![Partially-reflected-plutonium-sphere](https://upload.wikimedia.org/wikipedia/commons/1/13/Partially-reflected-plutonium-sphere.jpeg)
The pit, named after the hard core found in fruits such as
peach
The peach (''Prunus persica'') is a deciduous tree first domesticated and cultivated in Zhejiang province of Eastern China. It bears edible juicy fruits with various characteristics, most called peaches and others (the glossy-skinned, non-fu ...
es and
apricot
An apricot (, ) is a fruit, or the tree that bears the fruit, of several species in the genus ''Prunus''.
Usually, an apricot is from the species '' P. armeniaca'', but the fruits of the other species in ''Prunus'' sect. ''Armeniaca'' are also ...
s,
is the core of an
implosion nuclear weapon
Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three existing basic design types:
* pure fission weapons, the simplest and least technically ...
– the
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 ...
and any
neutron reflector
A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, tungsten carbide, gold, or other materials. A neutron reflector ...
or
tamper bonded to it. Some weapons tested during the 1950s used pits made with
U-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 exist ...
alone, or in
composite
Composite or compositing may refer to:
Materials
* Composite material, a material that is made from several different substances
** Metal matrix composite, composed of metal and other parts
** Cermet, a composite of ceramic and metallic materials
...
with
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 ...
, but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.
Pit designs
Christy pits
The pits of the first nuclear weapons were solid, with an
''urchin'' neutron initiator in their center.
The Gadget
Trinity was the code name of the first detonation of a nuclear weapon. It was conducted by the United States Army at 5:29 a.m. on July 16, 1945, as part of the Manhattan Project. The test was conducted in the Jornada del Muerto desert abo ...
and
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 ...
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
Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...
with 0.13 mm of
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
; the layer, however, developed blistering and the blisters had to be ground and plated with
gold leaf
Gold leaf is gold that has been hammered into thin sheets (usually around 0.1 µm thick) by goldbeating and is often used for gilding. Gold leaf is available in a wide variety of karats and shades. The most commonly used gold is 22-kara ...
before the test. The Fat Man pit, and those of subsequent models, were all plated with
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
. 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 source
A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear p ...
s, and with boosted fission weapons.
The solid-cores were known as the "Christy" design, after
Robert Christy
Robert Frederick Christy (May 14, 1916 – October 3, 2012) was a Canadian-American theoretical physicist and later astrophysicist who was one of the last surviving people to have worked on the Manhattan Project during World War II. He briefly ...
who made the solid pit design a reality after it was initially proposed by
Edward Teller
Edward Teller ( hu, Teller Ede; January 15, 1908 – September 9, 2003) was a Hungarian-American theoretical physicist who is known colloquially as "the father of the hydrogen bomb" (see the Teller–Ulam design), although he did not care fo ...
. Along with the pit, the whole
physics package
Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three existing basic design types:
* pure fission weapons, the simplest and least technically ...
was also informally nicknamed "Christy
sGadget".
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 back on to the problem of the hollow pit, and for the rest of the year they were headed by
Hans Bethe
Hans Albrecht Bethe (; July 2, 1906 – March 6, 2005) was a German-American theoretical physicist who made major contributions to nuclear physics, astrophysics, quantum electrodynamics, and solid-state physics, and who won the 1967 Nobel Prize ...
, his 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
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
and
tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...
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
Variable yield, or dial-a-yield, is an option available on most modern nuclear weapons. It allows the operator to specify a weapon's yield, or explosive power, allowing a single design to be used in different situations. For example, the Mod-10 ...
weapons.
Composite cores and uranium pits
At that time, 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
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
. The composite cores were available for
Mark 3 nuclear bomb
"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 firs ...
s 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
Operation Sandstone was a series of nuclear weapon tests in 1948. It was the third series of American tests, following Trinity in 1945 and Crossroads in 1946, and preceding Ranger. Like the Crossroads tests, the Sandstone tests were carried ou ...
.
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 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
The Mark 4 nuclear bomb was an American implosion-type nuclear bomb based on the earlier Mark 3 Fat Man design, used in the Trinity test and the bombing of Nagasaki. With the Mark 3 needing each individual component to be hand-assembled by only h ...
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
Mark 4 is the fourth chapter of the Gospel of Mark in the New Testament of the Christian Bible. It tells the parable of the Sower, with its explanation, and the parable of the Mustard Seed. Both of these parables are paralleled in Matthew ...
bomb used the Type C and Type D pits, which were insertable manually in flight. The
Mark 5
Mark 5 is the fifth chapter of the Gospel of Mark in the New Testament of the Christian Bible. Taken with the calming of the sea in , there are "four striking works hichfollow each other without a break": an exorcism, a healing, and the raisin ...
bomb used Type D pits, with automated in-flight insertion; the W-5 warhead used the same. Its successor, the
Mark 6
Mark 6 is the sixth chapter of the Gospel of Mark in the New Testament of the Christian Bible. In this chapter, Jesus goes to Nazareth and faces rejection by his own family. He then sends his Apostles in pairs to various cities in the region wh ...
bomb, presumably used the same or similar pits.
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
Violet may refer to:
Common meanings
* Violet (color), a spectral color with wavelengths shorter than blue
* One of a list of plants known as violet, particularly:
** ''Viola'' (plant), a genus of flowering plants
Places United States
* Vio ...
bomb and
Orange Herald
Orange Herald was a British nuclear weapon, tested on 31 May 1957. At the time it was reported as an H-bomb, although in fact it was a large boosted fission weapon and remains to date, the largest fission device ever detonated.
Technical
Oran ...
warhead used massive hollow pits, consisting of 87 and 117 kg (98 and 125 kg according to other sources) of
highly enriched uranium
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
. The
Green Grass fission core consisted of a sphere of highly enriched uranium, with inner diameter of 560 mm, wall thickness of 3.6 mm and 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 The Mark 18 nuclear bomb, also known as the SOB or Super Oralloy Bomb, was an American nuclear bomb design which was the highest yield fission bomb produced by the US. The Mark 18 had a design yield of 500 kilotons. Noted nuclear weapon designer ...
, 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
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
–
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
chain inserted in the pit.
A composite pit of plutonium and
uranium-233
Uranium-233 (233U or U-233) is a fissile Isotopes of uranium, isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a Nuclear fuel, reactor fuel. It ha ...
, based on the plutonium-U235 core from TX-7E
Mark 7 nuclear bomb
Mark 7 "Thor" (or Mk-7') was the first tactical fission bomb adopted by US armed forces. It was also the first weapon to be delivered using the toss method with the help of the low-altitude bombing system (LABS). The weapon was tested in Operati ...
, was tested in 1955 during the
Operation Teapot
Operation Teapot was a series of 14 nuclear test explosions conducted at the Nevada Test Site in the first half of 1955. It was preceded by ''Operation Castle'', and followed by ''Operation Wigwam''. ''Wigwam'' was, administratively, a part of ...
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
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...
, but
beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...
,
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, and possibly
vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...
are also used. Beryllium is brittle, toxic, and expensive, but is an attractive choice due to its role as a
neutron reflector
A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, tungsten carbide, gold, or other materials. 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
The Rocky Flats Plant was a U.S. manufacturing complex that produced nuclear weapons parts in the western United States, near Denver, Colorado. The facility's primary mission was the fabrication of plutonium pits, which were shipped to ot ...
.
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
A bowling ball is a hard spherical ball used to knock down bowling pins in the sport of bowling.
Balls used in ten-pin bowling and American nine-pin bowling traditionally have holes for two fingers and the thumb. Balls used in five-pin bowling, ...
, with a channel for injection of
tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...
(in the case of
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 ...
s), with the internal surface lined with plutonium. The size, usually between a bowling ball and a
tennis ball
A tennis ball is a ball designed for the sport of tennis. Tennis balls are fluorescent yellow in organised competitions, but in recreational play can be virtually any color. Tennis balls are covered in a fibrous felt which modifies their aerodyna ...
, accuracy of sphericity, and weight and isotopic composition of the fissile material, the principal factors influencing the weapon properties, are often classified. The hollow pits can be made of half shells with three joint
welds around the equator, and a tube
brazed
Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.
Brazing differs from we ...
(to beryllium or aluminium shell) or
electron beam
Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to ele ...
or
TIG-welded
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the welding, weld. The weld area and electrode are protected from oxidation or ot ...
(to stainless steel shell) for injection of the boost gas.
[BREDL Southern Anti-Plutonium Campaign](_blank)
. Bredl.org (1995-08-22). Retrieved on 2010-02-08. Beryllium-clad pits are more vulnerable to fracture, more sensitive to temperature fluctuations, more likely to require cleaning, susceptible to
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
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.
[Nuclear Wastelands: A Global Guide to Nuclear Weapons Production and Its Health and Environmental Effects](_blank)
by Arjun Makhijani, Katherine Yih, MIT Press, 2000 , p. 58
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 when the explosive gets 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
The W89 was an American thermonuclear warhead design intended for use on the AGM-131 SRAM II air to ground nuclear missile and the UUM-125 Sea Lance anti-submarine missile.
What was to become the W89 design was awarded to the Lawrence Livermore ...
warhead is said to reuse pits from the
W68
The W68 warhead was the warhead used on the UGM-73 Poseidon SLBM missile. It was developed in the late 1960s at Lawrence Livermore National Laboratory.
Specifications
The W68 weighed and had an official design yield of .
The design was revol ...
s. 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.
The sealed pits can be classified as bonded or non-bonded. Non-bonded pits can be disassembled mechanically; a
lathe
A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to c ...
is sufficient for separating the plutonium. Recycling of bonded pits requires chemical processing.
Pits of modern weapons are said to have radii of about 5 cm.
Weapons and pit types
Safety considerations
![One-Point Safety Test](https://upload.wikimedia.org/wikipedia/commons/d/db/One-Point_Safety_Test.svg)
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.
Many cases of accidental bomb losses and explosions therefore led only to dispersal of uranium from the bomb's tamper. Later hollow-pit designs, where there is no space between the pit and the tamper, however, made this impossible.
The pits of earlier weapons had accessible inner cavities. For
safety
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to risk management, the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are ...
, objects were inserted into the pit and removed only when needed. Some larger pits, e.g. the British
Green Grass, had their inner cavity lined with rubber and filled with metal balls; this design was improvised and far from optimal, for example in that subjecting the safed pit with balls inside to vibration, e.g. in an airplane, could lead to its damage. A fine metal chain from a neutron-absorbing material (the same used for reactor
control rod
Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing ...
s, e.g.
cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
), can be used instead. The
W47
The W47 was an American thermonuclear warhead used on the Polaris A-1 sub-launched ballistic missile system. Various models were in service from 1960 through the end of 1974. The warhead was developed by the Lawrence Radiation Laboratory between 1 ...
warhead had its pit filled with a cadmium-
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
wire when it was manufactured; on arming the weapon, the wire was pulled out to a spool by a small motor and could not be reinserted. However, the wire tended to become brittle and break during removal, making its complete removal impossible and rendering the warhead a dud.
The switch from solid to hollow pits caused a work safety issue; the larger surface-to-mass ratio led to comparatively higher emission of gamma rays and necessitated the installation of better radiation shielding in the Rocky Flats production facility. The increased amount of rolling and machining required led to higher consumption of machining oil and
tetrachloromethane
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemica ...
, used for degreasing the parts afterwards and creating a large amount of contaminated waste. The
pyrophoric
A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
plutonium shavings also posed a risk of self-ignition.
["Making a Real Killing: Rocky Flats and the Nuclear West"](_blank)
, Len Ackland, p. 131, University of New Mexico Press, 2002
Sealed pits require a different method of safing. Many techniques are used, including
Permissive Action Link
A permissive action link (PAL) is an access control security device for nuclear weapons. Its purpose is to prevent unauthorized arming or detonation of a nuclear weapon.
The United States Department of Defense definition is:
The earliest PALs ...
s and
strong link weak link systems, designed to fail in case of an accident or improper arming sequence; these include mechanical interlocks, critical parts designed to malfunction in case of fire or impact, etc.
Beryllium cladding, while advantageous technically, poses risk for the weapon plant employees. Machining the tamper shells produces beryllium and
beryllium oxide
Beryllium oxide (BeO), also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and exceeds that of m ...
dust; its inhalation can cause
berylliosis
Berylliosis, or chronic beryllium disease (CBD), is a chronic allergic-type lung response and chronic lung disease caused by exposure to beryllium and its compounds, a form of beryllium poisoning. It is distinct from acute beryllium poisoning, wh ...
. 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.
After the
1966 Palomares B-52 crash
The 1966 Palomares B-52 crash, also called the Palomares incident, occurred on 17 January 1966, when a B-52G bomber of the United States Air Force's Strategic Air Command collided with a KC-135 tanker during mid-air refueling at over the Me ...
and the
1968 Thule Air Base B-52 crash
On 21 January 1968, an aircraft accident, sometimes known as the Thule affair or Thule accident (; da, Thuleulykken), involving a United States Air Force (USAF) B-52 bomber occurred near Thule Air Base in the Danish territory of Greenland. The ...
, the safety of weapons against accidental plutonium dispersal became a concern of US military.
Fire-resistant pits (FRP) are a safety feature of modern nuclear weapons, reducing plutonium dispersal in case of fire. The current pits are designed to contain molten plutonium in temperatures up to 1000 °C, the approximate temperature of a burning aircraft fuel, for several hours. Fire-resistant pits would be of no help in cases where pits were scattered around by an explosion; they are used therefore together with
insensitive high explosive
Insensitive munitions are munitions that are designed to withstand stimuli representative of severe but credible accidents. The current range of stimuli are shock (from bullets, fragments and shaped charge jets), heat (from fires or adjacent the ...
s, which should be resistant to accidental detonation by impact or fire, and undetonable propellants when used in missiles. Vanadium cladding was tested for design of fire-resistant pits, but it is unknown if it is in use or only experimental. The
W87
The W87 is an American thermonuclear missile warhead formerly deployed on the LGM-118A Peacekeeper ("MX") ICBM. 50 MX missiles were built, each carrying up to 10 W87 warheads in multiple independently targetable reentry vehicles (MIRV), and were ...
warhead is an example of a FRP-employing assembly. FRP does not, however, provide protection if the pit cladding is mechanically damaged, and may fail if subjected to missile fuel fire, which has a higher burning temperature (about 2000 °C) than does aircraft fuel.
Severe weight and size constraints may preclude the use of both FRP and insensitive explosives.
SLBM
A submarine-launched ballistic missile (SLBM) is a ballistic missile capable of being launched from Ballistic missile submarine, submarines. Modern variants usually deliver multiple independently targetable reentry vehicles (MIRVs), each of whic ...
s, with their size considerations and more energetic and vulnerable fuel, tend to be less safe than
ICBM
An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than , primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons c ...
s.
Other
energetic material Energetic materials are a class of material with high amount of stored chemical energy that can be released.
Typical classes of energetic materials are e.g. explosives, pyrotechnic compositions, propellants (e.g. smokeless gunpowders and rocket ...
s in the vicinity of the pit also influence its safety. US missile propellants come in two general classes. The class 1.3, fire hazard but very difficult to impossible to detonate; an example is 70%
ammonium perchlorate
Ammonium perchlorate ("AP") is an inorganic compound with the formula NH4 ClO4. It is a colorless or white solid that is soluble in water. It is a powerful oxidizer. Combined with a fuel, it can be used as a rocket propellant called ammonium pe ...
, 16%
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, and 14% binder. The class 1.1, both fire and detonation hazard, is a
double-base propellant based on
cross-linked
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
polymer, containing 52%
HMX, 18%
nitroglycerine
Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating ...
, 18% aluminium, 4% ammonium perchlorate, and 8% binder. The 1.1 propellant has 4% higher specific impulse (about 270 s versus 260 s), giving an 8% longer range for constant burning time. The insensitive high explosives are also less powerful, necessitating larger and heavier warheads, which reduces the missile range – or sacrificing some yield. The safety/performance tradeoff is especially important for e.g.
submarine
A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
s.
As of 1990, the
Trident
A trident is a three- pronged spear. It is used for spear fishing and historically as a polearm.
The trident is the weapon of Poseidon, or Neptune, the God of the Sea in classical mythology. The trident may occasionally be held by other marine ...
SLBMs used both detonable fuel and non-insensitive explosives.
Material considerations
Casting and then machining plutonium is difficult not only because of its toxicity, but because plutonium has many different
metallic phases, also known as
allotrope
Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: the ...
s. As plutonium cools, changes in phase result in distortion and cracking. This distortion is normally overcome by alloying it with 3–3.5 molar% (0.9–1.0% by weight)
gallium
Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
, forming a
plutonium-gallium alloy, which causes it to take up its delta phase over a wide temperature range.
When cooling from molten it then suffers only a single phase change, from epsilon to delta, instead of the four changes it would otherwise pass through. Other
trivalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, or affinity of an ...
metals would also work, but gallium has a small neutron
absorption cross section
Absorption cross section is a measure for the probability of an absorption process. More generally, the term cross section is used in physics to quantify the probability of a certain particle-particle interaction, e.g., scattering, electromagne ...
and helps protect the plutonium against
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
. A drawback is that gallium compounds themselves are corrosive and so if the plutonium is recovered from dismantled weapons for conversion to
plutonium dioxide
Plutonium(IV) oxide or (plutonia) is the chemical compound with the formula Pu O2. This high melting-point solid is a principal compound of plutonium. It can vary in color from yellow to olive green, depending on the particle size, temperature a ...
for
power reactors, there is the difficulty of removing the gallium.
Because plutonium is chemically reactive it is common to plate the completed pit with a thin layer of inert metal, which also reduces the toxic hazard.
[Fissionable Materials](_blank)
section of th
Nuclear Weapons FAQ
Carey Sublette. Retrieved Sept 23, 2006. The Gadget
Trinity was the code name of the first detonation of a nuclear weapon. It was conducted by the United States Army at 5:29 a.m. on July 16, 1945, as part of the Manhattan Project. The test was conducted in the Jornada del Muerto desert abo ...
used galvanic silver plating; afterwards,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
deposited from
nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
vapors was used,
but
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
is now preferred.
To produce the first pits,
hot pressing
Hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. This is achieved by the simultaneous application of heat a ...
was used to optimally employ the scarce plutonium. Later designs used
machined
Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...
pits, but
turning
Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helix toolpath by moving more or less linearly while the workpiece rotates.
Usually the term "turning" is reserved for the generation of ...
produces a large amount of waste, both as
pyrophoric
A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
turnings
Swarf, also known as chips or by other process-specific names (such as turnings, filings, or shavings), are pieces of metal, wood, or plastic that are the debris or waste resulting from machining, woodworking, or similar subtractive (material-r ...
of plutonium and plutonium-contaminated oils and
cutting fluid
Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining and stamping. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists) ...
s. The goal for the future is direct
casting
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
of the pit. In the absence of nuclear testing, however, the slightly different nature of cast and machined surfaces may cause difficult to predict performance differences.
Corrosion issues
Both uranium and plutonium are very susceptible to
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
. A number of the problem-plagued
W47
The W47 was an American thermonuclear warhead used on the Polaris A-1 sub-launched ballistic missile system. Various models were in service from 1960 through the end of 1974. The warhead was developed by the Lawrence Radiation Laboratory between 1 ...
UGM-27 Polaris
The UGM-27 Polaris missile was a two-stage solid-fueled nuclear-armed submarine-launched ballistic missile (SLBM). As the United States Navy's first SLBM, it served from 1961 to 1980.
In the mid-1950s the Navy was involved in the Jupiter missile ...
warheads had to be replaced after corrosion of the fissile material was discovered during routine maintenance. The
W58
The W58 was an American nuclear bomb, thermonuclear warhead used on the UGM-27 Polaris, Polaris A-3 submarine-launched ballistic missile. Three W58 warheads were fitted as multiple warheads on each Polaris A-3 missile.
The W58 was in diameter a ...
pits also suffered corrosion problems. The
W45 pit was prone to corrosion that could alter its geometry. The
Green Grass pit was also corrosion-prone. The radioactivity of the materials used can also cause
radiation corrosion in the surrounding materials. Plutonium is highly susceptible to humidity; moist air increases corrosion rate about 200 times. Hydrogen has strong catalytic effect on corrosion; its presence can accelerate corrosion rate by 13 orders of magnitude. Hydrogen can be generated from moisture and nearby organic materials (e.g. plastics) by
radiolysis
Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is ...
. These factors cause issues with storage of plutonium. The volume increase during oxidation can cause rupture of storage containers or deformation of pits.
Contamination of the pit with deuterium and tritium, whether accidental or if filled by design, can cause a hydride corrosion, which manifests as
pitting corrosion
Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation re ...
and a growth of a surface coating of
pyrophoric
A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
plutonium hydride
Plutonium hydride is a non-stoichiometric chemical compound with the formula PuH2+x. It is one of two characterised hydrides of plutonium, the other is PuH3.Gerd Meyer, 1991, Synthesis of Lanthanide and Actinide Compounds Springer, . PuH2 is non- ...
. It also greatly accelerates the corrosion rates by atmospheric oxygen.
Deuterium and tritium also cause
hydrogen embrittlement
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed ...
in many materials.
Improper storage can promote corrosion of the pits. The AL-R8 containers used in the
Pantex
Pantex is the primary United States nuclear weapons assembly and disassembly facility that aims to maintain the safety, security and reliability of the U.S. nuclear weapons stockpile. The facility is located in the Texas Panhandle, Panhandle of T ...
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.
Beryllium cladding can be corroded by some solvents used for cleaning of the pits. Research has shown that
trichloroethylene
The chemical compound trichloroethylene is a halocarbon commonly used as an industrial solvent. It is a clear, colourless non-flammable liquid with a chloroform-like sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, ...
(TCE) causes beryllium corrosion, while
trichloroethane Trichloroethane (CHCl) may refer to either of two isomeric chemical compounds:
* 1,1,1-Trichloroethane (methyl chloroform, CClCH)
* 1,1,2-Trichloroethane
1,1,2-Trichloroethane, or 1,1,2-TCA, is an organochloride solvent with the molecular formula ...
(TCA) does not.
Pitting corrosion
Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation re ...
of beryllium cladding is a significant concern during prolonged storage of pits in the
Pantex
Pantex is the primary United States nuclear weapons assembly and disassembly facility that aims to maintain the safety, security and reliability of the U.S. nuclear weapons stockpile. The facility is located in the Texas Panhandle, Panhandle of T ...
facility.
Isotopic composition issues
The presence of
plutonium-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 ...
in the pit material causes increased production of heat and neutrons, impairs fission efficiency and increases the risk of predetonation and
fizzle.
Weapon-grade plutonium therefore has plutonium-240 content limited to less than 7%.
Supergrade plutonium
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 ...
has less than 4% of the 240 isotope, and is used in systems where the radioactivity is a concern, e.g. in the
US Navy
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage of ...
weapons which have to share confined spaces on ships and submarines with the crews.
Plutonium-241
Plutonium-241 (241Pu or Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes (especially 239Pu), 241Pu is fissile, with a neutron absorption cross section about one-third greater t ...
, commonly comprising about 0.5% of weapon-grade plutonium, decays to
americium-241
Americium-241 (, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of . is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is com ...
, which is a powerful
gamma radiation
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
emitter. After several years, americium builds up in the plutonium metal, leading to increased gamma activity that poses an occupational hazard for workers. Americium should therefore be separated, usually chemically, from newly produced and reprocessed plutonium.
However, in around 1967 the
Rocky Flats Plant
The Rocky Flats Plant was a U.S. manufacturing complex that produced nuclear weapons parts in the western United States, near Denver, Colorado. The facility's primary mission was the fabrication of plutonium pits, which were shipped to ot ...
stopped this separation, blending up to 80% of old americium-containing pits directly to the foundry instead, in order to reduce costs and increase productivity; this led to higher exposure of workers to gamma radiation.
Aging issues
Metallic plutonium, notably in the form of the plutonium-gallium alloy, degrades chiefly by two mechanisms: corrosion, and self-irradiation.
In very dry air, plutonium, despite its high chemical reactivity, forms a passivation layer of
plutonium(IV) oxide
Plutonium(IV) oxide or (plutonia) is the chemical compound with the formula Pu O2. This high melting-point solid is a principal compound of plutonium. It can vary in color from yellow to olive green, depending on the particle size, temperature a ...
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
Plutonium hydride is a non-stoichiometric chemical compound with the formula PuH2+x. It is one of two characterised hydrides of plutonium, the other is PuH3.Gerd Meyer, 1991, Synthesis of Lanthanide and Actinide Compounds Springer, . PuH2 is non- ...
. 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 Pu
2O
3 layer, which then rapidly oxidizes to PuO
2, 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
alpha-decay. The decaying atom of
plutonium-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 ...
liberates an
alpha particle
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...
and a
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 ...
nucleus. The alpha particle has an energy of more than 5
MeV
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...
and in the metal lattice has range of about 10 micrometers; then it stops, acquires two electrons from nearby atoms, and becomes a
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
atom. The contaminant
plutonium-241
Plutonium-241 (241Pu or Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes (especially 239Pu), 241Pu is fissile, with a neutron absorption cross section about one-third greater t ...
beta-decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
s to
americium-241
Americium-241 (, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of . is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is com ...
, which then alpha-decays to
neptunium-237
Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be sy ...
.
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
Frenkel pair
In crystallography, a Frenkel defect is a type of point defect in crystalline solids, named after its discoverer Yakov Frenkel. The defect forms when an atom or smaller ion (usually cation) leaves its place in the lattice, creating a vacancy a ...
s and a local thermal spike lasting few picoseconds, during which the newly formed defects recombine or migrate. In a typical weapons-grade bulk material, each atom gets displaced in average once per 10 years.
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/cm
3. 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 cm
3 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.
Production and inspections
The
Radiation Identification System
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
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
Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics.
Most radioactive sources produce gamma rays, which are of various energi ...
with high-resolution
germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
detectors. The 870.7 keV line in the spectrum, corresponding to the first excited state of
oxygen-17
Oxygen-17 (17O) is a low-abundance, natural, stable isotope of oxygen (0.0373% in seawater; approximately twice as abundant as deuterium).
As the only stable isotope of oxygen possessing a nuclear spin (+5/2) and a favorable characteristic of fi ...
, indicates the presence of
plutonium(IV) oxide
Plutonium(IV) oxide or (plutonia) is the chemical compound with the formula Pu O2. This high melting-point solid is a principal compound of plutonium. It can vary in color from yellow to olive green, depending on the particle size, temperature a ...
in the sample. The age of the plutonium can be established by measuring the ratio of
plutonium-241
Plutonium-241 (241Pu or Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes (especially 239Pu), 241Pu is fissile, with a neutron absorption cross section about one-third greater t ...
and its decay product,
americium-241
Americium-241 (, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of . is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is com ...
. 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 Rocky Flats Plant was a U.S. manufacturing complex that produced nuclear weapons parts in the western United States, near Denver, Colorado. The facility's primary mission was the fabrication of plutonium pits, which were shipped to ot ...
; the plant was later closed due to numerous safety issues. The
Department of Energy A Ministry of Energy or Department of Energy is a government department in some countries that typically oversees the production of fuel and electricity; in the United States, however, it manages nuclear weapons development and conducts energy-rel ...
attempted to restart pit production there, but repeatedly failed. In 1993, the DOE relocated
beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...
production operations from defunct Rocky Flats Plant to
Los Alamos National Laboratory
Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, ...
; in 1996 the pit production was also relocated there. The reserve and surplus pits, along with pits recovered from disassembled nuclear weapons, totalling over 12,000 pieces, are stored in the
Pantex
Pantex is the primary United States nuclear weapons assembly and disassembly facility that aims to maintain the safety, security and reliability of the U.S. nuclear weapons stockpile. The facility is located in the Texas Panhandle, Panhandle of T ...
plant.
5,000 of them, comprising about 15 tons of plutonium, are designated as strategic reserve; the rest is surplus to be withdrawn. 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 The Reliable Replacement Warhead (RRW) was a proposed new American nuclear warhead design and bomb family that was intended to be simple, reliable and to provide a long-lasting, low-maintenance future nuclear force for the United States. Initiated ...
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
The Chemistry and Metallurgy Research Replacement Facility, usually referred to as the CMRR, is a facility under construction at Los Alamos National Laboratory in New Mexico which is part of the United States' nuclear stockpile stewardship program ...
(CMMR) will expand this capability, but it is not known by how much. An
Institute for Defense Analyses
The Institute for Defense Analyses (IDA) is an American non-profit corporation that administers three federally funded research and development centers (FFRDCs) – the Systems and Analyses Center (SAC), the Science and Technology Policy Institute ...
report written before 2008 estimated a “future pit production requirement of 125 per year at the CMRR, with a surge capability of 200."
Russia stores the material from decommissioned pits in the
Mayak
The Mayak Production Association (russian: Производственное объединение «Маяк», , from 'lighthouse') is one of the biggest nuclear facilities in the Russian Federation, housing a reprocessing plant. The closest ...
facility.
Pit recycling
Recovery of plutonium from decommissioned pits can be achieved by numerous means, both mechanical (e.g. removal of cladding by a
lathe
A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to c ...
) 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
Plutonium hydride is a non-stoichiometric chemical compound with the formula PuH2+x. It is one of two characterised hydrides of plutonium, the other is PuH3.Gerd Meyer, 1991, Synthesis of Lanthanide and Actinide Compounds Springer, . PuH2 is non- ...
, 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.
See also
*
Beryllium nuclear properties
*
Charles Allen Thomas
Charles Allen Thomas (February 15, 1900 – March 29, 1982) was a noted American chemist and businessman, and an important figure in the Manhattan Project. He held over 100 patents.
A graduate of Transylvania College and Massachusetts Institute ...
*
Dayton Project
The Dayton Project was a research and development project to produce polonium during World War II, as part of the larger Manhattan Project to build the first atomic bombs. Work took place at several sites in and around Dayton, Ohio. Those workin ...
*
Edward Condon
Edward Uhler Condon (March 2, 1902 – March 26, 1974) was an American nuclear physicist, a pioneer in quantum mechanics, and a participant during World War II in the development of radar and, very briefly, of nuclear weapons as part of the ...
*
Eugene Wigner
Eugene Paul "E. P." Wigner ( hu, Wigner Jenő Pál, ; November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist who also contributed to mathematical physics. He received the Nobel Prize in Physics in 1963 "for his con ...
*
George Kistiakowsky
George may refer to:
People
* George (given name)
* George (surname)
* George (singer), American-Canadian singer George Nozuka, known by the mononym George
* George Washington, First President of the United States
* George W. Bush, 43rd Preside ...
*
James L. Tuck
James Leslie Tuck Order of the British Empire, OBE, (9 January 1910 – 15 December 1980) was a British physicist. He was born in Manchester, England, and educated at the Victoria University of Manchester. Because of his involvement with the Manh ...
*
Modulated neutron initiator
A modulated neutron initiator is a neutron source capable of producing a burst of neutrons on activation. It is a crucial part of some nuclear weapons, as its role is to "kick-start" the chain reaction at the optimal moment when the configuration i ...
*
Munroe effect
A shaped charge is an explosive charge shaped to form an explosively formed penetrator (EFP) to focus the effect of the explosive's energy. Different types of shaped charges are used for various purposes such as cutting and forming metal, ini ...
*
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character ...
*
Supergrade plutonium
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 ...
*
Urchin
References
{{DEFAULTSORT:Pit (Nuclear Weapon)
Nuclear weapons
Plutonium
Nuclear weapon implosion
Nuclear weapon design