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

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)_δ and (220)_δ diffraction peaks of the ZrH_1.5 + x (0 ≤ x≤ 0.16) δ-phase in unirradiated Zircaloy-4 samples having a hydrogen concentration as low as 0.2 at.% (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 ZrH_1.5 + x δ-phase particles precipitated in samples having hydrogen isotope concentration lower than 2 at.% (220 wppm). A significant increment of the (111)_δ and (220)_δ peak areas were observed at room temperature after post-irradiation thermal treatments at 600 °C during 4 h. 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)_δ peak area versus the bulk H concentration regression line, made with unirradiated samples. These results get support to a hydrogen trapping hypothesis proposed in previous works. In addition, it has been shown that the zirconium hydride that precipitate in the irradiated samples have the equilibrium δ-ZrH_1.5 + x cubic crystalline structure.

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