before its deployment. The ongoing miniaturization process led to design changes, whereby the pit could be inserted in the factory during the device assembly. This necessitated safety testing to make sure that accidental detonation of the high explosives would not cause a full-scale nuclear explosion; Project 56 was one of such a series of tests.

Accidental high-yield detonation was always a concern. The levitated pit design made it practical to allow in-flight insertion of pits to the bombs, separating the fissile core from the explosives around it.

Beryllium cladding, while advantageous technically, poses risk for the weapon plant employees. Machining the tamper shells produces beryllium and beryllium oxide dust; its inhalation can cause berylliosis. By the 1996, the US Department of Energy identified more than 50 cases of chronic berylliosis among nuclear industry employees, including three dozen in the Rocky Flats Plant; several died.^[18]

After the

Improper storage can promote corrosion of the pits. The AL-R8 containers used in the Pantex facility for storage of the pits are said to promote instead of hinder corrosion, and tend to corrode themselves. The decay heat released by the pits is also a concern; some pits in storage can reach temperatures as high as 150 °C, and the storage facilities for larger numbers of pits may require active cooling. Humidity control can also present problems for pit storage.^[44]

Beryllium cladding can be corroded by some solvents used for cleaning of the pits. Research has shown that trichloroethylene (TCE) causes beryllium corrosion, while trichloroethane (TCA) does not.^[45] Pitting corrosion of beryllium cladding is a significant concern during prolonged storage of pits in the Pantex facility.

The presence of

In very dry air, plutonium, despite its high chemical reactivity, forms a passivation layer of plutonium(IV) oxide that slows down the corrosion to about 200 nanometers per year. In moist air, however, this passivation layer is disrupted and the corrosion proceeds at 200 times this rate (0.04 mm/year) at room temperature, and 100,000 times faster (20 mm/year) at 100 °C. Plutonium strips oxygen from water, absorbs the liberated hydrogen and forms plutonium hydride. The hydride layer can grow at up to 20 cm/hour, for thinner shells its formation can be considered almost instant. In presence of water the plutonium dioxide becomes hyperstoichiometric, up to PuO_2.26. Plutonium chips can spontaneously ignite; the mechanism involves formation of Pu₂O₃ layer, which then rapidly oxidizes to PuO₂, and the liberated heat is sufficient to bring the small particles with low thermal mass to autoignition temperature (about 500 °C).

The self-irradiation occurs as the plutonium undergoes

The alpha-particles lose most of their energy to electrons, which manifests as heating the material. The heavier uranium nucleus has about 85 keV energy and about three quarters of it deposit as a cascade of atomic displacements; the uranium nucleus itself has the range of about 12 nanometers in the lattice. Each such decay event influences about 20,000 other atoms, 90% of which stay in their lattice site and only are thermally excited, the rest being displaced, resulting in formation of about 2500

At cryogenic temperatures, where next to no annealing occurs, the α-phase of plutonium expands (swells) during self-irradiation, the δ-phase contracts markedly, and the β-phase contracts slightly. The electrical resistance increases, which indicates the increase of defects in the lattice. All three phases, with sufficient time, converge to amorphous-like state with density averaging at 18.4 g/cm³. At normal temperature, however, most of the damage is annealed away; above 200K vacancies become mobile and at around 400K the clusters of interstitials and vacancies recombine, healing the damage. Plutonium stored at non-cryogenic temperatures does not show signs of major macroscopic structural changes after more than 40 years.

After 50 years of storage, a typical sample contains 2000 ppm of helium, 3700 ppm americium, 1700 ppm uranium, and 300 ppm neptunium. One kilogram of material contains 200 cm³ of helium, which equals three atmospheres of pressure in the same empty volume. Helium migrates through the lattice similarly to the vacancies, and can be trapped in them. The helium-occupied vacancies can coalesce, forming bubbles and causing swelling. Void-swelling is however more likely than bubble-swelling.^[46]

The Radiation Identification System is among a number of methods developed for nuclear weapons inspections. It allows the fingerprinting of the nuclear weapons so that their identity and status can be verified. Various physics methods are used, including gamma spectroscopy with high-resolution germanium detectors. The 870.7 keV line in the spectrum, corresponding to the first excited state of oxygen-17, indicates the presence of plutonium(IV) oxide in the sample. The age of the plutonium can be established by measuring the ratio of plutonium-241 and its decay product, americium-241.^[47] However, even passive measurements of gamma spectrums may be a contentious issue in international weapon inspections, as it allows characterization of materials used e.g. the isotopic composition of plutonium, which can be considered a secret.

Between 1954 and 1989, pits for US weapons were produced at the Rocky Flats Plant; the plant was later closed due to numerous safety issues. The Department of Energy attempted to restart pit production there, but repeatedly failed. In 1993, the DOE relocated beryllium production operations from defunct Rocky Flats Plant to Los Alamos National Laboratory; in 1996 the pit production was also relocated there.^[48] The reserve and surplus pits, along with pits recovered from disassembled nuclear weapons, totalling over 12,000 pieces, are stored in the Pantex plant.^[19] 5,000 of them, comprising about 15 tons of plutonium, are designated as strategic reserve; the rest is surplus to be withdrawn.^[49] The current LANL production of new pits is limited to about 20 pits per year, though NNSA is pushing to increase the production, for the Reliable Replacement Warhead program. The US Congress however has repeatedly declined funding.

Up until around 2010, Los Alamos National Laboratory had the capacity to produce 10 to 20 pits a year. The Chemistry and Metallurgy Research Replacement Facility (CMMR) will expand this capability, but it is not known by how much. An Institute for Defense Analyses report written before 2008 estimated a “future pit production requirement of 125 per year at the CMRR, with a surge capability of 200."^[50]

Russia stores the material from decommissioned pits in the Mayak facility.^[51]

Recovery of plutonium from decommissioned pits can be achieved by numerous means, both mechanical (e.g. removal of cladding by a lathe) and chemical. A hydride method is commonly used; the pit is cut in half, a half of the pit is laid inside-down above a funnel and a crucible in a sealed apparatus, and an amount of hydrogen is injected into the space. The hydrogen reacts with the plutonium producing plutonium hydride, which falls to the funnel and the crucible, where it is melted while releasing the hydrogen. Plutonium can also be converted to a nitride or oxide. Practically all plutonium can be removed from a pit this way. The process is complicated by the wide variety of the constructions and alloy compositions of the pits, and the existence of composite uranium-plutonium pits. Weapons-grade plutonium must also be blended with other materials to alter its isotopic composition enough to hinder its reuse in weapons.

• Beryllium nuclear properties – chemical element with symbol Be and atomic number 4Pages displaying wikidata descriptions as a fallback
• Charles Allen Thomas – American chemist (1900–1982)
• Dayton Project
• Edward Condon – American nuclear physicist (1902–1974)
• Eugene Wigner – Hungarian-American physicist and mathematician (1902–1995)
• George Kistiakowsky – Ukrainian-American physical chemistry professor
• James L. Tuck – British physicist (1910–1980)Pages displaying wikidata descriptions as a fallback
• Modulated neutron initiator – Neutron source used in some nuclear weapons
• Munroe effect – Explosive with focused effect
• Polonium – chemical element with symbol Po and atomic number 84Pages displaying wikidata descriptions as a fallback
• Supergrade plutonium – Isotope of plutonium
• Urchin – Neutron source used in some nuclear weapons