Plutonium–gallium alloy

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. This alloy was developed during the Manhattan Project.

Metallic plutonium has several different solid allotropes. The δ phase is the least dense and most easily machinable. It is formed at temperatures of 310–452 °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.

Pu–Ga has many practical advantages:

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 plutonium 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. Under 4 mol.% gallium the pressure-induced phase change is irreversible.

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.

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 to become 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.

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