Sixteen spheres the size of billiard balls. Inside each of them are twelve thousand microscopic particles of uranium encased in layers of carbon and ceramic. They are now at the heart of theAdvanced Test Reactor Idaho, the most powerful experimental reactor in the world. For the next thirteen months, they will be bombarded with neutrons, heated, cooled, pushed to the limit. The goal? To demonstrate that this fuel, called TRISO-X (just “TRISO” to his friends), can’t melt. Ever. Not even if he wanted to. X-energy has just started the first large-scale irradiation test of TRISO in the United States. This is not a laboratory experiment: it is the final test before commercialization. If it works, the modular reactors they will have their ultimate fuel.
Thirteen months under neutron bombardment
The test, called X-energy Pebble Reactor Test (XPeRT), started on November 6, 2025 at theIdaho National LaboratoryThe sixteen TRISO-X spheres will be subjected to conditions simulating virtually every imaginable operational scenario: various power levels, extreme temperatures, and different burnup conditions. This is the first test of its kind in the US since 2020 and marks a turning point for advanced nuclear fuel.
After the irradiation cycle in theAdvanced Test Reactor, the spheres will go through post-irradiation testing at both Idaho andOak Ridge National LaboratoryScientists will measure the retention of fission products and the structural stability of the material. The data collected is essential for obtaining approval from the Nuclear Regulatory Commission (NRC), without which no advanced reactor can be commercialized.
Dan Wachs, national technical director for the Department of Energy’s Advanced Fuels Campaign, called this trial run “huge for advanced nuclear advocates.”
The test marks the first irradiation of TRISO fuel for advanced reactors since 2020 and the first use of a new test capability atAdvanced Test Reactor which makes these experiments possible.
Why TRISO is different from everything else
The fuel sad (TRi-structural ISOtropic) is composed of microscopic particles with a nucleus of uranium, carbon, and oxygen. This nucleus is surrounded by three layers of carbon- and ceramic-based materials that prevent the release of radioactive fission products. The particles are the size of poppy seeds and are assembled into spheres the size of a billiard ball.
The triple-coated structure makes each particle a self-contained containment systemUnlike traditional fuel, TRISO can withstand temperatures well above 1.600°C, far above the threshold of current nuclear fuels. According to the Italian Nuclear Association, this feature makes the risk of meltdown physically impossible.
The American race to modular nuclear power
X-energy is building TX-1, a fuel fabrication facility in Oak Ridge, Tennessee. It could become the first U.S. facility authorized by the NRC for the production of advanced nuclear fuel in more than fifty years. The TRISO-X fuel is intended for Xe-100, the small modular reactor (SMR) designed by the company.
The first Xe-100 reactors are planned for the production site of Dow in Seadrift, Texas, to provide electrical power and high-temperature heat for industrial use. Amazon It has also signed an agreement with X-energy to build up to twelve SMRs in Washington state, with the goal of achieving more than 5 GW of new nuclear capacity by 2039.
In parallel, Standard Nuclear and the French giant Framatome have formed a joint venture to produce TRISO fuel on a commercial scale. The new entity, called Standard Nuclear-Framatome (SNF), plans to begin production at its Richland, Washington, facility in 2027. The initial goal is to produce 2 metric tons of TRISO per year to fuel the emerging fleet of advanced reactors.
“TRISO-X embodies decades of American innovation in fuel design,” said J. Clay Sell, CEO of X-energy.
“This testing program brings us one step closer to redefining the standard for safety and reliability in nuclear power.”
In thirteen months we will have the definitive answer. China has already demonstrated that reactors using TRISO fuel can shut down safely without human intervention. The United States wants to certify that its fuel can do so, too. And then build the factory that will mass-produce it.
