Performance evolution and influencing factors of hold-down springs in nuclear reactor internals under high-temperature cyclic loading conditions

As a critical component in reactor internals, the performance of hold-down springs directly affects operational safety. However, most existing studies are conducted under room-temperature conditions, which is difficult to reflect the performance evolution law in actual high temperature service. In this work, using a self-developed high-temperature test rig, we carried out 200-cycle comparative tests on three scaled models (L160R50, L180R50, L200R50) at both room temperature and 330°C. After 200 cycles, the stiffness of the three models increased by 45.5%, 35.3%, and 23.7% at room temperature, but only by 2.8%, 9.1%, and 7.2% at 330°C. Meanwhile, the initial stiffness at 330°C was 54.6%, 23.4%, and 31.4% higher than that at room temperature, respectively. The friction coefficient shows a similar trend: it gradually rises to 0.52–0.58 at room temperature, while at high temperature it starts higher but fluctuates little. Microscopic analysis further revealed that the wear mechanisms were dominated by abrasive and adhesive wear at room temperature, and changed to oxidative and fatigue wear at 330°C. This mechanism shift leads to an increase in contact area and frictional resistance at high temperature, accompanied by a more obvious hysteresis effect. In addition, increasing the contact width reduces the preload variation during cyclic loading. Overall, this study provides experimental evidence and theoretical support for the design, preload configuration, and safety assessment of hold-down springs under high-temperature conditions.