The U.S. Department of Energy’s (DOE’s) MARVEL microreactor achieved 90% final design, a key step that will allow the project to move forward with preparation for fabrication and construction.

“This is an incredible milestone for the Department of Energy,” John Jackson, national technical director for DOE’s Microreactor Program headquartered at Idaho National Laboratory (INL), told POWER during an exclusive interview. “This is the first new reactor under the Department of Energy’s regime to achieve 90% final design. It enables us to proceed as a project and begin the transition from development and design to execution and construction.”

Although 52 mostly first-of-their-kind reactors have been built at INL since the site was established in 1949, MARVEL will be the first new reactor to be constructed there in more than four decades. The DOE expects MARVEL to help industry partners demonstrate microreactor applications, evaluate systems for remote monitoring, and develop autonomous control technologies for new reactors.

“MARVEL is not intended to be a commercial concept. It is built for a different mission—a different objective—for the Department of Energy,” explained Jackson. “At a very high level, it’s about moving from a paper or PowerPoint reactor to a real reactor. It’s about establishing a precedent. It’s about building capability. It’s about training our R&D [research and development] staff in preparation for commercial demonstrations that are expected to follow. It’s about polishing off the rust from decades of inactivity in this space. It’s about building processes and procedures, again, to help pave the pathway for commercial demonstrations. It’s about answering the fundamental question that we get asked all the time and that is: ‘When can we see one of these in operation?’ MARVEL will fill that void,” he said.

A Unique Reactor Design

MARVEL is an acronym, which stands for Microreactor Applications Research Validation and EvaLuation. The MARVEL design is a sodium-potassium-cooled microreactor that will be built inside the Transient Reactor Test (TREAT) facility at INL. It will generate 85 kW of thermal energy and about 20 kW of electrical output. To be clear, that’s pretty small—only enough power to serve about 10 homes—but the experience of constructing and operating the unit could be crucial for future microreactor developers and microgrid designers, as future plans are to connect it to a microgrid.

“MARVEL is a very descriptive name,” Yasir Arafat, former technical lead of the MARVEL project, said during a presentation given to reporters at the TREAT facility earlier this year (Figure 1). “MARVEL is an applications test reactor. We’re going to extract heat and energy from the reactor, and combine it with solar, wind, and other energy sources. We also will generate data that we will be using to validate our computational models,” he said.

1. The Transient Reactor Test (TREAT) facility provides a transformational research platform that links science and engineering capabilities for the advancement of fundamental science and nuclear technology. Source: POWER

Arafat noted that microreactors have three key features: they can be built in factories, transported over standard roads, and are self-regulating, that is, they’re designed to operate safely without human interaction. Arafat described some of the less conventional features of the design. “This reactor, unlike a larger system, does not have a containment building. It only has an upper confinement, meaning, we don’t need a giant amount of concrete and steel and rebar, the way we do for larger systems,” he said. Arafat also noted that the power conversion, which takes the heat and converts it to electricity, is integrated with the reactor—it is not a separate system, as is typical of larger-scale nuclear units.

The MARVEL design is cooled naturally—there are no pumps required. “We have a single riser where the fuel sits, and essentially four downcomer locations. We have the Stirling engines sitting on those four locations,” explained Arafat. Liquid flows up through the center riser, where it is heated by the fission of nuclear fuel. The liquid is then directed to the Stirling engines, where the heat is extracted to produce electricity, which causes the liquid to become denser. The heavier liquid flows down through the four downcomers, creating natural circulation through the reactor.

The MARVEL design is primarily based on existing technology and will be built using off-the-shelf components, allowing for faster construction. High-assay, low-enriched uranium (HALEU) from available research materials will be used to make the fuel, known as TRIGA fuel, an acronym that stands for Training, Research, Isotopes General Atomics. TRIGA International, a joint venture between Framatome and General Atomics, will handle fuel production at its fuel fabrication facility in Romans, France.

TRIGA fuel is uranium zirconium hydride, and the hydride is the defining characteristic for moderation, that is, the slowing down of neutrons to facilitate fission. Whereas, commercial light water reactors are moderated by water, MARVEL will have self-moderating fuel.

The Path Forward for MARVEL

There has been a steady progression to get to 90% final design. “The project kicked off in about June of 2020, so we’ve been at it for a little over three years now,” Jackson said. “One of the objectives of this project is to do it as rapidly as possible, which leads to some of the design choices, for example, Stirling engines for power conversion and utilization of TRIGA fuel.”

The 90% threshold allows for minor changes that might arise due to unforeseen complexities during construction and assembly. With the design at this point, the project team can now focus on constructability and fabricability. Researchers are currently testing a full-scale, electrically heated replica of MARVEL (Figure 2) that will help verify the performance of the microreactor’s cooling system.

2. The primary coolant apparatus test, also known as PCAT, fired up for the first time in September at Creative Engineers Inc.’s manufacturing facility in New Freedom, Pennsylvania. The company installed the PCAT in May and loaded the system with sodium-potassium and lead-bismuth coolants to demonstrate heat removal from its electrically heated core. DOE is now working toward collecting new data on the system’s temperatures and coolant flow to ensure the MARVEL reactor will perform as expected. Courtesy: U.S. DOE Office of Nuclear Energy

“We do have authorization to fabricate parts to a limited extent,” Jackson said. “Some of the authorization process has to occur before we’re full bore on fabrication, but we’ve got initial authorization through long-lead procurement processes to fabricate certain large parts.” Among the long-lead items is fuel fabrication.

A preliminary documented safety analysis is expected to be completed by spring or early summer next year. Jackson expects fuel will be received from TRIGA International in late 2024. Then, the team would focus on fuel loading and criticality, that is, achieving a stable, self-sustaining nuclear chain reaction, which could happen in early 2025.

Valuable Lessons to Be Gained

MARVEL will go through its entire lifecycle at INL, which includes design, development, construction, operation, deactivation, and decommissioning. INL will get learnings in every phase of its life.

“We’ll get experience in all of those things,” Jackson said, suggesting the industry would benefit from every activity. “From a developer perspective, not only do we get trained staff to help them enable their demonstrations that they choose to demonstrate on the National Laboratory campus, but they also get the learnings from the MARVEL microreactor. One of these hasn’t been started up in decades. So, nuances associated with the startup operation of a microreactor are going to be very valuable lessons learned for the industry. Response times—if we get into something like load following, for instance—this real, true operation, operational characteristics will be useful for the developer community,” he said.

“We envision MARVEL being connected to a local microgrid. That provides an opportunity for developers of microgrid technology to connect to a nuclear-powered microgrid and understand the nuances associated with that sort of connection and that sort of demonstration,” said Jackson. “We’re building this for the U.S. taxpayer, which includes the developers. We’ve got to be successful in our mission, so we will license the technology as appropriate and share to the extent possible.”

Still, Jackson stressed that MARVEL isn’t yet ready for primetime. “I know it’s tempting to envision a MARVEL microreactor plopped in front of a city block and servicing 10 houses, but we’re not quite there yet. We’ve got some ways to go,” he said. “Microreactors will probably be sited in places like remote Alaska and other places like that, mostly owing to the economics of the situation. Because the first of a kind is always very expensive relative to the hundredth or thousandth of a kind.”

An Exciting Time for the Nuclear Industry

The fact that hundredth and thousandth of a kind are even something to be contemplated should be encouraging to nuclear proponents. Like other demonstration reactors that have come before it at INL, MARVEL could be a stepping stone to bigger and better things.

“Those initial deployments, and in particular MARVEL, are very important for there to be a springboard from which to jump for the microreactor industry,” said Jackson. “I’m also fond of pointing out that we’re not developing microreactors to take over the world. From a strategic perspective, it’s best to have a diverse portfolio of options from which to choose, depending on your situation. Microreactors will likely fill a niche market. They’re not going to replace gigawatt-scale reactors, because those have a very specific purpose and are very well-established, and very safe and reliable. But this is about expanding the portfolio of options.”

And there are plenty of other options close behind MARVEL in the development pipeline. “I think it’s an exciting time for nuclear energy. We have bipartisan support,” Jackson noted. “I feel blessed to be a part of this new nuclear revolution, if you will. I think we have got a great facility at Idaho National Laboratory. We’ve got a great sponsor in the Department of Energy. This is an accomplishment for the nation and we hope it’s a game changer. Shame on us if we don’t take advantage of the momentum,” Jackson concluded.

Aaron Larson is POWER’s executive editor (@POWERmagazine).