TRISO Fuel: Built-In Safety

TRISO (TRi-structural ISOtropic) particles are advanced nuclear fuel particles designed for exceptional containment. Unlike conventional fuel, where uranium pellets are stacked inside metal cladding, TRISO distributes the fuel into thousands of coated particles. Each particle acts as a small containment system, with multiple carbon and ceramic layers surrounding the fuel kernel [1].

This design makes safety part of the fuel itself. The layered structure helps retain fission products at the particle scale, including under high-temperature conditions up to 1600 °C [2]. As advanced nuclear reactors move toward deployment, fuels with strong inherent safety characteristics will play an important role in building confidence and supporting licensing.

Below is a simplified cross-sectional visualization of a TRISO particle.

STRATOMIC

TRISO Fuel

01

Outer Pyrolytic Carbon

Structural shell & mechanical protection

PyC Outer coating

02

Silicon Carbide Layer

Primary ceramic containment barrier

β-SiC Primary barrier

03

Inner Pyrolytic Carbon

Fission product retention & SiC interface

PyC Gas retention

04

Buffer Layer

Porous carbon — absorbs gas & swelling

Porous C Fission gas volume

05

Fuel Kernel

Uranium oxycarbide fissile core

UCO / UO₂ ≤ 19.75% EU

Selected Layer

Fuel Kernel

The fissile core of the TRISO particle. Uranium oxycarbide or uranium dioxide is formed into a dense sphere and sintered at high temperature. During fission, it generates heat and releases gaseous fission products retained by the surrounding layers.

UCO / UO₂ ≤ 19.75% EU

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References

[1] U.S. Nuclear Regulatory Commission, TRISO Fuel: Properties and Failure Modes, Washington, DC, USA, 2021.

[2] U.S. Nuclear Regulatory Commission, Final Safety Evaluation for EPRI TRISO Topical Report, ADAMS Accession No. ML20216A453, Washington, DC, USA, 2020.