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The Westinghouse AP1000 nuclear re- actor uses 157 fuel assemblies. The top portion of the assembly is shown here.


the part count from nine to two by combining the top portion of the nozzle into a single CF-3 stainless steel investment casting that was then welded onto a machined Type 304L stainless bottom plate. Te metals were selected for their suitability to maintaining their properties while in an aqueous solution at moderately elevated temperatures. Reducing the part count to two helped reduce the value stream for the part, but the sup- ply chain team knew the design still had room for improvement. For ease of manufacturability,


flow slots in the bottom plate were machined in one direction with a leading chamfer to minimize blocked area and pressure drop. However, the unidirectional pattern resulted in a weak and strong direction for load distribution not optimal for the part’s application. Additionally, the chamfer generated a flow gradient that led to a higher pressure drop across the subassembly and total reactor core. To accommodate the high pressure drop, the fuel assembly needed to include either a higher capacity reactor cool- ant pump or less flow through the steam generator and turbine. Te design team developed a nozzle design with an as-cast optimized flow plate pattern to reduce the pressure drop. Te engineers designed the flow slots as multiple shapes in a symmetri- cal pattern to assist in load distribu- tion during worst-case shipping and handling and potential accidents. “We wanted to relax the stress level of the pumps,” Peterson said. “When we were able to cast those slots and configurations, we could optimize the design with full radii and drop the pressure further by about 50%.” The initial design was developed as a solid model, which was loaded into finite element modeling (FEM) software to validate load distribu- tions. From there, the design team began tweaking the model for fur- ther improvements before sending it to GSC for review. Due to concerns regarding voids in the flow plate


22 | METAL CASTING DESIGN & PURCHASING | Jan/Feb 2013


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