A Study of Flow-Induced Grid-to-Rod Fretting Wear in PWR Fuel Assemblies

It is generally known that flow-induced vibration at the nuclear fuel assembly structures may cause grid-to-rod fretting wear and subsequently rod failure. The flow-induced grid-to-rod fretting wear is found to initiate at a certain critical grid-to-rod gap that is strongly correlated with the extent of flow-induced vibration. In this paper, three vibration drivers acting on the grid-to-rod fretting wear are proposed, based on various fretting wear experience in commercial reactors as well as on various flow-induced fretting wear test results. The first active vibration driver is high turbulence-induced excessive fuel rod vibration with the combination of excessive grid-to-rod gap. The second active vibration driver is self-excited fuel assembly vibration in a low frequency range caused by hydraulically unbalanced mixing vanes of the spacer grid assembly. The third active vibration driver is self-excited spacer grid strap vibration in quite a high frequency range caused by some spacer grid designs. Each vibration driver on the grid-to-rod fretting wear damage is discussed. On the other hand, simplified three formulas for the grid-to-rod fretting wear progress such as constant work rate, constant stress rate and linear stress rate have been derived and used in predicting the rod failure time. It can be said that the linear stress rate is the most effective in predicting the rod failure time, while the constant work rate the least effective. In addition, spacer grids with the larger grid-to-rod contact area seem to generate the less fretting wear rate.