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

阿海珐的流固耦合技术团队发现的一个重大技术问题与受到地震激励的燃料组件中的燃料棒的振动和流固耦合有关。燃料棒是加压的细长金属管，里面装有可裂变物质的颗粒，这些颗粒被组装成束，或者是由金属网格结构（间隔网格）支撑的燃料组件。在辐照周期中，棒和间隔网格之间的连接松弛，允许两个结构之间的振荡和随后的冲击。地震激发引起的过度振动，大的管道断裂等，可能导致棒和间隔网格之间的微动，最终可能破坏核裂变材料和冷却剂之间的安全屏障。因此，认识到燃料组件和燃料棒中的能量耗散机制对于核工程师来说是至关重要的，以确保燃料组件组件在最恶劣的地震条件下能够承受并保持功能。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之前与加拿大AREVA的合作（项目EG 462720-14和EGP 476586-14），包括设计一个数值程序来识别复杂系统中嵌入的结构阻尼的非线性行为。该研究项目将(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非线性阻尼和刚度演变的能力。这将有助于在地震或失去冷却剂条件下对这些组件进行验证和性能评估，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.