Mechanical properties of zirconium alloy components, influence of irradiation and hydrogen

Operation of the active tensile-testing machine; active sample handling using manipulators.
Once hydrogen has diffused into the cladding, it influences the mechanical properties. Hydrogen in solution can enhance the creep rate of the cladding at high temperature. A concentration above the hydrogen solution limit leads to precipitation into hydrides. The hydrides are brittle and reduce the ductility of the material. If hydrides are oriented in the unfavorable radial direction of the cladding, the tendency to brittle failure can be enhanced. The distribution/location, length and orientation of each individual hydride on a 360° cross section of a fuel rod and related statistical information can be obtained with the PSI image analysis software HYDIVA. The hydrides distribution and morphology can closely be linked with the prevailing stress conditions.

Another hydrogen effect is the so-called Delayed Hydride Cracking (DHC) where hydrogen diffuses to stress raisers (e.g. scratches, small cracks), precipitates in the stress field, fails and extends a crack. All these phenomena are investigated with thermo-mechanical testing (hydrides re-orientation, DHC), hydrogenation and diffusion experiments, accompanied by modelling of stress fields in the non-standard samples and by modelling of hydrogen diffusion and precipitation.
Schematic view of the setup used to carry thermo-mechanical tests of a cladding tube sample; cladding fracture toughness testing.
Load-Displacement curves for un-irradiated and irradiated cladding sections during fracture toughness testing.
Left – Hoop stress state during tensile testing of a cladding tube sample: this setup is particularly used to test the reorientation properties of zirconium hydrides. Right – Hydrides distribution in a part of a cladding cross section.