# TRISO fuel
**TRISO fuel** (tristructural isotropic fuel) is a nuclear fuel made of uranium-bearing microspheres, each about the size of a poppy seed, encapsulated in four ceramic coating layers of pyrolytic carbon (dense carbon layer deposited from hot hydrocarbon gas) and silicon carbide that retain radioactive fission products[1](https://www.energy.gov/ne/articles/nuclear-101-what-high-temperature-gas-reactor). The particles are embedded in a graphite matrix to form cylindrical compacts for prismatic reactors or billiard-ball-sized pebbles for [pebble-bed reactors](https://www.edgepedia.org/pebble-bed-reactor), and they are the reference fuel for high-temperature gas-cooled reactors (HTGRs) and many microreactor designs[2](https://art.inl.gov/programs/triso-fuel-qualification/). The U.S. Department of Energy describes the form as the most robust nuclear fuel on Earth because the coated particles cannot melt in a reactor under design-basis conditions[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth).
| Key fact | Detail |
|---|---|
| Particle structure | Fuel kernel surrounded by a porous carbon buffer, inner pyrolytic carbon (IPyC), silicon carbide (SiC), and outer pyrolytic carbon (OPyC)[2](https://art.inl.gov/programs/triso-fuel-qualification/) |
| Kernel composition | Commonly uranium oxycarbide (UCO) or uranium dioxide (UO₂)[2](https://art.inl.gov/programs/triso-fuel-qualification/) |
| Size scale | Coated particle about 0.92 mm with a roughly 0.5 mm kernel; a pebble carries a 5 mm graphite layer around embedded particles[4](https://www.sandia.gov/app/uploads/sites/273/2024/07/Advanced-Reactors-Spent-Fuel-and-Waste-Streams-Disposition-Strategies-2335764.pdf) |
| Burnup record | 19% fissions per initial metal atom achieved in a three-year test at Idaho National Laboratory in 2009, about three times the burnup of light-water fuels[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth) |
| Accident testing | Irradiated fuel held for more than 300 hours at up to 1,800 °C with no to minimal particle damage and low fission product release[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth) |
| Enrichment | Advanced reactor fuel designs may use enrichments up to 20 weight percent U-235, typically high-assay low-enriched uranium (HALEU)[5](https://www.nrc.gov/materials/new-fuels/fabrication) |
| Commercial status | TRISO-X holds a 40-year NRC special nuclear material license for its Oak Ridge fabrication facility, with fuel production expected to begin in early 2028[6](https://www.energy.gov/ne/articles/triso-x-receives-nrc-special-nuclear-material-license-advanced-fuel-fabrication) |
## Structure of the particle
A standard [TRISO particle](https://www.edgepedia.org/triso-particle) is built from the center outward in five layers[2](https://art.inl.gov/programs/triso-fuel-qualification/):
1. **Fuel kernel**, commonly uranium oxycarbide (UCO) or uranium dioxide (UO₂), which contains the fissile material.
2. **Porous carbon buffer**, which accommodates [fission gas release](https://www.edgepedia.org/fission-gas-release) and protects the inner pyrolytic carbon layer from fission recoils.
3. **Inner pyrolytic carbon (IPyC)**, which contains fission gas and provides structural support.
4. **Silicon carbide (SiC)**, the main structural layer and the primary barrier to fission-product release.
5. **Outer pyrolytic carbon (OPyC)**, which protects the SiC layer and contributes to overall particle integrity.
The DOE describes these layers as acting like individualized containment domes that largely retain radioactive fission products[1](https://www.energy.gov/ne/articles/nuclear-101-what-high-temperature-gas-reactor). The ceramic fuel form means the particles cannot melt in a high-temperature reactor under any design-basis condition, and they are structurally resistant to neutron irradiation, corrosion, and high temperatures[1](https://www.energy.gov/ne/articles/nuclear-101-what-high-temperature-gas-reactor).
## Fuel forms and reactor types
TRISO particles are embedded in a graphite-based matrix in one of two common forms[2](https://art.inl.gov/programs/triso-fuel-qualification/). Cylindrical fuel compacts, stacks of which sit in channels drilled through hexagonal graphite blocks, are associated with prismatic HTGR concepts. Spherical fuel pebbles, roughly billiard-ball sized, are associated with pebble-bed reactor concepts[1](https://www.energy.gov/ne/articles/nuclear-101-what-high-temperature-gas-reactor).
In a pebble-bed core, helium is blown down through a bed of TRISO pebbles surrounded by graphite reflector blocks, and the reactor is refueled continuously by adding fresh pebbles at the top while older ones are discharged at the bottom; discharged pebbles are evaluated for reinsertion or on-site storage[1](https://www.energy.gov/ne/articles/nuclear-101-what-high-temperature-gas-reactor). A single TRISO-X pebble designed for the Xe-100 reactor contains over 18,000 TRISO particles[6](https://www.energy.gov/ne/articles/triso-x-receives-nrc-special-nuclear-material-license-advanced-fuel-fabrication).
The fuel enables high reactor outlet temperatures, with HTGR coolant outlet temperatures approaching 1,000 °C, supporting efficient electricity generation and industrial heat uses such as petrochemical production, oil refining, and clean hydrogen production[7](https://www.ornl.gov/publication/coated-particle-fuel-historical-perspectives-and-current-progress) • [1](https://www.energy.gov/ne/articles/nuclear-101-what-high-temperature-gas-reactor). TRISO particles can also be fabricated into compacts or pebbles for molten salt-cooled reactors[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth).
## Manufacturing
Manufacturing proceeds in three broad stages: kernel formation, coating deposition, and fabrication into compacts or pebbles, with quality control applied at each stage[2](https://art.inl.gov/programs/triso-fuel-qualification/).
**Kernel formation.** UO₂ and UCO microspheres are produced by external gelation, internal gelation, or a combination of the two, followed by sintering and upgrading to remove undersized, oversized, and high-aspect-ratio kernels before quality control against fuel specifications[8](https://www.sciencedirect.com/science/article/abs/pii/S002954931100906X).
**Coating deposition.** The TRISO coatings are deposited by fluidized bed chemical vapor deposition. At [Oak Ridge National Laboratory](https://www.edgepedia.org/oak-ridge-national-laboratory)'s laboratory-scale coater, kernels are measured by optical shadow imaging and then coated in a 50-mm diameter conical chamber, with computer control of temperature and gas composition and typical batch sizes of 60 to 100 g[9](https://www.ornl.gov/research-highlight/fabrication-and-characterization-uranium-based-high-temperature-reactor-fuel). In the AP account of TRISO-X production, the black kernel spheres, about the size of poppy seeds, pass through coating machines at temperatures as high as 1,800 °C to receive their protective carbon coatings[10](https://apnews.com/article/b60efd5433ca4f336e2f7f035cd37538).
**Overcoating and compacting.** A protective overcoat of resinated graphite matrix precursor is applied to the coated particles, which are then pressed into compacts[9](https://www.ornl.gov/research-highlight/fabrication-and-characterization-uranium-based-high-temperature-reactor-fuel). The overcoat provides the matrix that forms the compact, protects particles during handling, separates them evenly into a homogeneous array, and creates a stable phase with good thermal conductivity[8](https://www.sciencedirect.com/science/article/abs/pii/S002954931100906X).
In U.S. Department of Energy programs, fabrication was progressively scaled from laboratory to engineering scale through successive experiments, with improvements aimed at reducing coating defects and tightening property distributions[2](https://art.inl.gov/programs/triso-fuel-qualification/).
## Performance and safety testing
TRISO fuel was first developed in the United States and United Kingdom in the 1960s with uranium dioxide fuel; coated particle concepts date back roughly 60 years, evolving from primitive pyrocarbon-coated kernels to the modern particle[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth) • [7](https://www.ornl.gov/publication/coated-particle-fuel-historical-perspectives-and-current-progress). In 2002 the DOE focused on improving TRISO fuel using uranium oxycarbide kernels, and in 2009 the improved fuel set an international record by achieving 19% maximum burnup with excellent fuel performance during a three-year test at [Idaho National Laboratory](https://www.edgepedia.org/idaho-national-laboratory), nearly double the previous German mark from the 1980s and about three times the burnup of current light-water fuels[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth).
The irradiated fuel was then exposed to more than 300 hours of testing at temperatures up to 1,800 °C, exceeding predicted worst-case accident conditions for HTGRs, with no to minimal particle damage and low fission product release[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth).
Detailed results come from the AGR-1 post-irradiation heating tests. Fourteen fuel compacts, about 58,000 particles with burnup from 13.4% to 19.1% fissions per initial metal atom, were heated at 1,600 to 1,800 °C in flowing helium while fission product release was monitored[11](https://inl.elsevierpure.com/en/publications/performance-of-agr-1-high-temperature-reactor-fuel-during-post-ir/). Cesium was very well retained by intact SiC layers, with fractional release below 5 × 10−6 after 300 hours at 1,600 °C or 100 hours at 1,800 °C, and no complete coating failures were observed after 300 hours at 1,600 or 1,700 °C[11](https://inl.elsevierpure.com/en/publications/performance-of-agr-1-high-temperature-reactor-fuel-during-post-ir/). Krypton release from coating failures was observed only after about 210 hours at 1,800 °C in one compact, and release of silver, europium, and strontium through intact coatings may become significant after about 100 hours at 1,800 °C[11](https://inl.elsevierpure.com/en/publications/performance-of-agr-1-high-temperature-reactor-fuel-during-post-ir/).
## Qualification and commercialization
In DOE fuel qualification programs, TRISO fuel is evaluated under representative reactor conditions through irradiation testing, post-irradiation examination, and safety testing, with goals including high burnup and assessments of fission product retention at elevated temperatures; the resulting data inform design, safety analysis, and licensing of gas-cooled reactor concepts[2](https://art.inl.gov/programs/triso-fuel-qualification/). The DOE's Advanced Gas Reactor (AGR) Fuel Development and Qualification program pursues qualification of TRISO coated-particle fuel for HTGRs, with experiments AGR-1 through AGR-8 planned to provide irradiation performance data, qualify fuel for normal operating conditions, and support fuel performance and fission product transport models[12](https://www.energy.gov/sites/prod/files/CX-013314.pdf). Irradiation experiments are conducted in the [Advanced Test Reactor](https://www.edgepedia.org/advanced-test-reactor) at Idaho National Laboratory[2](https://art.inl.gov/programs/triso-fuel-qualification/).
Commercial fabrication is now moving from government programs to industry. In February 2026 the NRC approved a 40-year Part 70 special nuclear material license for TRISO-X, a wholly owned subsidiary of X-energy, authorizing commercial fuel production at the TX-1 facility in [Oak Ridge, Tennessee](https://www.edgepedia.org/oak-ridge-tennessee), where fuel fabrication is expected to begin in early 2028[6](https://www.energy.gov/ne/articles/triso-x-receives-nrc-special-nuclear-material-license-advanced-fuel-fabrication). TRISO-X and X-energy have operated a pilot facility at Oak Ridge National Laboratory since 2016, and irradiation testing of TRISO-X fuel is underway at the Advanced Test Reactor[6](https://www.energy.gov/ne/articles/triso-x-receives-nrc-special-nuclear-material-license-advanced-fuel-fabrication). The Xe-100 reactor the pebbles are designed for is an 80-megawatt-electric, 200-megawatt-thermal HTGR, and in May 2025 the NRC accepted a construction permit application from Dow to build the first four Xe-100 units at its Seadrift, Texas manufacturing site[6](https://www.energy.gov/ne/articles/triso-x-receives-nrc-special-nuclear-material-license-advanced-fuel-fabrication).
Fuel supply depends on high-assay low-enriched uranium, enriched between 5% and 19.75% U-235[13](https://www.energy.gov/ne/articles/what-high-assay-low-enriched-uranium-haleu). In January 2026 the fuel fabrication company Standard Nuclear received its first shipment of HALEU from DOE, which it will use to produce TRISO fuel for a Radiant Industries microreactor[14](https://files.gao.gov/reports/GAO-26-107385/index.html). Reactor vendors including X-energy and Kairos Power, along with the Department of Defense, plan to use TRISO fuel in small modular reactor and microreactor designs[3](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth).
## References
1. [Nuclear 101: What Is a High-Temperature Gas Reactor? U.S. Department of Energy](https://www.energy.gov/ne/articles/nuclear-101-what-high-temperature-gas-reactor)
2. [TRISO Fuel Qualification, INL Advanced Reactor Technologies](https://art.inl.gov/programs/triso-fuel-qualification/)
3. [TRISO Particles: The Most Robust Nuclear Fuel on Earth, U.S. Department of Energy](https://www.energy.gov/ne/articles/triso-particles-most-robust-nuclear-fuel-earth)
4. [Advanced Reactors Spent Fuel and Waste Streams Disposition Strategies, Sandia National Laboratories](https://www.sandia.gov/app/uploads/sites/273/2024/07/Advanced-Reactors-Spent-Fuel-and-Waste-Streams-Disposition-Strategies-2335764.pdf)
5. [Fuel Fabrication of New Fuels, U.S. Nuclear Regulatory Commission](https://www.nrc.gov/materials/new-fuels/fabrication)
6. [TRISO-X Receives NRC Special Nuclear Material License for Advanced Fuel Fabrication Facility, U.S. Department of Energy](https://www.energy.gov/ne/articles/triso-x-receives-nrc-special-nuclear-material-license-advanced-fuel-fabrication)
7. [Coated particle fuel: Historical perspectives and current progress, Oak Ridge National Laboratory (Journal of Nuclear Materials, 2019)](https://www.ornl.gov/publication/coated-particle-fuel-historical-perspectives-and-current-progress)
8. [Fabrication of uranium oxycarbide kernels and compacts for HTR fuel, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/S002954931100906X)
9. [Fabrication and Characterization of Uranium-based High Temperature Reactor Fuel, Oak Ridge National Laboratory](https://www.ornl.gov/research-highlight/fabrication-and-characterization-uranium-based-high-temperature-reactor-fuel)
10. [A look inside a lab making the advanced fuel to power growing US nuclear energy ambitions, Associated Press](https://apnews.com/article/b60efd5433ca4f336e2f7f035cd37538)
11. [Performance of AGR-1 high-temperature reactor fuel during post-irradiation heating tests, Nuclear Engineering and Design 306 (2016)](https://inl.elsevierpure.com/en/publications/performance-of-agr-1-high-temperature-reactor-fuel-during-post-ir/)
12. [CX-013314: Categorical Exclusion Determination, AGR Fuel Development and Qualification Program, U.S. Department of Energy](https://www.energy.gov/sites/prod/files/CX-013314.pdf)
13. [What is High-Assay Low-Enriched Uranium (HALEU)? U.S. Department of Energy](https://www.energy.gov/ne/articles/what-high-assay-low-enriched-uranium-haleu)
14. [GAO-26-107385, Nuclear Fuel: Actions Needed to Enhance Cost Reporting and Economic Analysis for Federal Uranium Supply Efforts](https://files.gao.gov/reports/GAO-26-107385/index.html)
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*Topic: Encyclopedia › Technology and the built world › Energy technology › Nuclear power › Reactor designs and fuel technology*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026; Oct 25, 2026 · Last review: Oct 15, 2026*
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