Industry and INL collaborate to test new varieties of TRISO fuel

At 250 MW, INL's ATR remains the world's gold standard for nuclear fuel testing, with nine large flux traps and 68 drop-in positions within the core and neutron reflector to accommodate dozens of experiments during each 60-day operating cycle. (Photo: INL)

In 1979, the Fort St. Vrain nuclear power plant in Colorado used TRISO fuel in a commercial reactor for the first time. TRISO fuel research and development continues today, and it ranks among the most extensively studied advanced fuel forms.

As the United States embarks on a new era of commercial nuclear energy deployment, numerous reactor developers are using these ultra robust, poppy seed-sized fuel particles to power their designs, especially high-­temperature, gas-­cooled reactors (HTGRs).

Now, reactor companies are working with researchers at Idaho National Laboratory to irradiate their own fuel and, in some cases, explore variations of TRISO fuels that will give performance benefits comparable to existing uranium dioxide fuel pellets used in today's reactors.

Specifically, fuel developed by X-­energy is currently being irradiated at the lab, and both BWX Technologies and Radiant Industries are preparing to irradiate various forms and formulations of TRISO fuels in INL's Advanced Test Reactor.

The ATR provides researchers and industry with capabilities to irradiate new nuclear fuels, materials, and components in a high flux environment that can accelerate the testing of their designs. The data from those experiments will help qualify TRISO fuels for three HTGRs: X-­energy's Xe-­100, the BWXT Advanced Nuclear Reactor (BANR), and Radiant's Kaleidos.

The companies are testing types of TRISO fuel that include uranium nitride fuels, pebble fuel forms, annular fuel forms, and other variations-some of which could allow TRISO fuels to run in small advanced reactors at higher power densities or for longer durations before needing to refuel.

AGR experiments

This new round of TRISO tests in the ATR builds on the series of experiments performed by the Advanced Gas Reactor (AGR) Fuel Development and Qualification Program.

Starting in 2002, BWXT, the Department of Energy, INL, and Oak Ridge National Laboratory began a new push to qualify TRISO fuel for a new generation of high-­temperature gas reactors in the United States.

Researchers and industry collaborated to design, fabricate, irradiate, and conduct postirradiation examination of TRISO fuels in parallel with the Next Generation Nuclear Plant reactor design program, which focused on larger HTGRs.

For the first AGR TRISO fuel irradiation, BWXT made uranium oxycarbide (UCO) kernels, and ORNL applied the coatings and compacted the particles and graphite into the cylindrical fuel form. They were then transported to INL for irradiation in the ATR and postirradiation examination.

Over the next decade, the collaboration continued and the manufacturing process advanced from lab to production scale, with BWXT making the fuel kernels, applying the protective TRISO layers, and fabricating fuel compacts composed of thousands of TRISO fuel particles.

"The postirradiation examination of those fuels showed that the scaled-­up fuel form performs well, compared to the smaller lab-­scale produced fuel," said John Stempien, an INL researcher who served as technical lead for the AGR fuel postirradiation examination.

In 2019, when the Department of Defense first issued the call for Project Pele, a small, portable nuclear reactor that could replace or augment diesel generators and renewables at U.S. military bases, BWXT won the contract and started adding TRISO manufacturing line capacity.

Containment at high temperatures

Today's TRISO particles consist of tiny spheres of high-­assay, low-­enriched uranium-defined as uranium enriched to more than 5 percent but less than 20 percent U-­235-encapsulated in multiple ceramic and carbon layers. These particles are then embedded into a graphitic matrix and formed into a "pebble" the size of a pool ball or into a cylindrical compact.

The ceramic and carbon layers maintain dimensional stability and structural integrity, transfer nuclear heat to the coolant, and help keep radioactive fission products contained at very high temperatures.

"In conventional light water fuels, if the fuel starts melting, that's when the fission products start escaping the fuel form," said Joe Palmer, an experiment design engineer at INL. "With TRISO fuel's silicon carbide and ceramic layers, the fission products can be contained at extraordinarily high temperatures-much higher than the melting point of steel."

He continued, "The great thing about TRISO fuel is that when the temperature starts going up, it's able to liberate that energy without releasing those fission products."

Indeed, TRISO fuels can withstand temperatures higher than 1,600°C, the worst-­case accident scenario for some HTGR designs.

However, TRISO fuels can impose limits on reactor design. In some cases, industry is looking for solutions so the fuel can be used in smaller reactors.

"One of the things that I find interesting is that microreactor developers have latched on to TRISO, but it was not originally designed for these small reactors," Stempien said. "One of the criticisms of TRISO is that it is a low-­density fuel form. In a large reactor, that's okay. It makes accidents slower and would lower peak temperatures."

However, Stempien continued, "That lower density means they're having to do a fair amount of design work to make TRISO work in more-­compact microreactors and SMRs."

All of the small reactors that use TRISO fuel rely on passive safety systems that move coolant through the core using convection instead of coolant pumps.

The experiments currently underway at INL build on the AGR tests and could help make TRISO-­powered nuclear reactors more economical, including widespread commercial deployment of smaller reactors.

Palmer has spent decades designing test vessels for insertion into the ATR. To conduct this latest batch of experiments, he and his colleagues reestablished a capability at ATR called the Auxiliary Lead-­Out Experiment House, which began operation in late 2025.

ALE House allows researchers to control the irradiation of TRISO fuels with real-­time monitoring and gas management.

Each test rig, which is about 26 feet long, has multiple chambers to test the samples in different temperature and gas environments simultaneously.

"Because this is a very high-­temperature fuel, we have to design a containment vessel that can irradiate fuel up to 1,500°C while protecting the test rig and ATR itself," Palmer said. "To do that, there's an insulating jacket of noble gases between the stainless steel test rig pressure boundary and the fuel holder. The machined graphite fuel holders can be almost as hot as the fuel."

The XPeRT experiment

The X-­energy Pebble Reactor Test (XPeRT) experiment is the first irradiation and postirradiation examination of a nearly full-­sized TRISO fuel pebble, which will power the company's commercial 80-­MWe/200-­MWt Xe-­100 HTGR.

The 60-­millimeter-­diameter pebbles consist of a graphitic matrix embedded with thousands of TRISO particles. In X-­energy's Xe-­100, approximately 220,000 of these pebbles will circulate through the reactor core. As used pebbles reach the bottom of the core, they are measured for burnup. Pebbles that have not reached full discharge burnup are recirculated back to the top of the core, cycling through the reactor multiple times until fully depleted. Spent pebbles are removed from the reactor automatically.

In the XPeRT experiment, 16 pebbles are being irradiated, said INL researcher Heather Chichester, program technical lead for industry partner programs. The test pebbles used for the experiment are slightly undersized (at 50 mm) to fit within the experiment assembly, which was placed in one of ATR's large flux traps.

"The X-­energy experiment mostly falls within the bounds of the AGR tests," Chichester said. "The big difference is AGR tested a cylindrical compact, and X-energy wants to test fuel performance in the pebble."

"Thermocouples measure the temperature during the test," she continued. "The design allows us to change the gas blend to maintain the temperature we want. The gas that is flowing through the experiment goes to a fission-­product monitor to see if any of the fuel particles have failed and released their fission gas."

TRISO tests for Kaleidos

The Radiant ATR Fuel Test (RAFT) uses HALEU TRISO fuel fabricated by Standard Nuclear and will provide data for the qualification of TRISO fuel for Radiant's Kaleidos microreactor, a helium-­cooled, graphite-­moderated HTGR designed to produce approximately 3 MWt that the company is developing as a transportable alternative to diesel generators in remote and military applications.

TRISO testing for RAFT will occur in parallel with Kaleidos's development unit demonstration in the Demonstration of Microreactor Experiments (DOME) facility at INL. In July 2025, the DOE selected Radiant to perform one of the first fueled microreactor tests in DOME.

Because the initial demonstration in DOME is scheduled to last months, not years, the experiments in ATR will provide Radiant with a better idea of how Kaleidos's TRISO fuel will perform over the long term.

"They want to test the fuel to higher burnup and under more prototypic conditions," Chichester said.

TRISO tests for BANR

BWXT's BANR is a 75-­MWt HTGR designed for producing electricity, industrial heat, and cogeneration with renewables and fossil sources.

The reactor's initial TRISO fuel formulation is UCO HALEU, which allows for higher burnup than uranium dioxide fuel. However, BWXT is testing a range of new TRISO formulations and designs, including uranium nitride (UN) TRISO fuel.

"[UN] fuel kernels are 20 percent to 25 percent denser than UCO," said Stempien. "You're getting more fuel in the same volume. That does a couple of things. At the same power level, they can run longer, and it allows more fuel in the core. Another option is to make the core smaller [to make a reactor easier to transport]."

He continued, "That's a different kernel chemistry. We're not sure what to expect from it. It has good thermal properties, but it is still largely untested."

In total, Stempien and his colleagues are testing five variations of UN and UCO fuel, including UN in a somewhat traditional carbon matrix produced using a nontraditional fabrication process, UN in a silicon carbide matrix, and variations of those two fuels in an annular compact form. These variations all offer theoretical advantages that remain untested under real-­world conditions.

BWXT's BANR tests bring the company's long history of collaboration with the TRISO development process full-­circle.

"BWXT is proud to have played a foundational role in the development and successful irradiation testing of high-­performance TRISO fuel under the U.S. Advanced Gas Reactor program," said Joe Miller, BWXT president for government operations. "The UCO TRISO fuel we manufactured achieved exceptional results at Idaho National Laboratory's Advanced Test Reactor, providing critical data that gives advanced-­reactor developers confidence in TRISO's robustness and safety."

Miller continued, "As irradiation testing at the ATR resumes for gas-­cooled reactor designs, we're building on this strong heritage. Through our [Advanced Reactor Demonstration Program] award with DOE, we are advancing uranium nitride TRISO variants that offer technical advantages for future applications, including potential use in reactors like our BANR microreactor. These efforts underscore the vital importance of INL's world-­class testing capabilities in driving the next generation of advanced nuclear technologies forward."

Postirradiation examination

After irradiation in the ATR, these experiments will undergo postirradiation examination at INL's Materials and Fuels Complex. In addition to typical analyses, such as examining particles using X-­ray tomography and microscopy, researchers will conduct accident tests using high-­temperature furnaces.

Researchers will use the Fuels Accident Condition Simulator to heat the TRISO fuel to between 1,600°C and 1,800°C in a controlled helium environment, targeting the peak temperatures in a reactor accident. They then will evaluate the fission gas releases from the test to see how many TRISO particles survive.

Likewise, the Air Moisture Ingress Experiment allows researchers to heat TRISO fuels in the presence of air or moisture to make sure the fuel is resistant to oxidation reactions under severe accident conditions.

Evolving TRISO fuel

These experiments build on the AGR tests and mark several important steps in the evolution of TRISO fuel at a time when these fuels will power a new wave of advanced reactors.

Because the AGR tests focused on larger reactors, some microreactor or small modular reactor companies are interested in pursuing slightly different designs than the original AGR UCO TRISO fuel. In particular, some smaller reactors could benefit from a higher uranium density than was tested in the AGR experiments.

"Some companies are tweaking the dimensions of the [TRISO particle's] kernel and layer dimensions but are still using UCO," Chichester said. "That's one type of change that feels more evolutionary than revolutionary."

"Switching over to uranium nitride is a bigger step with bigger risk and potentially bigger rewards," she continued. "Uranium nitride has a much higher fuel density, which has a bigger effect on a reactor design. The fabrication takes more work-more development-and we have to see how it behaves."

Cory Hatch is a science writer for Idaho National Laboratory, which is managed by Battelle Energy Alliance for the Department of Energy's Office of Nuclear Energy.