  The stability properties of vacancy clusters in hexagonal close-packed Zr,
cavities and dislocation loops, are investigated at the atomic scale, with a
modeling approach based on density functional theory and empirical potentials.
Considering the vacancy-vacancy interactions and the stability of small vacancy
clusters, we establish how to build the larger clusters. The study of extended
vacancy clusters is then performed using continuous laws for defect energetics.
Once validated with an empirical potential, these laws are parameterized with
ab initio data. Our work shows that the easy formation of loops can be
explained by their thermodynamic properties.
