The Study on the Impact of FSI on the Fuel Assembly Design Optimization

The fluid-induced fuel rod fretting wear in PWRs mainly proceeds in the grid-to-rod contact areas. This grid-to-rod fretting wear observed in various PWR fuel assembly designs may depend on external and internal vibration sources. The extent of the external vibration may be governed by reactor coolant flow velocity, non-uniform flow profiles caused by the reactor internals, and inter-fuel assembly gaps and fuel assembly-shroud gaps. The extent of the internal vibration may be controlled by spacer grid mixing vane pattern, irradiation-induced spring force relaxation and/or growth rate differences in fuel rod and guide tube. In this study, the internal vibration sources only are focused and its impact on the grid-to-rod fretting wear is evaluated, based on verification test results of a newly developed advanced fuel assembly as well as their operating experiences in various reactor conditions. In addition, key guidelines of fuel assembly design optimization are provided to prevent the grid-to-rod fretting wear-induced failure.