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NNS Reactor Coolant Pump Impeller

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Framatome / Aubert & Duval
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NNS Reactor Coolant Pump Impeller
Benefit: Cost Saving
Main Forming Process: Hot Isostatic Pressing
End Use Sector: Energy
Material: Stainless Steel

Description

Framatome, fabricates and supplies reactor coolant pump (RCP) sets covering the worldwide nuclear market (Figure 1). In this RCP, a component with large-dimension and complex geometry puts real production difficulties. Moreover, the manufacturer aims at improving its mechanical performances in connection with the nuclear power plants lifetime extension politics.

Large components with complex shapes, such as impellers, were traditionally manufactured by casting but the lifetime of these products was limited by the ageing of the material due to the presence of ferrite years, alternative approaches consisting of machining impellers from a forged ingot have appeared in the market place. Of course, this solution offers an improvement of the mechanical characteristics but, in the case of a RCP impeller, it implies to start with a 4000kg ingot to finish with a 600kg impeller, resulting in the scrapping of 85% of the material. This observation is also valid in the field of aeronautics where impellers for gas turbines are elaborated by a forging/machining process with a mass ratio of 10 to 100 between the starting ingot and the final part. On the basis of this analysis, a consortium composed of Framatome, Aubert & Duval, Ventana Group, Metalscan and institutional laboratories (Université de Bourgogne, ARTS and CEA) established the manufacturing sequence of a large-dimension impeller in 316L austenitic stainless steel by means of PM- HIP processes with a Near Net Shape (NNS) approach.

The tooling of the impeller has been designed by 2D/3D simulation and then machined in low carbon steel elements. Once assembled and welded, the low-carbon steel container is filled with 316L powder and prepared according to classical HIP capsule preparation procedures. After the HIP cycle, a rough machining was performed to open hydraulic channels and then facilitate the chemical pickling. This approach aims at reducing as much as possible the machining and finishing operations after HIP, in particular on the blades of the impeller, to limit the final cost of the part and reduce the fabrication times.

Component Specification

Density (g/cm3): 7.96
Tensile Strength (MPa): 580
Yield Strength (MPa): 290
Elongation (% strain): 57
Hardness (HV): 220

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