  Void swelling can develop in materials under persistent irradiation when
non-equilibrium vacancy and self-interstitial populations migrate under
sufficiently asymmetric interaction biases. In conventional metals, the
propensity is determined to first approximation by comparing point-defect
relaxation strains. We thus present DFT-based calculations of structures and
formation energies and volumes of point defects in the $\alpha$ and the
$\delta$-phases of plutonium. Our results show that lattice defects in
$\delta$-Pu, in contrast to most fcc metals, have surprisingly small formation
volumes. Equally unexpected are the large defect formation volumes found in the
low-symmetry $\alpha$-Pu phase. Both these unusual properties can be
satisfactorily explained from defect-induced spin/orbital moment formation and
destruction in the Pu phases. When we use the calculated defect properties to
estimate the classic void swelling bias in each of the phases, we find it to be
unusually small in $\delta$-Pu, but likely much larger in $\alpha$-Pu. Hence,
swelling rates and mechanisms can diverge dramatically between the different
phases of Pu. Especially in the transient regime before formation of large
defect clusters, the swelling rate of $\alpha$-Pu can reliably be expected to
be much larger than $\delta$-Pu. However, accurate forecasts over longer times
will require the conventional void-swelling theory to be modified to handle the
complexities presented by the different Pu phases. As a case in point, we show
possible anomalous temperature dependence of vacancy properties in $\delta$-Pu,
caused by entropic contributions from defect-induced spin-lattice fluctuations.
Such complications may affect defect-defect interactions and thus alter the
void swelling bias.

Materials Science