Printing nuclear parts4 July 2017
NEI assesses the opportunities for additive manufacturing in the nuclear sector, ongoing initiatives and hurdles that still need to be overcome.
Siemens announced in March that it had achieved the first successful commercial installation and continuing safe operation of a 3D-printed part in a nuclear power plant. The replacement part, a metallic 108mm-diameter impeller for a fire protection pump that is in constant operation, has been in place at Krško in Slovenia since January.
The original impeller had been in operation since the plant was commissioned in 1981 and the manufacturer is no longer in business. Siemens’ team of experts in Slovenia reverse- engineered and created a “digital twin” of the part. The company’s additive manufacturing (AM) facility, which has been operating at Finspång, Sweden, since 2009, then applied its advanced AM process, using a 3D printer to produce the part.
3D printing is a way of manufacturing metal or plastic parts directly from design data, using lasers to fuse together high- performance materials layer by layer. The technique enables components, including one-off specialist items, to be made relatively quickly from 3D design data, which could be obtained by scanning an existing item. Metal parts can be printed to a high resolution in a wide range of materials, including titanium, stainless steel and brass.
Meeting Krško’s stringent quality and safety assurance requirements required extensive testing, which was performed jointly with plant operator Nuklearna Elektrarna Krško (NEK) over several months. Further material testing at an independent institute, and CT scans, showed that the material properties of the 3D-printed part were superior to those of the original.
Krško provides more than 25% of Slovenia’s and 15% of Croatia’s electricity. Siemens has been performing modifications and providing service on the plant’s non-nuclear side, including turbine, generator and auxiliary equipment, for more than 10 years.
“Obsolete, non-OEM parts are particularly well-suited for this new technology as they and their designs are virtually impossible to obtain,” Siemens says. “This technology thus allows mature operating plants to continue operating and achieving – or, as in the Krško case, even extending – their full life expectancy.”
Siemens says it plans to continue its research and development with Krško into the use of 3D-printed parts. It is, “looking at advancing the design of parts that are most difficult to produce using classical manufacturing techniques, such as lightweight structures with improved cooling pattern.”
Hydro Inc, the pump engineering and service company, also uses reverse engineering processes to support customers who need difficult-to-find, long-lead time or obsolete parts. However it takes a slightly different approach. Rather than printing the metal, Hydro scans the components to be replaced using a laser coordinate measuring machine, creates a 3D solid model, and then 3D prints a mould from the model file. The mould is sent to a foundry, where they pour the metal to create the component.
“The big difference is that with 3D printed metal there is a question that is out there whether the material properties will support the quality assurance requirements,” says Faisal Salman, nuclear market manager at Hydro.
“With the 3D mould process it is the best of both worlds: high quality geometric part, fast delivery, and proven casting processes.”
Hydro Inc has already supplied safety related 3D printed mould parts to Krško on an emergency service water pump and it has reverse engineered and supplied an impeller to the plant.
In another instance, the technique was used to repair a safety-related pump at a nuclear facility within 12 weeks. “This special casting, meeting nuclear quality standards, simply could not be sourced from the OEM in the time frame required by the customer,” says Salman. The traditional manufacturing process of a cast component can take from nine months to a year. “Under emergency conditions Hydro Inc was able to reverse- engineer and manufacture a 3D model, including engineering analysis, and the pouring of the metal,” Salman adds.
In addition to reducing lead time, scientific developments in additive manufacturing are allowing foundry engineers to make significant changes in the way tooling is developed and castings are created. Since most major pump parts come from castings, pump users will continue to see improvements in critical areas such as price, delivery and quality of their spare parts, Hydro Inc says.
Westinghouse told NEI it is using binder-jetting additive manufacturing to cut costs and lead times associated with some difficult-to-procure parts identified as offering the greatest savings to the nuclear industry. “Benefits will be immediate because we are combining binder-jetting additive manufacturing for safety-related components with traditional casting processes, which avoids the need to modify or create new nuclear regulations to apply it,” says Clint Armstrong, advanced manufacturing subject matter expert, global
technology development, at Westinghouse Electric Company. “We’ve estimated as much as 50 percent cost and 75 percent lead-time reductions for certain replacement casting by making use of modern, digital manufacturing processes,” Armstong adds.
Looking beyond replacement
There are a number of applications for additive manufacturing in the nuclear sector, including in new-build, for fuel and for in-reactor components. However, work still needs to be done to qualify material and demonstrate that components can meet nuclear codes and standards.
“We see opportunity in a variety of nuclear applications including fuel components, reactor services, new plant components and replacement parts,” says Fran Bolger, manager, new product introduction, at GE Hitachi Nuclear Energy. But Bolger stresses the strengths of the 3D printing process, “need to be aligned well with the application” for there to be a positive cost-benefit outcome. These include size limitations, opportunity for performance improvements and delivery speed requirements.
Last summer, GEH was chosen to lead a $2 million additive manufacturing research project on behalf of the US Department of Energy (DOE). As part of the project GEH is using direct material laser melting (DMLM) to produce replacement parts for nuclear power plants. Samples of 316L and Inconel 718 have been 3D printed in metal at the GE Power Advanced Manufacturing Works facility in Greenville, South Carolina and shipped to the Idaho National Laboratory (INL) for irradiation in the Advanced Test Reactor. Once irradiation is completed, the samples will be removed, tested and compared with an analysis of unirradiated material, which has mostly been completed.
GEH tells NEI it has also been looking at in-reactor applications including fuel debris filters and jet pump repair parts. Non-safety parts are another potential opportunity.
Looking at the challenges ahead, Bolger says: “In the short term our customers need to see that we have adequately demonstrated material quality and that we can satisfy code and regulatory requirements. Longer term we need to equip our 3D printing factories with the right machines and tools to meet cost, productivity and quality goals.”
Armstrong, advanced manufacturing expert at Westinghouse, also stresses that in general digital, automated processes for manufacturing nuclear components must first be included in the American Society of Mechanical Engineers (ASME) code or other associated codes and standards.
To facilitate the rapid qualification of additively manufactured parts, Westinghouse is collaborating with Oak Ridge National Laboratory’s Manufacturing Demonstration Facility, the University of Tennessee in Knoxville and Rolls-Royce on another project being led by the Electric Power Research Institute and funded by the US DOE. “The goal is to show that we can continually reproduce the same properties in components created with additive manufacturing through integrated computational materials engineering and in-situ process monitoring, rather than much slower, repetitive destructive testing,” Armstrong says.
Westinghouse has already successfully irradiated and conducted post-irradiation tests on components made with laser powder-bed fusion additive manufacturing.
It is also irradiating an alloy for fuel components and plans to have a low-risk fuel component in a commercial reactor by early 2018. Westinghouse is seeking to improve fuel performance by applying laser powder-bed fusion to build components using designs not possible with traditional manufacturing.
“Additive manufacturing has powerful potential for the nuclear industry,” says Armstrong. “It frees us from the limitations of traditional manufacturing, liberating creativity in component design.”
With powder-bed fusion, the company says it can improve performance by creating more complex components that are not possible with traditional methods. “For example, we can build new internal passageways for improved flow characteristics and matrix structures for lighter, stronger components. We can also build custom and low-volume parts; prototypes and mockups; and jigs, fixtures and tooling, affordably.”
Binder jetting additive manufacturing results in significant cost and schedule savings, because it can eliminate the need to manually create intricate casting moulds for replacement parts. It can also cut the lead-time on prototypes and non-metallic and matrix materials for next-generation plant components.
Direct-energy deposition can be used as an alternative to large castings and forgings or as an addition to them. This process can reduce lead-time, machining and material waste. Certain direct-energy deposition processes also offer improved material properties versus traditional methods; for example, they can be used for localised customisation of materials such as hard- facing for resistance to corrosion and wear; and they can be applied to repair high-value components, tooling and bearing surfaces.
Armstrong says: “Additive manufacturing can produce alloys with properties that cannot be produced using traditional methods, including improved alloys crucial for next generation plants.” For example, the process can use materials such as silicon that cannot be worked using traditional metal-bending methods. “It is an exciting technology that can greatly change what can be done for both operating plants and future plants, especially as designers embrace the freedom this technology offers to deliver improved component designs,” says Armstrong.
Promise for the next generation
In the UK, the Nuclear Advanced Manufacturing Research Centre (AMRC) is carrying out R&D on bulk additive manufacture (BAM). The site has a dedicated facility, which uses arc welding and wire, and is also investigating BAM applications using electron beam and diode laser facilities. The electron beam facility has been used to produce Trefoil plates used in steam generators to support the tubes. The early test piece was made of 316L stainless steel deposited at 1.6kg/hr.
Nuclear AMRC is also investigating the potential for 3D printing in the manufacture of reactor pressure vessels or the addition of components such as nozzles or lifting lugs. “The technology could transform manufacturing in terms of capacity, material yield and transportation,” says Udisien Woy, bulk additive manufacturing technology lead at Nuclear AMRC.
However, there is still work to be done. “Regardless of what application we select, it is critical to achieve the right properties. In the initial stages we want to demonstrate we can achieve the required geometries. Then we want to demonstrate we can achieve material structure. And the process needs to be repeatable,” Woy adds.
In a separate project, an international research consortium led by the Nuclear AMRC has shown the way forward for developing powder metallurgy techniques for the civil nuclear industry. The PowderWay project was funded by the European Commission through Nugenia, the European association for R&D in nuclear fission technologies, and led by the Nuclear AMRC. Partners include reactor developer Areva, utility EDF, French nuclear suppliers association PNB, French energy commission CEA, and Swedish materials research group Swerea.
The team worked with suppliers and end users to identify a range of nuclear parts that could be produced by powder metallurgy. They also identified three promising techniques for maturity and potential application in nuclear manufacturing: hot isostatic pressing (HIP); additive manufacture using powder bed or blown powder; and spark plasma sintering.
These processes can produce near-net-shape parts with excellent material properties, which avoid many potential weak points such as welds. They are already used in sectors such as aerospace and for some naval reactor components, but are yet to be embraced or approved by the civil nuclear industry.
“We developed a priority list of around half a dozen component types, from primary pipework of a metre diameter with elbow shapes that could be produced by HIP, to small additive manufactured complex structures that go in the fuel filter to catch debris,” says Will Kyffin, PM technology lead at the Nuclear AMRC. “We also looked at what capabilities are available around Europe in commercial organisations and R&D institutions, and merged that with the parts data to determine whether the European supply chain can produce the priority components at this point, or whether there’s additional work needed.”
There is also the potential to use 3D printing for radioactive waste containers. The UK’s Sellafield Ltd announced in May 2014 it was working with 3D specialist companies 3T RPD and Central Scanning to create metal and plastic components, parts and one-offs to help meet the challenges of decommissioning.
As part of the effort, the companies saved £25,000 by using the technology to design and produce a replacement lid for a 40t solid waste exporter flask, which is used to ship radioactive sludge across the Sellafield site.
Setting the standard
One pressing issue is standards. Armstrong notes that additive manufacturing is not fully included in the ASTM International standards or in the ASME code. He says that Westinghouse is participating in ASTM International’s F42 subcommittee, which is researching, developing and implementing additive manufacturing standards for industry. The next step is developing nuclear codes and standards through the ASME.
“In addition, while additive manufacturing has great potential for saving money for industry, currently the cost of the equipment is high and the size restrictive for larger nuclear components,” Armstong says. “There is also a lack of standardisation for processes, equipment types and production parameters. Process, powder and design variability results in each part requiring extensive testing and qualification before being produced for commercial use.”
And we are not quite there yet. “At this point in the technology’s development, most parts are built to near-net shape and require additional polishing or machining to meet dimension and surface finish requirements,” notes Armstong. “Likewise, components normally go through a heat treatment or hot isostatic pressing process to achieve the required material properties.”
While these temporary barriers exist, organisations and researchers working to resolve them.