Plutonium–gallium alloy
alloy used in nuclear weapon pits
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.
Overview
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:
stable between −75 and 475 °C,
very low thermal expansion,
low susceptibility to corrosion (4% of the corrosion rate of pure plutonium),
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.
Use in nuclear weapons
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.
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