TRISO-X: Advanced Fuel for Next-Generation Nuclear Energy

The Most Robust Nuclear Fuel on Earth

The U.S. Department of Energy describes TRISO particle fuel as “the most robust nuclear fuel on Earth.” The distinction is well-earned. Tri-Structural Isotropic fuel is a complete reimagination of nuclear fuel design, and a central pillar of the intrinsic safety and high outlet temperature that advanced reactors aim to achieve.

While its form and composition have evolved over time, the theory of the case has always been clear: Nuclear energy can do more, and it starts with the fuel.

Inside the TRISO Particle

Conventional nuclear fuel typically consists of uranium pellets stacked inside long metal tubes, called fuel rods. which are bundled into fuel assemblies, and inserted into the reactor core. This technology has powered water-cooled reactors safely since the 1960’s by relying on the integrity of the fuel’s metal cladding, backup cooling systems, and large external containment structures.

For TRISO-fueled reactors, the fuel itself becomes the primary containment system. Rather than concentrating bulk uranium – and irradiation – in stacked pellets, TRISO distributes the fuel across thousands of coated particles, each roughly one millimeter in diameter.

Each TRISO particle consists of four microscopic layers that serve a distinct role:

• Uranium Kernel: At the center of every particle is a kernel of High-Assay Low-Enriched Uranium (HALEU), the engine of nuclear fission. HALEU allows for significantly higher fuel burnup, allowing the Xe-100 to extract more energy from every gram of uranium than conventional reactors.
• Carbon Buffer: Surrounding the kernel is a porous carbon layer that provides a plenum for fission gases to collect without building up excessive pressure, and shielding the outer layers from potential damage caused by recoiling fission fragments.
• Inner and Outer Pyrolytic Carbon: These dense carbon layers provide a gas-tight seal and a structural foundation, helping maintain the integrity of the particle over long periods of irradiation.
• Silicon Carbide: This is the primary structural shell of the particle, a ceramic material with exceptional strength and thermal resistance. This layer acts as the primary barrier against the release of fission products, designed to withstand extreme temperatures without melting.

These particles are then embedded in a graphite matrix to create fuel forms suited to different reactor designs. In pebble-bed reactors like X-energy’s Xe-100, approximately 18,000 particles are pressed into billiard-ball-sized spheres that circulate slowly through the core. In prismatic block reactors like X-energy’s XENITH microreactor, particles are bonded into cylindrical compacts stacked within hexagonal graphite blocks. Same technology, different geometry—both designed to optimize for their reactor’s thermal and operational requirements.

The TRISO Safety Case: Physics vs. Redundancy

TRISO fuel offers a fundamentally different approach to nuclear safety, and it comes down to a basic question that extends beyond just fuel: How does a nuclear technology ensure safety? Conventional nuclear fuel relies on the integrity of the fuel’s metal cladding, mechanical backup systems that require operator intervention, and external containment structures. Safety is ensured through active redundancy.

TRISO fuel is designed to ensure safety through passive physics, precision-engineered to function as a self-reliant multi-layered vault that traps and retains harmful radionuclides at the source, exceeding the work of thousands of tons of concrete and steel in an area the size of a poppy seed.

In the Xe-100, if core temperature rises unexpectedly, three things are designed to happen automatically:

1. Graphite moderates the reaction: Nuclear fission requires neutrons to be slowed down (moderated) before they can split uranium atoms. As temperature increases, graphite slightly expands and becomes less dense, meaning neutrons have to travel farther to find another uranium atom to split. This naturally slows the fission rate.
2. Fission rate decreases: Higher temperatures cause neutrons to move faster on average. Faster neutrons are less likely to cause fission in the fuel. The hotter the core gets, the less efficient fission becomes — a built-in brake pedal controlled by physics, not computers or humans.
3. The reactor shuts down automatically: These two effects combine to create what physicists call a “negative temperature coefficient”—meaning higher temperature naturally reduces power output. The reactor doesn’t need control rods, emergency systems, or operator commands to shut down. These still exist, but physics does the work.

The four particle layers are designed to remain structurally sound at temperatures exceeding 1,600°C. To put this in perspective, the maximum temperature the Xe-100 can reach, even if all cooling systems were to fail and all operators were to leave the site, is well below this threshold. This is intrinsic safety in principle: innovative design, underpinned by the immovable laws of physics and thermodynamics.

Intrinsic Safety in Nuclear Licensing

In nuclear licensing, this basic question of how your technology ensures safety informs much of the downstream regulatory process. By systematically demonstrating our safety case through pre-application engagement with the NRC, our TRISO-X fuel has helped to unlock new efficiencies in nuclear licensing:

• Smaller emergency planning zone (EPZ): 400 meters for the Xe-100 vs. 10 miles for conventional light-water reactors.
• Industrial Co-Location: Compact footprint enables siting at manufacturing facilities, such as our first project with Dow under ARDP, delivering high thermal output directly at the point of need.
• Environmental Assessment: In May 2026, the Xe-100 became the first commercial reactor in U.S. history to receive NRC environmental approval through an Environmental Assessment rather than a multi-year Environmental Impact Statement, in-part due to the strong safety profile of our fuel.

U.S. Leadership: How Concept Becomes Commercial

The safety case for TRISO fuel is not a theoretical aspiration. The first TRISO fuels date back to the 1960s, and its design has been iterated on over decades of government-led programs testing, refinement, and validation, most significantly in the United States with the U.S. Department of Energy’s Advanced Gas Reactor Program (AGR) through Idaho National Laboratory (INL) and Oak Ridge National Laboratories (ORNL).

Since 2002, the AGR program has conducted a systematic irradiation campaign that has made TRISO the most thoroughly tested and validated advanced nuclear fuel form in the world, setting world-records for nuclear fuel performance:

• AGR-1 achieved peak burnup of 19% FIMA (a measure of how much energy is extracted from a given amount of nuclear fuel) compared to an average 5% FIMA for LWR fuel, and more than double the previous TRISO record.
• Zero coating failures across 300,000 particles after three years of testing at 1250°C 
• Retained fission products at temperatures up to 1,800°C, validated by post-irradiation testing.

TRISO-X Fuel Qualification

Since 2016, X-energy and TRISO-X have used production specifications established by the AGR program to develop our proprietary TRISO-X fuel. Today, that same fuel is building on what the AGR program started, with a batch of fuel pebbles currently undergoing confirmatory irradiation testing in INL’s Advanced Test Reactor, the same reactor where modern TRISO fuel proved its potential.

This 13-month campaign is designed to evaluate how TRISO-X performs under a wide range of power levels, temperatures, and burnup conditions. By testing the fuel to the point of failure in a controlled environment, our team can validate the safety margins and performance metrics required for commercial licensing, giving us data-driven confidence in our fuel’s performance. 

TRISO-X: Closing the Commercial Gap

For decades, TRISO fuel existed primarily in research quantities. Demonstration reactors and test programs operated on fuel produced in pilot facilities, but no commercial-scale manufacturing capability existed in the United States to bridge the gap between laboratory validation and fleet deployment.

X-energy and TRISO-X are working to change that, and are currently constructing TX-1, a first-in-the-nation fuel fabrication facility in Oak Ridge, Tennessee, to establish a secure, domestic source of high-quality commercial TRISO fuel for the first time in U.S history. TX-1 is the first of three expected facilities in a world-class nuclear fuel campus dedicated solely to nuclear fuel innovation:

• TX-1: Expected to be the first purpose-built commercial TRISO fuel fabrication facility in the United States, currently under construction, and designed to establish domestic production of TRISO-X fuel.
• TX-2: A second, larger fuel fabrication facility to significantly expand capacity, with enhanced automation and production efficiency based on TX-1 operational experience, currently in the design phase.

In February of 2026, TX-1 and TX-2 received a first-ever Category II Special Nuclear Material License under 10 CFR Part 70, granting federal authorization for commercial fabrication of TRISO-X fuel under an initial 40-year license, effective upon completion and final inspection. Part 70 approval makes TX-1 and TX-2 the first new fuel facilities licensed in the United States in over 50 years.

A Foundation for the Future

TRISO fuel stands as one of the great technical achievements of the nuclear industry. We believe the opportunity to commercialize and scale it should be equally regarded as one of its great opportunities, not just for innovation’s sake, but because of the urgent need that demands this innovation.

The theory has always been that nuclear energy can do more. Today, the reality is that nuclear energy has to do more. The global economy is changing. Heavy industry, high-scale manufacturing, and the growing infrastructure of artificial intelligence and cloud computing require modern solutions to meet the challenges of the modern economy.

The ability for any new nuclear technology to meet those challenges rests firmly on a secure, domestic fuel supply, designed from the atom up to solve for what’s next.