Experimental and theoretical study on nonlinear vibration of a reduced-scale PWR fuel assembly with GTR sliding

During the operation of a pressurized water reactor (PWR), the fuel assembly (FA) experiences axial flow-induced vibrations, leading to wear at the contact interfaces between the fuel rod (FR) cladding and the grids. Additionally, during seismic events or loss-of-coolant accidents (LOCAs), the FA may undergo severe deformation, potentially resulting in structural failures. Therefore, establishing a dynamic model of the FA to investigate its structural behavior under these conditions is essential for ensuring reactor safety. Experimental studies have demonstrated that FAs display nonlinear behavior under loading, including bilinear hysteresis in quasi-static experiments and stiffness softening in forced vibration experiments. Previous quasi-static studies have revealed that the stiffness softening and hysteresis behavior of FAs are primarily induced by relative axial sliding and rotational interactions at the grid-to-rod (GTR) joints. Building upon this foundation, the Euler–Bernoulli beam theory with bending-axial deformation coupling is employed to develop a theoretical nonlinear FA dynamic model that extends the previous quasi-static analysis to transient dynamic conditions. To further investigate the structural characteristics of the FA and provide insights for modeling, vibration experiments were conducted on a reduced-scale 3 × 3 FA with both ends rigidly fixed. The theoretical model successfully reproduced the structural response characteristics observed in both quasi-static and dynamic experiments by incorporating relative axial sliding and relative rotational sliding. The stiffness transition phase observed under quasi-static cyclic loading has been demonstrated to play a critical role in simulating the nonlinear dynamic response of the fuel assembly. Furthermore, using the developed nonlinear dynamic model, the GTR fretting (GTRF) wear characteristics of the FA are simulated and analyzed. The simulations identified wear patterns at different GTR joints, with the distribution of high-wear-rate GTR joints aligning with experimental observations. This study provides a reliable structural dynamic modeling framework for FA.