Additive Manufacturing, specifically Laser Powder Bed Fusion, is being explored by academic, industrial, and regulatory entities for technical feasibility, cost effectiveness, and safety of fabricating components for nuclear power plant applications. Laser Powder Bed Fusion is capable of fabricating highly complex geometries, and in some applications can reduce assemblies of 10s of parts to a single as- fabricated component. In conventional manufacturing the material properties are well characterized, and the difficulty lies in machining and forming the desired geometry. In laser powder bed fusion, the difficulty lies in characterizing material properties while geometric freedom is nearly unlimited.
In general, laser powder bed fusion is a reproducible process with material properties equivalent or superior to conventional manufacturing, provided that appropriate calibration, technician training, and feedstock tracking is followed. However, quantitative data on part to part variability in a production setting, powder feedstock lot to lot variability, and machine to machine variability is not readily available due to corporate confidentiality. Academic studies have attempted to address the knowledge gap in property variability, but frequently suffer from incomplete reporting, narrow focus, and lack of replicate specimens. As a result, reported material property values for laser powder bed fusion components vary widely in common measurements such as yield strength (YS) and ultimate tensile strength (UTS). Specific properties such as irradiation stress corrosion cracking (SCC) crack growth rates may only have one or two published journal articles.
Many of the codes and standards indirectly supporting laser powder bed fusion are well established (e.g. powder measurement, laser calibration). Codes and standards specifically on laser powder bed fusion vary widely in quality, and some standards explicitly contradict on a few important details such as feedstock recycling criterion. The National Aeronautics and Space Administration (NASA) has published MSFC- STD-3716, which provides a statistically rigorous framework for determining material properties and design values in the context of a laser powder bed fusion production setting. AWS and ASTM standards on laser powder bed fusion specify collecting material property data from simplified geometries such as cylinders and bars. However, the authors recommend that material properties initially not be evaluated from simplified geometries but instead from sectioned end-use geometry components when possible.
Material properties are a function of geometry in laser powder bed fusion, and the use of simplified geometries may give erroneous values. In the context of light water reactors, additional studies are needed on the dendritic microstructure of laser powder bed fusion 316L, on the acceptable chemical composition range of laser powder bed fusion 316L (which may not be identical to conventional 316L), and on differences in welding laser powder bed fusion materials relative to conventional materials. Some of above studies are already in progress or under consideration with ASME Code Cases.
In summary, additive manufacturing represents a significant opportunity for American manufacturing and nuclear electrical energy generation. Some codes and standards details must be resolved empirically as production data becomes available, and additional studies on the microstructure and welding of laser powder bed fusion 316L are needed. However, early studies indicate that laser powder bed fusion 316L can offer equivalent or superior performance relative to conventional 316L given appropriate manufacturing parameters and heat treatments, while significantly improving cost effectiveness and reducing assembly complexity. The NRC has developed a companion document to this report (ML20351A204) that provides context to the gaps identified in this report from a regulatory perspective and highlights key technical information related to LPBF-fabricated components in nuclear power plants.
To download the Review of Advanced Manufacturing Techniques and Qualification Processes for Light Water Reactors—Laser Powder Bed Fusion Additive Manufacturing document in its entirety, please click on the pdf file below. For more information on the Oak Ridge National Laboratory or the United States Nuclear Regulatory Commission, visit their websites at https://www.ornl.gov/ or https://www.nrc.gov/