During the corrosion process of zirconium alloys, hydrogen is picked up by the cladding, precipitating into hydrides when the solubility limit is surpassed. The orientation of the hydrides in either tangential or radial directions has an impact on the fuel cladding ductility with radially oriented hydrides being the most detrimental to the cladding ductility. Recent results from Japan Atomic Energy Agency (JAEA) suggested that a Boiling Water Reactor (BWR) fuel rod operated in the Oskarshamn 3 BWR, with alumina and chromia doped (ADOPT™) pellets had a greater concentration of radially oriented hydrides in one position for the particular segment tested as compared to a sibling fuel rod with un-doped pellets. However, recent hot cell work on BWR fuel rods with Zircaloy-2 re-crystalized annealed (RXA) (LK3) liner cladding at end of life from both Leibstadt and Forsmark Nuclear Power Plants (NPPs) have indicated no such correlation of radial hydrides with ADOPT pellets.

Three rods, two with ADOPT pellets and one with un-doped pellets, were irradiated for eight annual cycles in Leibstadt NPP and cladding samples from three different elevations were investigated via Scanning Electron Microscope (SEM) or Light Optical Microscope (LOM) for hydride orientation. Although the hydrogen content remained below 250 w-ppm, some radial hydrides were nonetheless seen in the sample with the highest hydrogen content. Similarly, with the Oskarshamn 3 rod, some clusters of radial hydrides were observed in this sample. This fuel rod contained only un-doped pellets. The fuel rods with the ADOPT pellets had only insignificant amounts of radial hydrides, also for the samples with hydrogen above the solubility limit during operation. Additionally, cladding samples from an ADOPT fueled rod from the Forsmark NPP was examined with SEM. The fuel rod had operated for a similar time as the Oskarshamn 3 rod in the JAEA study, while the power history was more typical to normal operation. The hydrogen content was low, with only limited amount of hydrogen above the solubility limit, and the hydride orientation in the investigated samples were only in tangential direction.