Wear inspection for fuel channels in HANARO

It has been observed that fuel assemblies have mechanical damage on some components due to the flow-induced vibration in the fuel channels. The major damage was the fretting-wear on the bottom end plates and spacer plates. Both the components are aluminum but cause the fretting-wear on the corresponding surfaces of the zirconium fuel channels. Also the bottom guide arms and top springs of the fuel are considered to be major parts that cause wear on the fuel channels. Therefore, the inspection of the inner surfaces of the fuel channels is required from a lifetime point of view. It is very difficult and time-consuming work to remove and install the fuel channels because of their inherent characteristics and the physical interference of other components in the reactor core. Thus we developed special tools for the inspection of the fuel channels by using an impression material without the removal of the reactor components. The impression material is a compound to replicate the damage of the fuel channels within a limited working time considering the hardening time as well as the radiation effect. The wear-inspection tool is a mechanical tool to press the impression compound against the inner walls of the fuel channels where the fuel components contact. The inspection tool has 36 molding cups that are operated in radial directions by turning the central rod with a fuel-handling tool from the pool top 12m above the reactor core. The same concept but a more compact design is applied to the cylindrical fuel channel. The wear inspection was successfully accomplished for a few fuel channels. The results show visible wear marks on a hexagonal fuel channel at the positions corresponding to fuel components such as the bottom guide arms and top springs. The wear damage is slight, approximately 0.2mm depth, in comparison with the thickness (1.6mm) of the fuel channel. No visible wear mark has been found in the cylindrical flow tubes so far. The wear inspection is being continued for all the remaining fuel channels to get valuable results for estimation of the lifetime of the fuel channels. Also, we have another plan to inspect the deformation and/or wear of the spider pin of the fuel channel by using the impression material and proper tools. These inspections would also be helpful for the design improvement of the fuel assemblies. (author)