Impact of Nuclear Fuel Assembly Design on Grid-to-Rod Fretting Wear

The fluid-induced vibration at the nuclear fuel assembly may cause grid-to-rod fretting wear, and subsequently, fuel rod perforation. The fluid-induced grid-to-rod fretting wear occurs with a certain fuel assembly design and/or in certain nuclear power plants, which may be strongly correlated with external and internal vibration causes acting on the fuel assembly. The external vibration causes may include reactor coolant flow velocity, nonuniform radial flow profiles caused by reactor internals, and interfuel assembly gap and fuel assembly-shroud gap, whereas the internal vibration causes may include asymmetric mixing vane pattern across the spacer grid assembly and inadequate spacer grid design. In this study, the external and internal vibration causes are described including three internal vibration mechanisms acting on the fuel assembly. The impact of internal vibration on the grid-to-rod fretting wear was investigated, based on out-of-pile vibration and grid-to-rod fretting wear test results for two kinds of fuel assembly designs as well as their operating experiences in commercial reactors. In addition, fuel assembly design optimization guidelines are proposed to eliminate the grid-to-rod fretting wear-induced failure.