Vibrations and damping identification of non-linear systems under periodic excitation for nuclear applications

阿海珐流体-结构相互作用技术小组确定的一个重要技术问题与燃料组件中燃料棒的振动和流体-结构相互作用有关，这些燃料棒受到地震激励。燃料棒是加压的细长金属管，其中包含组装成束的可裂变材料颗粒，或由金属网格结构（间隔网格）支撑的燃料组件。在辐照循环期间，棒和间隔栅之间的连接松弛，允许两个结构之间的振荡和随后的冲击。地震激励、大型管道破裂等引起的过度振动，可能导致燃料棒和定位格架之间的磨损，最终可能使核裂变材料和冷却剂之间的安全屏障失效。因此，认识到燃料组件和燃料棒中耗散能量的机制对于核工程师来说至关重要，以确保燃料组件部件能够承受最严重的地震条件情景并在地震条件情景期间和之后保持功能。Amabili教授的团队进行的研究表明，随着振荡幅度的增加，结构阻尼比大幅增加。拟议的研究计划继续我们以前的合作与阿海珐加拿大（项目EG 462720-14和EGP 476586-14），包括设计一个数值程序，以确定嵌入在复杂系统中的结构阻尼的非线性行为。本研究项目将（i）通过进行实验和推进该方法的理论公式来研究和扩展非线性阻尼识别方法，以包括不同类型的几何非线性振动响应。(ii)这些实验的结果将用于验证增强参数识别方法描述燃料组件部件非线性阻尼和刚度演变的能力。这将有助于在地震或冷却剂损失条件下对这些部件进行验证和性能评价，提高其安全性，并提高最终核燃料产品在加拿大核市场上的竞争力。

A significant technical issue identified by AREVA's fluid-structure interaction technical team is related to the vibrations and fluid-structure interaction of fuel rods, found in fuel assemblies, subjected to seismic excitations. Fuel rods are pressurized, long slender metal tubes containing pellets of fissionable material that are assembled into bundles, or fuel assemblies supported by metallic grid structures (spacer grids). During the irradiation cycle the connection between the rods and the spacer grids relaxes allowing for oscillations and subsequent impacts between the two structures. Excessive vibrations caused by seismic excitations, large pipe breaks, etc., could lead to fretting between the rods and the spacer grids that could eventually fail the safety barrier between nuclear fission material and the coolant. Therefore, recognizing the mechanisms that dissipate energy in fuel assemblies and fuel rods is of paramount importance for nuclear engineers in order to make sure that fuel assembly components can withstand and remain functional during and after the worst earthquake condition scenarios. Studies performed by Professor Amabili's team demonstrated a substantial increase of the structural damping ratio as the amplitude of the oscillations increases. The proposed research plan continues our previous collaboration with AREVA Canada (projects EG 462720-14 and EGP 476586-14) consisting in designing a numerical procedure to identify the non-linear behavior of structural damping embedded in complex systems. This research project will (i) investigate and extend the non-linear damping identification methodology by performing experiments and advancing the theoretical formulation of the methodology to include different types of geometrical non-linear vibrational responses. (ii) The results from these experiments will be used to validate the ability of the enhanced parameter identification methodology to describe the evolution of non-linear damping and stiffness of fuel assembly components. This will assist in the validation and performance evaluation of these components under seismic or loss-of-coolant conditions increasing their safety and improving competitiveness of the final nuclear fuel product in the Canadian nuclear market.