Hydride Reprecipitation in Prototypical 17×17 PWR Fuel Rods after Simulated Vacuum Drying Conditions

Light water reactor (LWR) fuel rod cladding picks up hydrogen as it oxidizes in reactor. Once the hydrogen concentration exceeds the solubility limit, zirconium hydride platelets precipitate, generally in the circumferential direction. In preparation for dry storage, elevated temperatures can dissolve some of the circumferential hydrides, which during cooling can then reprecipitate in a direction perpendicular to the hoop stress (radially). From 2019 to 2024, Oak Ridge National Laboratory (ORNL) published a series of reports that included metallographic observations of cladding after a thermal transient to 400°C followed by slow cooling. The simulated dry storage transient was performed on three full-length 17×17 LWR fuel rods at their as-discharged rod internal pressure. There were two important observations from the metallographic examinations of the heat-treated rods: (1) the propensity for hydride reorientation is strongly influenced by nearby as-discharged precipitated hydrides, (2) the radial hydrides observed near the cladding inner diameter were often associated with cracks in the pellet. While mechanical testing found that the radial hydrides in the cladding did not decrease ductility at 20°C, subsequent finite element modeling supports the conclusion that pellet cracks enhance the local hoop stress in the cladding creating preferential sites for hydride reorientation and that the influence of the pellet cannot be ignored in accurate fuel rod performance predictions.