Plutonium-gallium alloy
Encyclopedia
Plutonium-gallium alloy is an alloy
of plutonium
and gallium
, used in nuclear weapon pits
– the component of a nuclear weapon where the fission chain reaction is started.
Metallic plutonium has several different solid allotropes
. The δ phase is the least dense and most easily machinable. It is formed at temperatures of 310 °C to 452 °C at ambient pressure (1 atmosphere), and is thermodynamically unstable at lower temperatures. However, plutonium can be stabilized in the δ phase by alloying it with a small amount of another metal. The preferred alloy is 3.0–3.5 mol.% (0.8–1.0 wt.%) gallium
. This alloy was developed during the Manhattan Project
.
Pu-Ga is stable between at least −75 to 475 °C, and has very low thermal expansion
. The presence of gallium also significantly lowers the susceptibility of plutonium to corrosion, to 4% of the corrosion rate of pure plutonium. The alloy is more dense in the molten state than in the solid state, which poses an advantage for casting as the tendency to form bubbles and internal defects is decreased.
Stabilized δ-phase Pu-Ga is ductile, and can be rolled into sheets and machined by conventional methods. It is suitable for shaping by hot pressing
at about 400 °C. This method was used for forming the first nuclear weapon pits.
More modern pits are produced by casting. Subcritical testing showed that wrought and cast plutonium performance is the same. As only the ε-δ transition occurs during cooling, casting Pu-Ga is much less problematic than casting pure plutonium.
δ phase Pu-Ga is still thermodynamically unstable, so there are concerns about its aging behavior. There are substantial differences of density (and therefore volume) between the various phases. The transition between δ-phase and α-phase Pu, occurs at a low temperature of 115 °C and can be reached by accident. Prevention of the phase transition and the associated mechanical deformations and consequent structural damage and/or loss of symmetry is of critical importance.
However, the phase change is useful during the operation of a nuclear weapon. As the reaction starts, it generates enormous pressures, in the range of hundreds of gigapascals. Under these conditions, δ phase Pu-Ga transforms to α phase, which is 25% denser and thus more critical
.
Gallium tends to segregate in plutonium, causing "coring" – gallium-rich centers of grains and gallium-poor grain boundaries. To stabilize the lattice and reverse and prevent segregation of gallium, annealing
is required at the temperature just below the δ-ε phase transition, so gallium atoms can diffuse through the grains and create homogeneous structure. The time to achieve homogenization of gallium increases with increasing grain size of the alloy and decreases with increasing temperature. The structure of stabilized plutonium at room temperature is the same as unstabilized at δ-phase temperature, with the difference of gallium atoms substituting plutonium in the fcc lattice.
Under 4 mol.% gallium the pressure-induced phase change is irreversible.
Plutonium in its α phase has a low internal symmetry, caused by uneven bonding between the atoms, more resembling (and behaving like) a ceramic than a metal. Addition of gallium causes the bonds becoming more even, increasing the stability of the δ phase. The α phase bonds are mediated by the 5f shell
electrons, and can be disrupted by increased temperature or by presence of suitable atoms in the lattice which reduce the available number of 5f electrons and weaken their bonds. The alloy is more dense in molten state than in solid state, which poses an advantage for casting as the tendency to form bubbles and internal defects is decreased.
During the Manhattan Project, the maximum amount of diluent atoms for plutonium to not affect the explosion efficiency was calculated to be 5 mol.%. Two stabilizing elements were considered, silicon
and aluminium
. However, only aluminium produced satisfactory alloys. But the aluminium tendency to react with α-particles and emit neutrons limited its maximum content to 0.5 mol.%; the next element from the boron group
of elements, gallium, was tried and found satisfactory.
There are several plutonium and gallium intermetallic compounds: PuGa, Pu3Ga, and Pu6Ga.
During aging of the stabilized δ alloy, gallium segregates from the lattice, forming regions of Pu3Ga (ζ'-phase) within α phase, with the corresponding dimensional and density change and buildup of internal strains. The decay of plutonium however produces energetic particles (alpha particle
s and uranium-235
nuclei) that cause local disruption of the ζ' phase, and establishing a dynamic equilibrium
with only a modest amount of ζ' phase present, which explains the alloy's unexpectedly slow, graceful aging. The alpha particles are trapped as interstitial helium
atoms in the lattice, coalescing into tiny (about 1 nm diameter) helium-filled bubbles in the metal and causing negligible levels of void swelling; the size of bubbles appears to be limited, though their number increases with time.
Addition of 7.5 wt.% of plutonium-238
, which has significantly faster decay rate, to the alloy increases the aging damage rate by 16 times, assisting with plutonium aging research. The Blue Gene
supercomputer aided with simulations of plutonium aging processes.
Presence of gallium in plutonium signifies its origin from weapon plants or decommissioned nuclear weapons. The isotopic signature
of plutonium then allows rough identification of its origin, manufacturing method, type of the reactor used in its production, and rough history of the irradiation, and matching to other samples, which is of importance in investigation of nuclear smuggling.
For reprocessing of surplus warhead pits into MOX fuel
, the majority of gallium has to be removed as its high content could interfere with the fuel rod cladding (gallium attacks zirconium
) and with migration of fission products in the fuel pellets. In the ARIES process, the pits are converted to oxide by converting the material to plutonium hydride
, then optionally to nitride, and then to oxide. Gallium is then mostly removed from the solid oxide mixture by heating at 1100°C in a 94% argon 6% hydrogen atmosphere, reducing gallium content from 1% to 200 ppm. Further dilution of plutonium oxide during the MOX fuel manufacture brings gallium content to levels considered negligible. A wet route of gallium removal, using ion exchange
, is also possible. Electrorefining is another way to separate gallium and plutonium.
For weapons use, the plutonium pit parts have to be coated with a layer of another metal. The first attempts used galvanically deposited silver. Subsequent pits were coated with nickel
, by exposing the plutonium parts to nickel tetracarbonyl gas, which reacts with the plutonium surface and deposits a thin layer of nickel. Evaporation coating with aluminium and electroplating with zinc
were shown to not work.
Plutonium alloys can be produced by adding a metal to molten plutonium. However, if the alloying metal is sufficiently reductive, plutonium can be added in the form of oxides or halides. The δ phase plutonium-gallium and plutonium-aluminium alloys are produced by adding plutonium(III) fluoride
to molten gallium or aluminium, which has the advantage of avoiding dealing directly with the highly reactive plutonium metal.
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
of plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
and gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
, used in nuclear weapon pits
Pit (nuclear weapon)
The pit is the core of an implosion weapon – the fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium, but all-plutonium pits are the smallest in diameter and have been the standard...
– the component of a nuclear weapon where the fission chain reaction is started.
Metallic plutonium has several different solid allotropes
Allotropes of plutonium
Even at ambient pressure, plutonium occurs in a variety of allotropes. These allotropes differ widely in crystal structure and density; the α and δ allotropes differ in density by more than 25% at constant pressure....
. The δ phase is the least dense and most easily machinable. It is formed at temperatures of 310 °C to 452 °C at ambient pressure (1 atmosphere), and is thermodynamically unstable at lower temperatures. However, plutonium can be stabilized in the δ phase by alloying it with a small amount of another metal. The preferred alloy is 3.0–3.5 mol.% (0.8–1.0 wt.%) gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
. This alloy was developed during the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
.
Pu-Ga is stable between at least −75 to 475 °C, and has very low thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
. The presence of gallium also significantly lowers the susceptibility of plutonium to corrosion, to 4% of the corrosion rate of pure plutonium. The alloy is more dense in the molten state than in the solid state, which poses an advantage for casting as the tendency to form bubbles and internal defects is decreased.
Stabilized δ-phase Pu-Ga is ductile, and can be rolled into sheets and machined by conventional methods. It is suitable for shaping by hot pressing
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 and pressure....
at about 400 °C. This method was used for forming the first nuclear weapon pits.
More modern pits are produced by casting. Subcritical testing showed that wrought and cast plutonium performance is the same. As only the ε-δ transition occurs during cooling, casting Pu-Ga is much less problematic than casting pure plutonium.
δ phase Pu-Ga is still thermodynamically unstable, so there are concerns about its aging behavior. There are substantial differences of density (and therefore volume) between the various phases. The transition between δ-phase and α-phase Pu, occurs at a low temperature of 115 °C and can be reached by accident. Prevention of the phase transition and the associated mechanical deformations and consequent structural damage and/or loss of symmetry is of critical importance.
However, the phase change is useful during the operation of a nuclear weapon. As the reaction starts, it generates enormous pressures, in the range of hundreds of gigapascals. Under these conditions, δ phase Pu-Ga transforms to α phase, which is 25% denser and thus more critical
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
.
Gallium tends to segregate in plutonium, causing "coring" – gallium-rich centers of grains and gallium-poor grain boundaries. To stabilize the lattice and reverse and prevent segregation of gallium, annealing
Annealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
is required at the temperature just below the δ-ε phase transition, so gallium atoms can diffuse through the grains and create homogeneous structure. The time to achieve homogenization of gallium increases with increasing grain size of the alloy and decreases with increasing temperature. The structure of stabilized plutonium at room temperature is the same as unstabilized at δ-phase temperature, with the difference of gallium atoms substituting plutonium in the fcc lattice.
Under 4 mol.% gallium the pressure-induced phase change is irreversible.
Plutonium in its α phase has a low internal symmetry, caused by uneven bonding between the atoms, more resembling (and behaving like) a ceramic than a metal. Addition of gallium causes the bonds becoming more even, increasing the stability of the δ phase. The α phase bonds are mediated by the 5f shell
Electron shell
An electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
electrons, and can be disrupted by increased temperature or by presence of suitable atoms in the lattice which reduce the available number of 5f electrons and weaken their bonds. The alloy is more dense in molten state than in solid state, which poses an advantage for casting as the tendency to form bubbles and internal defects is decreased.
During the Manhattan Project, the maximum amount of diluent atoms for plutonium to not affect the explosion efficiency was calculated to be 5 mol.%. Two stabilizing elements were considered, silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
. However, only aluminium produced satisfactory alloys. But the aluminium tendency to react with α-particles and emit neutrons limited its maximum content to 0.5 mol.%; the next element from the boron group
Boron group
The boron group is the series of elements in group 13 of the periodic table, comprising boron , aluminium , gallium , indium , thallium , and ununtrium . The elements in the boron group are characterized by having three electrons in their outer energy levels...
of elements, gallium, was tried and found satisfactory.
There are several plutonium and gallium intermetallic compounds: PuGa, Pu3Ga, and Pu6Ga.
During aging of the stabilized δ alloy, gallium segregates from the lattice, forming regions of Pu3Ga (ζ'-phase) within α phase, with the corresponding dimensional and density change and buildup of internal strains. The decay of plutonium however produces energetic particles (alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s and uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
nuclei) that cause local disruption of the ζ' phase, and establishing a dynamic equilibrium
Dynamic equilibrium
A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state...
with only a modest amount of ζ' phase present, which explains the alloy's unexpectedly slow, graceful aging. The alpha particles are trapped as interstitial helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
atoms in the lattice, coalescing into tiny (about 1 nm diameter) helium-filled bubbles in the metal and causing negligible levels of void swelling; the size of bubbles appears to be limited, though their number increases with time.
Addition of 7.5 wt.% of plutonium-238
Plutonium-238
-External links:**...
, which has significantly faster decay rate, to the alloy increases the aging damage rate by 16 times, assisting with plutonium aging research. The Blue Gene
Blue Gene
Blue Gene is a computer architecture project to produce several supercomputers, designed to reach operating speeds in the PFLOPS range, and currently reaching sustained speeds of nearly 500 TFLOPS . It is a cooperative project among IBM Blue Gene is a computer architecture project to produce...
supercomputer aided with simulations of plutonium aging processes.
Presence of gallium in plutonium signifies its origin from weapon plants or decommissioned nuclear weapons. The isotopic signature
Isotopic signature
An isotopic signature is a ratio of stable or unstable isotopes of particular elements found in an investigated material...
of plutonium then allows rough identification of its origin, manufacturing method, type of the reactor used in its production, and rough history of the irradiation, and matching to other samples, which is of importance in investigation of nuclear smuggling.
For reprocessing of surplus warhead pits into MOX fuel
MOX fuel
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material. MOX fuel contains plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the...
, the majority of gallium has to be removed as its high content could interfere with the fuel rod cladding (gallium attacks zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
) and with migration of fission products in the fuel pellets. In the ARIES process, the pits are converted to oxide by converting the material to plutonium hydride
Plutonium hydride
Plutonium hydride is the chemical compound with the formula PuH2. It is one of two characterised hydrides of plutonium, the other is PuH3. PuH2 is non-stoichiometric with a composition range of PuH2 – PuH2.7. Additionally metastable stoichiometries with an excess of hydrogen can be formed. PuH2...
, then optionally to nitride, and then to oxide. Gallium is then mostly removed from the solid oxide mixture by heating at 1100°C in a 94% argon 6% hydrogen atmosphere, reducing gallium content from 1% to 200 ppm. Further dilution of plutonium oxide during the MOX fuel manufacture brings gallium content to levels considered negligible. A wet route of gallium removal, using ion exchange
Ion exchange
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
, is also possible. Electrorefining is another way to separate gallium and plutonium.
For weapons use, the plutonium pit parts have to be coated with a layer of another metal. The first attempts used galvanically deposited silver. Subsequent pits were coated with nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, by exposing the plutonium parts to nickel tetracarbonyl gas, which reacts with the plutonium surface and deposits a thin layer of nickel. Evaporation coating with aluminium and electroplating with zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
were shown to not work.
Plutonium alloys can be produced by adding a metal to molten plutonium. However, if the alloying metal is sufficiently reductive, plutonium can be added in the form of oxides or halides. The δ phase plutonium-gallium and plutonium-aluminium alloys are produced by adding plutonium(III) fluoride
Plutonium(III) fluoride
Plutonium fluoride or plutonium trifluoride is the chemical compound composed of plutonium and fluorine with the formula PuF3. It forms violet crystals...
to molten gallium or aluminium, which has the advantage of avoiding dealing directly with the highly reactive plutonium metal.