Ab initio calculations for void swelling bias in α- and β-plutonium

Void swelling can develop in materials under persistent irradiation when nonequilibrium vacancy and self-interstitial populations migrate under sufficiently asymmetric interaction biases. In conventional metals, the propensity is determined to the 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. We discuss the pros and cons of various levels of electronic structure theory: spin polarization, spin-orbit coupling, and orbital polarization. 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. Surprisingly, the point defects in alpha-Pu are found to induce far larger transformation of the local electronic structure than in delta-Pu. 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 the 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 the 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.

Automated summary

This study presents DFT calculations of point defects in the alpha and the delta phases of plutonium and discusses the pros and cons of various levels of electronic structure theory. The results show that lattice defects in delta-Pu have small formation volumes, while those in alpha-Pu have large defect formation volumes. The influence of point defects on the local electronic structure is found to be far larger in alpha-Pu than in delta-Pu. Therefore, swelling rates and mechanisms can differ significantly between the different phases of Pu.