Written by Marc Albert, Principal Team Leader - Advanced Manufacturing and Materials
The manufacturing techniques and materials used by the nuclear industry have not been significantly updated since the 1970s and 80s. Recently, new fabrication methods have become available that could be applied for large cost and timeline reductions, but further research and development is needed to prove their effectiveness for adoption in the nuclear industry.
Advanced nuclear reactor designers are hopeful that these techniques will soon be available for nuclear plant construction, helping to eliminate costs and wait times for large forging, casting, and machining capacity. Doing so is expected to make advanced reactors significantly more competitive in the energy marketplace.
But before they can be used in constructing nuclear reactors, advanced manufacturing techniques must be included in the American Society of Mechanical Engineers (ASME) code, which approves standardized methods for specific materials and use cases.
Many advanced reactor designs make use of new fuel forms and non-light water coolants over long operational lifetimes that will also require new materials to be characterized and introduced to codes and standards.
Section III of the ASME code pertains to nuclear energy systems and is notably endorsed by the U.S. Nuclear Regulatory Commission (NRC). Specifically, ASME Section III Division 5 describes high temperature nuclear reactor pressure vessels and piping systems. For inclusion in the code and use in reactors, materials must meet robust pressure, strength, heat, and irradiation requirements over decades-long lifetimes.
Approving manufacturing techniques and materials for use in nuclear reactors can be a long and costly process. Much research remains, but EPRI is helping to organize efforts between stakeholders and push progress forward to meet the industry’s needs for these technologies.
Advanced Manufacturing Techniques for Advanced Reactor Fabrication
Advanced nuclear reactors (ARs) include non-light water moderated reactors, small modular reactors (SMRs), and microreactors.
A handful of new fabrication techniques developed and used in other industries stand out for their potential benefits in AR construction.
While these advanced manufacturing techniques show promise, they have not yet been proven for nuclear component fabrication, which will require larger components and production capacity.
Powder Metallurgy-Hot Isostatic Pressing
Powder metallurgy-hot isostatic pressing (PM-HIP) has been shown to produce high quality components at competitive cost with excellent inspectability. This technique has been used in the aerospace, automotive, and offshore oil and gas industries.
In PM-HIP, a capsule is fabricated in a similar geometry to the desired component. It is filled with metallic powder, degassed, and sealed; then, it is subject to high temperatures and pressures to consolidate the powder into solid alloy. The capsule is removed, leaving the component.
PM-HIP for Advanced Reactors
Creating components with PM-HIP has the potential to increase plant reliability and safety, reduce manufacturing energy use, improve inspectability, and provide a new domestic supply base for advanced light water reactor (ALWR) and SMR applications.
In ARs, PM-HIP may be used to greatly simplify the fabrication of the pressure vessel, steam generators, large valves, pump housing, and joints such as elbows – provided it can be included into ASME code for component materials.
EPRI and PM-HIP Demonstration
EPRI has already gathered a significant amount of data to add PM-HIP fabricated stainless 316L, nickel-based alloys, and Grade 91 creep-strength enhanced ferritic steels to ASME code for pressure vessels, which will also provide the boilerplate framework for future additions.
However, PM-HIP needs to be further optimized for large components such as those found in ARs. The largest HIP manufacturing capacity in the world is 72 inches in diameter, and many advanced reactors will require components larger than this.
More information on EPRI’s research on PM-HIP is available in 3002008030.
Electron Beam Welding
Electron beam welding (EBW) is an advanced autogenous, full-section, single-pass welding technique. It has been used in the aerospace, automotive, electronics, medicine, and defense industries, but mostly for small components.
EBW is highly automated and could lower weld times by 90% for large vessel girth welds, such as pressure vessels, compared to conventional techniques, such as submerged arc welding or gas tungsten arc welding.
EBW welds can also be eliminated to “reset the clock” on components with heat treatments, making it as if the component was simply fabricated as one single part without joining. This could remove the need for in-service inspection and save significant operational costs, although research is needed to confirm the properties and reliability first.
Remaining Challenges
One challenge for EBW is that it must be performed under vacuum. In theory, components and component pieces could be fabricated with PM-HIP and assembled using EBW, but this requires a large vacuum chamber, uses novel local reduced pressure vacuum systems, or requires a modular vacuum chamber approach (3002018146).
Nonetheless, the potential for modularization and schedule savings is immense if EBW can be demonstrated and added to the ASME code for AR materials.
More information on EPRI’s research on EBW is available in 3002005438.
Diode Laser Cladding
The pressure vessel and other components are exposed to coolants, corrosion, high pressures and temperatures, and irradiation. Depending on the reactor design, large surface areas need to be coated with cladding for component protection, often in unfavorable geometries.
Diode laser cladding (DLC) uses a diode laser instead of a welding arc to apply cladding materials, such as to the inside of a reactor pressure vessel.
DLC can be used out-of-position, requires 2-4x less cladding material, and substantially reduces weld dilution of the heat affected region compared to conventional welding methods for cladding.
DLC could significantly expedite and reduce costs for the cladding process and produce extremely smooth, highly inspectable cladding that requires less than half as much material as conventional techniques.
Additive Manufacturing
Additive manufacturing (AM) is a novel fabrication technology that has been a focus of significant and increasing research and development in recent years.
AM technologies, using both polymers and metal alloys, have been widely adopted in numerous industries outside of nuclear, in particular medical and dental devices, the automotive industry, and the aerospace industry.
AM can increase reliability, decrease part count, and reduce manufacturing costs by integrating part assemblies into a single, more complex part. It can help bring new reactor designs to market faster and simplify inventory and supply chain management.
The nuclear industry is now seeing additively manufactured components used in nuclear power plants and even within reactor cores. However, increased understanding of AM technologies is needed before the advantages can be fully realized.
Widespread deployment of many advanced manufacturing methods such as AM is also complicated by lack of standards, ASME acceptance and regulatory approval.
More information on EPRI’s development and research coordination for advanced additive manufacturing is available in our additive manufacturing roadmap.
Small Modular Reactor Vessel Manufacture and Fabrication
Advanced manufacturing techniques are expected to have great benefits for ARs. They are especially important for SMRs, which are expected to gain significant economic competitiveness from prefabrication, preassembly, and modularization (PPM) efforts that save costs by streamlining the construction process and reduce fabrication schedules as much as possible.
EPRI is currently collaborating on a project with NuScale Power, the U.K.’s Nuclear Advanced Manufacturing Research Centre (NAMRC), and other industry partners to apply and demonstrate advanced manufacturing techniques for fabrication of an SMR pressure vessel. The project is supported by the Department of Energy award DE-NE0008629.
The pressure vessel being fabricated is based on a ⅔ scale model of NuScale Power’s NuScale Power Module™, which has received design certification from the NRC and with plans to reach commercialization before 2030.
The project will fabricate the upper and lower reactor pressure vessel assemblies, including complex geometries for steam plenums, piping connections, and control rod inserts via advanced manufacturing methods. Producing these successfully will provide the foundation for using advanced manufacturing techniques throughout the construction process beyond the pressure vessel.
Using these techniques for NuScale’s SMR could save as much as 40% of the capital cost for the pressure vessel and reduce the component’s fabrication timeline to under a year. This would greatly enhance the design’s competitiveness and opportunities for PPM. Our latest update on this project is available in product 3002023900.
Materials Needed for Advanced Reactor Deployment
Beyond manufacturing, ARs will face two new materials challenges. First, many AR designs will operate at higher temperatures than traditional reactors, and new materials will need to be identified, researched, approved, and applied in designs for use at these higher temperatures.
Second, different coolant fluids will create new corrosion challenges that new materials must withstand.
Currently, the ASME Section III Division 5, which pertains to High-Temperature Reactors, includes only six materials for use above 427°C – and almost all non-water-cooled advanced reactor designs will operate above this temperature. While six materials is a decent starting point, more materials must be approved for AR designs to realize their full potential. Approving new materials requires significant investments of time and funding. Most recently, Alloy 617 was added to ASME Section III Division 5 in 2019 after 12 years and $15 million of research. This was the first material added to the Code in 30 years.
Materials also need to be studied for reliability against corrosive and radiation effects before they can be approved for use in nuclear reactors. Additional research includes collecting irradiation data, corrosion data, and thermal performance in specific reactor environments over long periods that can be extrapolated to ensure safety over a reactor’s lifetime.
Lastly, supply chains need to be established to guarantee sufficient material quality, consistency, and quantity. Supply chains remain a significant challenge for many new AR designs.
Material Needs for Advanced Nuclear Reactors
ARs can be grouped by design category to gain a better idea of their respective material needs. Some ARs, such as SMRs, benefit from many of the already approved materials that have been used by the nuclear industry for decades. Other reactor types will require significant materials research before they can be licensed and constructed.
Sodium Fast Reactors
Sodium fast reactors (SFRs) use liquid sodium as a coolant and operate at temperatures between 450-550°C. While austenitic Type 316 SS has already been approved, ferritic-martensitic (FM) steels are preferred for long-term use due to their stronger corrosion resistance. However, T91 is the only FM steel currently approved.
Molten Salt Reactors
Molten salt reactors (MSRs) use molten salt coolants, sometimes with fuel dissolved in a homogenous mixture. MSRs operate at a variety of temperatures, depending on the salts and fission isotopes used. In chloride and fluoride salt mixtures, Type 316H SS, Hastelloy N and its variants, and other Ni-based alloys are being considered, along with graphite moderators.
Even though Hastelloy N is a popular choice, including in the Molten Salt Reactor Experiment at Oak Ridge National Laboratory, Hastelloy N is still not ASME-approved for use in Section III Division 5.
Lead Fast Reactors
Lead fast reactors (LFRs) use either pure lead as coolant operating at 800°C or lead-bismuth eutectic operating at 550°C. Both of these materials can be highly corrosive to metal alloys, so FM materials, refractory metals, and non-metallic alloys are being considered. However, with the exception of T91, none of these materials have been approved.
High-Temperature Gas-Cooled Reactors
High-temperature gas-cooled reactors (HTGRs) typically use helium as a coolant, operating between 750-1000°C. Helium is an inert gas with favorable neutronics and corrosion properties.
HTGRs usually use graphite as a moderator and structural material. Graphite quality varies significantly based on how it is produced, so characterizing and securing supply chains will be an essential step.
Paving the Way for ARs with Advanced Manufacturing and Materials
Nuclear reactors operate in demanding environments for long lifetimes, with no margin for error. A substantial amount of research is needed to certify fabrication techniques and materials for use in ARs.
While the task may seem daunting, this research will pay dividends for decades to come by enabling a new generation of efficient and competitive nuclear reactors to bring clean electricity to the world.
EPRI is spearheading efforts by coordinating industry stakeholders and leading the way to success in advanced manufacturing and materials research. These efforts support efficient deployment of ARs and help create a clean energy future.
For more information on our projects and efforts, view our Advanced Manufacturing Methods Roadmap and our Advanced Reactor Materials Roadmap.