Research on Additive Manufacturing Technology for Bottom Nozzle of Fuel Assembly

Because of the outstanding advantages of additive manufacturing technology in improving efficiency, reducing weight and volume, more and more researchers have paid attention to it in the field of nuclear fuel assembly. The United States, France, and other countries have performed additive manufacturing technology research on multiple components such as the bottom nozzle, grid, and thimble plug assembly of nuclear fuel assemblies. The Westinghouse took the lead in achieving the application of 3D printing thimble plug assembly into commercial reactors in 2020. The product preparation based on additive manufacturing is quite different from the traditional process. At present, the research on additive manufacturing technology for nuclear fuel products is not so systematic, focusing on process experimentation and the material properties. In this paper, the whole process of additive manufacturing from powder preparation to product performance testing was systematically carried out with the bottom nozzle of fuel assembly as the object, and the powder was prepared by vacuum induction melting and atomization. The characteristic of this method is that the process is stable, the cost is low, and it is suitable for commonly used printing materials with low unit prices, such as stainless steel, aluminum alloy, cobalt alloy, etc. By strictly controlling the composition, structure and process flow of the powder raw materials, the obtained powder chemical composition meets relevant requirements. The selective laser melting (SLM) process is selected as the additive manufacturing process. The forming process does not require molds and is not limited by the complexity of the part structure. The components obtained have high density, high precision, and achieve metallurgical bonding. Detail process analysis such as support scheme was carried out through ANSYS additive software, and the final bottom nozzle product was obtained through solution heat treatment and surface treatment. The test of product performance shows that, the final product has good dimensional accuracy and surface roughness. The overall dimensions, guide tube aperture, and other dimensions meet the requirements of the design drawings. However, the roughness of the inner surface which the coolant flow through is slightly greater than that of the outer surface of the bottom nozzle. The mechanical properties of materials are quite well, and the strength in x/y direction is higher than that in z direction. The full process for bottom nozzle using 3D printing only took 14 days, which effectively saves the development cycle compared to traditional processes. The research can be applied to the additive manufacturing of fuel assembly bottom nozzle, and can also provide reference for other products.