Kalyana sougandhikam wiki
There are several plutonium and gallium intermetallic compounds: PuGa, Pu 3Ga, and Pu 6Ga.ĭuring aging of the stabilized δ alloy, gallium segregates from the lattice, forming regions of Pu 3Ga (ζ'-phase) within α phase, with the corresponding dimensional and density change and buildup of internal strains. 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. The presence of gallium in plutonium signifies its origin from weapon plants or decommissioned nuclear weapons. 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. The time to achieve homogenization of gallium increases with increasing grain size of the alloy and decreases with increasing temperature.
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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. Gallium tends to segregate in plutonium, causing "coring"-gallium-rich centers of grains and gallium-poor grain boundaries. The alloy is denser in molten state than in solid state, which poses an advantage for casting as the tendency to form bubbles and internal defects is decreased. 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. Addition of gallium causes the bonds to become more even, increasing the stability of the δ phase. 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. Under these conditions, δ phase Pu–Ga transforms to α phase, which is 25% denser and thus more critical. As the reaction starts, it generates enormous pressures, in the range of hundreds of gigapascals. However, the phase change is useful during the operation of a nuclear weapon. Under 4 mol.% gallium the pressure-induced phase change is irreversible. Prevention of the phase transition and the associated mechanical deformations and consequent structural damage and/or loss of symmetry is of critical importance. The transition between δ-phase and α-phase plutonium occurs at a low temperature of 115 ☌ and can be reached by accident. There are substantial differences of density (and therefore volume) between the various phases.
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Δ phase Pu–Ga is still thermodynamically unstable, so there are concerns about its aging behavior. As only the ε-δ transition occurs during cooling, casting Pu-Ga is easier than casting pure plutonium. Subcritical testing showed that wrought and cast plutonium performance is the same. More modern pits are produced by casting. This method was used for forming the first nuclear weapon pits. It is suitable for shaping by hot pressing at about 400 ☌. Stabilized δ-phase Pu–Ga is ductile, and can be rolled into sheets and machined by conventional methods. good castability since plutonium has the rare property that the molten state is denser than the solid state, the tendency to form bubbles and internal defects is decreased.low susceptibility to corrosion (4% of the corrosion rate of pure plutonium),.
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However, plutonium can be stabilized in the δ phase by alloying it with a small amount of another metal. It is formed at temperatures of 310–452 ☌ at ambient pressure (1 atmosphere), and is thermodynamically unstable at lower temperatures. The δ phase is the least dense and most easily machinable. Metallic plutonium has several different solid allotropes. This alloy was developed during the Manhattan Project. Plutonium–gallium alloy ( Pu–Ga) 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.