Equilibrium thermodynamics of helium in δ-phase Pu-Ga alloys

The paper presents a theoretical study of the response of δ-phase Pu-Ga alloys to self-irradiation. We investigate the long-term behaviour of He atoms in the face-centred cubic (fcc) lattice under ambient conditions by means of classical molecular dynamics. Because of the relatively low diffusive mobility of He atoms in the lattice their dynamics cannot be tracked directly in molecular dynamics simulations. Instead, we use the Helmholtz free energy function to investigate the equilibrium thermodynamics of metastable microconfigurations of perfect crystals with artificially introduced He atoms in different configurations. The Helmholtz free energy of the microconfigurations are calculated using the thermodynamic integration method. Based on the free energy evaluation, inferences about the relative thermodynamic stability of various microconfigurations under ambient conditions are drawn. Estimates of the equilibrium parameters of He bubbles in the lattice and the distribution of He bubble radii are calculated and compared to those observed experimentally.