Hydrogen diffusion and precipitation under non-uniform stress in duplex zirconium nuclear fuel cladding investigated by high-resolution neutron imaging

With the rising burn-up of nuclear fuel rods, the influence of hydrogen is of great importance when considering formation of brittle hydrides and their potential to the detriment of the mechanical integrity of nuclear fuel cladding. This work presents investigations on hydrogen diffusion and hydride precipitation in an unirradiated duplex cladding in response to thermo-mechanical cycles, by using high-resolution neutron imaging in correlation with finite element stress computation and hydride metallography analysis. The results demonstrate that the external stress, depending on the specific distribution, can work reversely or synergistically with the material heterogeneity in hydrogen migration. The material heterogeneity is identified as the prevailing driving force for hydrogen diffusion, whereas the applied stress contributes to hydrogen field variation only locally. Hydride precipitation is found to be significantly restrained in compressed area. In the presence of tensile hoop stress, hydride reorientation-induced elevation 40∼60 wppm of hydrogen concentration is identified, which could be attributed to the precipitation temperature difference between the radial and circumferential hydrides.