TRISO Fuel: The Key to Next-Generation Small Modular Reactors Faces Industrial Hurdles

A seemingly unassuming black sphere, roughly 6 cm in diameter, holds the promise of revolutionizing nuclear energy. These TRISO fuel balls, a cornerstone of advanced Small Modular Reactor (SMR) designs, are at the forefront of a new era in nuclear power. However, unlocking their full potential hinges on overcoming significant industrial challenges in production, fuel testing, and the crucial supply of High-Assay Low-Enriched Uranium (HALEU).

The global pursuit of cleaner, more efficient energy sources has placed nuclear power back in the spotlight. Among the most promising advancements are Small Modular Reactors (SMRs), designed for greater flexibility, enhanced safety, and potentially lower costs compared to traditional large-scale plants. Central to many of these advanced SMR concepts, particularly High-Temperature Reactors (HTRs), is TRISO fuel. This article delves into the intricate world of TRISO fuel, exploring its groundbreaking technology, the ambitious efforts underway to scale its production in the United States, and the critical bottlenecks that must be addressed for its widespread adoption.

The Promise of TRISO: A Fortified Fuel for Advanced Reactors

TRISO, an acronym for "Tristructural Isotropic," describes a highly resilient form of nuclear fuel. At its core, the TRISO fuel concept lies in its micro-particle structure. The actual nuclear fuel – a kernel of uranium, carbon, and oxygen – is minuscule, no larger than a poppy seed. This kernel is then encased in multiple protective layers of specialized materials.

Each TRISO particle boasts four distinct layers:

• Buffer Layer: A porous carbon layer that absorbs fission products and accommodates kernel swelling during operation.
• Inner Pyrolytic Carbon (PyC) Layer: Provides structural integrity and acts as a diffusion barrier.
• Silicon Carbide (SiC) Layer: A ceramic layer that is the primary containment for fission products, offering exceptional resistance to high temperatures and chemical attack.
• Outer Pyrolytic Carbon (PyC) Layer: Further enhances structural integrity and acts as an additional barrier.

The term "tristructural" refers to the three load-bearing layers (inner PyC, SiC, and outer PyC), though the buffer layer makes it technically a four-layered structure. These meticulously engineered particles are then embedded within a graphite matrix, which is then formed into a larger, approximately 6 cm diameter fuel ball. The sheer number of these particles within a single ball – over 18,000 – creates a formidable fuel form.

The genius of TRISO fuel lies in its inherent safety features. Each particle acts as its own miniature containment vessel. In the event of an accident or extreme operational conditions, the robust SiC layer is designed to retain fission products even at very high temperatures, significantly reducing the risk of their release into the environment. This contrasts with conventional light-water reactors, where the metallic cladding of fuel rods is the primary barrier.

Building the Industrial Backbone: The TRISO-X Facility in Oak Ridge

Recognizing the pivotal role of TRISO fuel in the future of nuclear energy, the United States is investing heavily in establishing a domestic industrial supply chain. A prime example is the TRISO-X fuel fabrication facility currently under construction at the Oak Ridge National Laboratory (ORNL) in Tennessee. This facility is poised to become the first commercial-scale production site for TRISO fuel in the USA.

The state of Tennessee is supporting the expansion of the Oak Ridge site with an $11 million investment, underscoring the strategic importance of this project. The ambitious TX-1 facility is designed to process approximately 5 tons of uranium annually and is slated to commence production in early 2028. This facility has already secured a significant milestone: in February 2026, the U.S. Nuclear Regulatory Commission (NRC) granted TRISO-X a 40-year license under 10 CFR Part 70. This license permits the company to possess, use, and manufacture special nuclear material for commercial fuel production, marking the first new U.S. fuel fabrication license of this category in approximately 50 years.

However, the path to full operational capacity is not without its challenges. The exact scope of the expansion, the total investment, and the final timeline for subsequent phases are still being determined. Furthermore, the first reactor facility slated to utilize this fuel, the X-energy Xe-100, is still navigating its own licensing and construction processes and has yet to receive a construction permit.

The TRISO-X campus plan encompasses three key pillars:

• Particle Manufacturing: Producing the individual TRISO fuel particles with their complex layered structure.
• Ball Fabrication: Encasing these particles within the graphite matrix to form the final fuel balls.
• Fuel Testing and Qualification: Rigorous testing to ensure the fuel’s performance under various operating conditions.

The HALEU Bottleneck: Fueling the Future Requires Enriched Uranium

Beyond the intricate manufacturing of the fuel balls themselves, a critical upstream challenge lies in the supply of enriched uranium. The advanced reactor designs that utilize TRISO fuel, such as the Xe-100, require High-Assay Low-Enriched Uranium (HALEU).

HALEU is defined as uranium enriched to contain more than 5% and up to nearly 20% of the fissile isotope Uranium-235. This is a higher enrichment level than that found in conventional low-enriched uranium (LEU), which typically contains up to 5% Uranium-235 and is used in most current light-water reactors.

The higher enrichment of HALEU offers several advantages for advanced reactor designs:

• Compact Core Designs: It allows for smaller, more efficient reactor cores.
• Extended Fuel Burnup: Reactors can operate for longer periods between refueling.
• Higher Operating Temperatures: It supports the higher operating temperatures characteristic of many advanced reactor concepts, enabling more efficient energy conversion and process heat applications.

Unfortunately, the domestic production of HALEU in the United States is currently insufficient to meet the projected demand. While the U.S. Department of Energy anticipates a domestic need of up to 50 tons per year by 2035, current enrichment capacity falls far short of this requirement. This shortfall creates a significant bottleneck for the planned deployment of advanced reactors.

The TRISO-X facility, while licensed to produce fuel, does not enrich uranium itself. It relies on an external supply of pre-enriched HALEU. The planned commencement of fuel fabrication at TX-1 in early 2028 is a tight timeline, as larger-scale U.S. HALEU enrichment capabilities are primarily announced for post-2028. Without a robust and readily available supply of HALEU, the TX-1 facility will be unable to operate at its full planned capacity, impacting the deployment schedules of SMRs like the Xe-100.

Fuel Qualification: Proving TRISO’s Mettle Under Fire

While TRISO fuel has a long history of research and development, the specific TRISO-X variant designed for the Xe-100 requires independent qualification for commercial use. This process involves rigorous testing to demonstrate its performance and safety under a wide range of operational and accident conditions.

Since November 2025, tests have been underway at the Advanced Test Reactor (ATR) at the Idaho National Laboratory (INL). Sixteen TRISO-X fuel balls are undergoing a 13-month irradiation campaign. The primary objectives of these tests are to:

• Assess Fission Product Retention: Evaluate the effectiveness of the TRISO particle layers in containing fission products at high temperatures and under irradiation.
• Monitor Structural Integrity: Examine how the fuel balls and their constituent particles withstand the stresses and strains of reactor operation, including thermal cycling and neutron flux.
• Evaluate Performance Against Design Specifications: Ensure the fuel meets the stringent performance requirements set forth by the reactor designers and regulators.

Following irradiation, detailed post-irradiation examinations will be conducted at INL and Oak Ridge National Laboratory. The findings from these tests are crucial for the formal qualification of the fuel for commercial deployment.

It is important to note that the licensing process for TRISO fuel involves three distinct, legally separated procedures:

1. Fuel Fabrication Facility License: This license, already granted to TX-1 by the NRC, pertains to the manufacturing and handling of enriched material.
2. Fuel Qualification: This process proves the fuel’s behavior under operational and accident scenarios.
3. Reactor License: This is the permit required for the construction and eventual operation of the Xe-100 reactor itself.

The license for the TX-1 fabrication facility does not automatically imply that the fuel has met all the requirements for use within a reactor.

Not All SMRs Need TRISO: A Diverse Technological Landscape

The term "Small Modular Reactor" encompasses a wide array of technological approaches. While TRISO fuel is a key component for many advanced reactor designs, it is not a universal requirement for all SMRs. Compact light-water reactors, for instance, continue to utilize traditional fuel assemblies with uranium oxide pellets and do not require TRISO balls.

TRISO fuel is particularly relevant for non-water-cooled reactors, including:

• High-Temperature Reactors (HTRs): These reactors use gas coolants (like helium) and operate at elevated temperatures, making TRISO’s thermal resilience highly advantageous.
• Salt-Cooled Reactors: Some molten salt reactor concepts can also benefit from the robust nature of TRISO fuel.
• Microreactor Concepts: The compact and inherently safe characteristics of TRISO fuel make it suitable for small, transportable microreactor designs.

While TRISO particles can be embedded in various forms, including cylindrical pellets and graphite blocks, the spherical fuel ball design is prominent in concepts like X-energy’s Xe-100. The Xe-100 is a helium-cooled HTR designed to produce 80 MW of electrical power or up to 200 MW of thermal power. A four-module plant could deliver 320 MW of electrical output, with the high outlet temperatures ideal for both electricity generation and industrial process heat.

A Continuous Cycle: How TRISO Fuel Moves Through the Reactor

The operational concept for reactors like the Xe-100 is distinct from traditional light-water reactors. Instead of requiring complete shutdown for refueling, TRISO fuel balls are designed to circulate continuously through the reactor core. In the Xe-100, an estimated 200,000 fuel balls would be in circulation.

The continuous refueling process unfolds as follows:

1. Fuel Ball Loading: Fresh fuel balls are loaded into the top of the reactor core.
2. Core Circulation: As the balls move down through the core, they are subjected to the reactor’s neutron flux, generating energy.
3. Spent Fuel Removal: Once their useful life is complete, spent fuel balls are automatically discharged from the bottom of the core.
4. Testing and Recycling: These discharged balls are then tested. If still viable, they can be reinserted into the reactor for further use, optimizing fuel utilization.

This dynamic process allows for continuous operation and efficient fuel management, a hallmark of advanced reactor designs.

Production Capacity: Meeting the Demand for a Growing Fleet

The TX-1 facility in Oak Ridge is projected to produce approximately 700,000 fuel balls annually. According to the U.S. Department of Energy, this output is intended to meet the ongoing demand for up to eleven Xe-100 modules. However, this figure represents the annual replenishment for operating reactors, not the initial core loading for eleven complete plants. Each reactor core requires a substantial initial quantity of fuel balls, and the annual production capacity is for ongoing fuel supply to maintain operation.

The Road Ahead: Challenges and Opportunities

The development of TRISO fuel and its associated industrial infrastructure represents a significant undertaking. The successful establishment of a robust supply chain, from HALEU enrichment to fuel fabrication and qualification, is paramount. The U.S. government’s commitment to fostering this sector through investments and regulatory support is a crucial factor.

The successful deployment of TRISO-based SMRs could have profound implications:

• Energy Security and Independence: Reducing reliance on fossil fuels and enhancing domestic energy production.
• Climate Change Mitigation: Providing a low-carbon source of electricity and heat.
• Industrial Decarbonization: Supplying high-temperature heat for industrial processes, enabling the decarbonization of sectors like chemical manufacturing.
• Economic Growth: Creating jobs in advanced manufacturing and the nuclear industry.

The journey from concept to widespread commercialization is complex and fraught with technical, regulatory, and economic hurdles. However, the innovative nature of TRISO fuel and the dedicated efforts to build the necessary industrial capacity signal a promising future for advanced nuclear energy. The world will be watching as these black spheres, once confined to laboratories, begin to power the next generation of clean energy solutions.

The German Roots of TRISO Technology

It is important to acknowledge that the foundational technology for TRISO fuel and pebble-bed reactors has significant roots in Germany. Decades ago, Germany pioneered research and development in this area, with notable projects like the AVR (Arbeitsgemeinschaft Versuchsreaktor) in Jülich and the THTR-300 (Thorium High-Temperature Reactor) in Hamm-Uentrop. These early experimental and prototype reactors demonstrated the viability of circulating spherical fuel elements.

While the THTR-300, Germany’s first large-scale pebble-bed power reactor, ceased operations in 1989 due to technical and economic challenges, the lessons learned were invaluable. Internationally, the concept continued to evolve, most notably with the HTR-PM (High Temperature Reactor-Pebble Module) project in China, which commenced commercial operation in December 2023. This facility features two reactors that collectively drive a steam turbine, demonstrating the ongoing global interest and development in this technology.

The U.S. effort, particularly with the TX-1 facility, represents a concerted push to establish a comprehensive, commercial-scale industrial supply chain for TRISO fuel, including the crucial HALEU component, which has been a significant bottleneck in previous iterations. The ambition is not just to build reactors but to create an entire ecosystem for advanced nuclear fuel production within the United States.