  The effect of solute hydrogen on the stability of vacancy clusters in
hexagonal closed packed zirconium is investigated with an ab initio approach,
including contributions of H vibrations. Atomistic simulations within the
density functional theory evidence a strong binding of H to small vacancy
clusters. The hydrogen effect on large vacancy loops is modeled through its
interaction with the stacking faults. A thermodynamic modeling of H segregation
on the various faults, relying on ab initio binding energies, shows that these
faults are enriched in H, leading to a decrease of the stacking fault energies.
This is consistent with the trapping of H by vacancy loops observed
experimentally. The stronger trapping, and thus the stronger stabilization, is
obtained for vacancy loops lying in the basal planes, i.e. the loops
responsible for the breakaway growth observed under high irradiation dose.
