Proton irradiation induced microstructural evolution in Nb containing and Nb free Zirconium alloys and the effect on mechanical properties

Abstract

Zirconium (Zr) alloys play a crucial role in the safe and efficient operation of light water reactors. Irradiation induces two types of dislocation loops in Zr, distinguished by their Burgers vectors. The formation of and type loops is directly related to the macroscopic elongation of fuel rods, a phenomenon known as irradiation-induced growth (IIG). IIG is a significant factor limiting the lifespan of nuclear reactors. Therefore, it is crucial to develop a comprehensive understanding of the underlying mechanisms governing dislocation loop formation and IIG. This thesis work was undertaken with the aim of enhancing our understanding of the mechanisms driving IIG. This study utilised proton irradiation to investigate the irradiation induced microstructural evolution in Zr and its alloys. Several techniques were employed for the characterisation of dislocation loop formation, chemical segregation, mechanical property change, and their overall relationship. The effect of individual alloying element additions to zirconium on its radiation performance was investigated through proton irradiating several binary, unalloyed Zr, and commercial Zircaloy-2 and Low-Sn ZIRLO alloys. This allowed separating the effects of each element on the overall microstructural evolution utilising XRD and TEM techniques and made a comparative study possible on different alloy systems. Proton irradiation depth dependent damage profile was exploited with spatially resolved microbeam synchrotron XRD experiments. Additionally, nanoindentation hardness with the same dpa dislocation density were correlated to produce a dislocation loop size dependent obstacle strengthening relationship. Further, proton irradiations looking into the effect of dose rate revealed no significant change in the dislocation density on Zircaloy-2 and Low-Sn ZIRLO. However, notable dislocation density difference between the two alloy were observed. Early Nb nanoclustering was confirmed thorough utilising APT technique and it was found that highest dose rate sample has the smallest cluster size. Additionally, clear loop alignment without any Fe/Sn banding was observed in Zircaloy-2 which was less pronounced in Low-Sn ZIRLO. Lastly, dislocation density evolution as a function of dpa was compared from XRD and TEM measurements utilising FIB sample preparation. The results indicate some discrepancy especially towards higher dpa.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philipp Frankel (Supervisor) & Robert Harrison (Supervisor)