Hydrogen in Zircaloy-4: effects of the neutron irradiation on the hydride formation

Abstract

X-ray diffraction patterns of neutron irradiated Zircaloy-4 samples were obtained at the Brazilian Synchrotron Light Laboratory (LNLS) to study the effects of the fast neutron fluxes and post-irradiation thermal treatments on the zirconium hydride evolution. The high intensity and resolution of the synchrotron beam allowed to detect the (111) and (220) diffraction peaks of the ZrH1.5+x d-phase in unirradiated Zircaloy-4 samples having a hydrogen concentration as low as 20 wppm. Then, irradiated samples, that were taken from the Zircaly-4 core components of the argentine HWPR Atucha I Nuclear Power Plant (CNA-1) were studied with that radiation to detect the diffraction peaks of ZrH1.5+x d-phase particles precipitated in samples having hydrogen isotope concentration lower than 2 at. %. A significant increment of the (111) and (220) peak areas were observed at room temperature after post-irradiation thermal treatments at 600ºC during 4 hours. These results indicate that after the annealing a hydrogen concentration between 35 to 70 wppm -which were apparently absent in the irradiated samples- has precipitated at room temperature as zirconium hydrides. That amount was estimated from a (220) peak area vs the bulk H concentration regression line, made with unirradiated samples. These results get support to a hydrogen trapping hypothesis proposed in previous works. It addition, it has been shown that the zirconium hydride that precipitate in the irradiated samples have the equilibrium d-ZrH1.5+x cubic crystalline structure.