Ab initio atomic-scale modelling of iodine effects on hcp zirconium

Abstract

In order to elucidate possible mechanisms of stress corrosion cracking of nuclear power plant cladding material, we have investigated the iodine–zirconium interactions using ab initio atomic-scale calculations. We show that for the gas pressure estimated during the reactor power transients, the reduction of the zirconium effective free surface energy induced by the adsorption of atomic iodine is significant (more than 80%). Furthermore, for given iodine partial pressure, the surface energy reduction for the basal planes is higher than for the prismatic ones, in agreement with the experimental observation of a cleavage along the basal planes. We have also estimated the iodine surface diffusion coefficient, and its high value (about 10⁻⁶ cm² s⁻¹ at 600 K) indicates that iodine is mobile enough to follow the crack tip during the cracking experiments reproduced in laboratory conditions. Our results clearly rule out the influence of absorbed iodine during the cracking process, its equilibrium concentration being totally negligible.

Acknowledgements

This work is part of the REVE (virtual reactor) project. Part of the calculations were made at C.I.N.E.S. and I.D.R.I.S. French national computational centres as well as at the C.R.I. of the U.S.T.L. supported by the Fonds Européens de Développement Régional. Part of this work was done on the supercomputers at CEA Grenoble in the framework of an EDF-CEA contract.