Plutonium - an element never at equilibrium

The relentless deposition of energy from the alpha-particle decay of plutonium damages its crystal lattice and transmutes plutonium into other elements over time (principally, helium, americium, uranium, and neptunium). At cryogenic temperatures (4 K), lattice damage causes significant volume expansion of pure plutonium and contraction of face-centered-cubic stabilized alloys, and both appear to lose crystallinity at long irradiation times. At room temperature, much of the lattice damage is annealed out because defects produced by self-irradiation are sufficiently mobile. Nevertheless, plutonium's delicate balance of stability with changes in temperature, pressure, or chemistry may be affected by self-irradiation. For example, at room temperature the lattice of fcc plutonium alloys expands and exhibits nanoscale bubbles at irradiation levels < 0.1 displacements per atom (dpa). In addition to self-irradiation damage, it is now generally agreed that most fcc alloys previously believed to be thermodynamically stable at room temperature are in fact metastable. They undergo eutectoidal decomposition to alpha-plutonium, plus the nearest intermetallic compound. However, for most practical purposes, the kinetics of phase decomposition are too slow to be of concern. So, although plutonium may not be far from equilibrium, it is never at equilibrium because of the very nature of its radioactive decay. Surface reactions in plutonium can be increased catastrophically by the presence of moist air or hydrogen.