Mitsubishi PWR fuel assembly ZDP-1 with high reliability to avoid GTRF wear

In recent years, high burn-up fuel (∼55 GWd/t (Assembly)) experienced some fuel leakages in Japanese PWRs. Grid-to-Rod fretting (GTRF) wear at the bottom grid position in some fuel assemblies was observed through fiberscope in onsite inspection and GTRF was considered to be cause of leakage issues. As a countermeasure to these fuel leakages, Mitsubishi Nuclear Fuel (MNF) has developed ZDP-1 fuel assembly. ZDP stands for "Zero Defect Performance". ZDP-1 is MNF's first design of ZDP fuel series. ZDP-1 fuel assembly is designed to make more margins against GTRF wear at the bottom grid. In this paper, the improvements of ZDP-1 and the wear evaluation results of ZDP-1 are explained, together with the situation of the wear issue and results of root cause analysis on the fuel leakages. (author)