The TRISO Fuel Revolution

This is a TRISO particle from Ultra Safe Nuclear. The central sphere is a nuclear fuel like Uranium Carbon Oxide (UCO), and the shells are made of various ceramics.

And it gets put into fuel pellets, usually cylinders, like this one from Ultra Safe Nuclear.

TRISO Fueled Reactors

There are more than a dozen reactors being designed to use TRISO fuel. All but three use helium coolant. My perspective is that using anything but helium for coolant reintroduces chemical reaction accidents that cripple other reactor designs, significantly detracting from the TRISO safety benefits. In any case, these three reactors are: 1) eVinci using TRISO in a graphite moderator cooled by heat pipes; 2) FHR using TRISO in hollow graphite pebbles that float in a big bath of molten salt coolant; and 3) BWXT PELE reactor designed for the US DOD. Knowing how uncreative military projects tend to be, I suspect it's a TRISO fuel in water moderator with nitrogen or helium gas cooling, like the original US Army ML-1 transportable reactor from the 1960s. The main benefit is that they don't have to haul the water in and out.

So it seems that TRISO-fueled reactors soon be all over the place. This post is aimed at showing people what TRISO particles are, how they work, and why they are different and better than traditional nuclear fuel. My intent is to provide a better resource compared to marketing or articles found online that are minimally researched and have little to no understanding concerning TRISO's technological basis and history.

Name	Full Name	Vendor	Status	Enrichment (%)
HTR-PM	High Temperature Reactor Pebble Module	DBD Limited / INET	Operating	9.08
HTTR	High Temperature Test Reactor	JAEA	Operating	9
BWXT PELE	BWXT's Advanced Nuclear Reactor for DOD PELE	BWX Technologies. Inc.	Stated 2024+ completion	19.75
MMR	Micro Modular Reactor	Ultra Safe Nuclear	Stated 2026 completion	9.9
FHR	Fluoride Salt-cooled high-temperature reactor	Kairos Power, LLC	Stated late 2020s	19.75
U-Battery	U-Battery HTGR	U-Battery/Urenco	Stated late 2020s	19.75
PYLON	Pylon	Ultra Safe Nuclear	Stated late 2020s completion	19.75
Kaleidos	Kaleidos	Radiant Nuclear	Stated 2026 initial core tests	19.75
eVicnci	eVinci heat-pipe micro reactor	Westinghouse	Stated 2028 completion	19.75
Xe-100	Xe-100	X-Energy, LLC	Stated 2030 completion	15.5
BANR	BWXT's Advanced Nuclear Reactor	BWX Technologies. Inc.	Concept	19.75
EM2	Energy Multiplier Module	General Atomics	Concept	14.5
MGHTR	Modular Integrated Gas-Cooled High Temperature Reactor	Boston Atomics LLC	Concept	19.75

Note on Heat Exchangers

Of the helium cooled reactor designs, most have abandoned the idea of a helium-steam heat exchanger which has historically led to serious maintenance and accident issues. The problem with a helium-steam heat exchanger is that water often leaks into the helium primary cooling circuit, where it can damage components. It seems that only the Xe-100, EM2, and MGHTR designs still intend to use helium-steam heat exchangers. All other concepts appear to use helium-molten salt (MMR), helium-nitrogen (U-Battery), helium-sCO2, or direct helium Brayton power conversion.

The TRISO Particle Fuel Concept

Originally conceived in 1957 ¹, TRistructural ISOtropic (TRISO) particle fuel consists of fissile Uranium or Thorium fuel kernels measuring 350 to 600 μm across and coated with layers of Pyrolytic Carbon and Silicon Carbide. TRISO particles are the fuel spheres with ceramic layers, while the matrix is the ceramic material that surroundings the TRISO particles to form a consolidated fuel pellet, usually a cylinder, measuring about 1-2 cm in diameter. The matrix is traditionally graphite but improved versions use SiC or even ZrC.

TRISO particle fuel is fundamentally different from conventional fuel which are centimeter sized pellets of Uranium Dioxide enclosed in Zircalloy tubes. The driving idea behind TRISO particle fuel is to give each 350 to 600 μm sized piece of nuclear fuel its own containment and pressure vessel to enhance the fuel’s ability to contain fission products at very high temperatures and neutron bombardment. The physical basis for miniaturizing the pressure vessel is twofold: fuel subdivision and pressure vessel enhancement.

First, subdividing the nuclear fuel by four to five orders of magnitude eliminates the risk of single point failures in the pressure vessel or fuel cladding. Ordinarily, a single failure in the reactor’s main steel pressure vessel or a fuel cladding can lead to a nuclear accident. With TRISO, single failures are unlikely, and when they occur, lead to a small radiation release into the fuel matrix where is likely stopped. The subdivision of fuel into particles is even believed to provide blast resistance as the particle may be small and durable enough to remain intact during explosive fractures like a direct missile hit. Indeed, with this reasoning in mind, the US Department of Defense began developing TRISO fueled reactors for forward bases in early 2019.²

The second advantage to miniaturizing the pressure vessel is the ability to enhance pressure vessel performance through use of thin layered ceramics in a mass manufactured, seamless, spherical design. Fabrication of millions of particle pressure vessels can be highly standardized and controlled in a mass manufacturing environment with defect rates of 1 in 100,000.³ Crucially, the millimeter scale vessel allows the use of highly pure brittle ceramics made through CVD techniques that maintain high strength and stability under high temperature and irradiation and have dual use as low reactivity fission product barriers. Normal reactor pressure vessels are made one at a time and have to be cylindrical, with various seaming and joining techniques, multi cm wall thickness, and cannot use brittle materials. As the pressure vessel size is reduced, the more efficient spherical geometry can be adopted, and the required wall thickness drops dramatically to the point that a 35 μm SiC layer can indefinitely contain gas pressures in excess of 200 MPa.⁴  This compares to the main reactor steel pressure vessel which may go as high as 10MPa or the Zircalloy cladding which can have pressures up to 100 MPa in limited conditions. As the nuclear fuel fissions, it continues to accumulate fission product gases and increases the gas pressure that must be contained. If the pressure vessel can handle higher pressures, the fuel can be burned more extensively and safely, which means more efficient use of fuel.

The attractiveness of TRISO-matrix fuel is derived from its ceramics' unusually high thermal conductivity, high fission product retention, and superior corrosion resistance across operating, accident, and storage conditions. This translates into better economic potential and safety for current civilian reactors (higher temperatures, burnups, and power densities with higher safety margins) as well as upcoming micro reactors, gas cooled reactors, and molten salt cooled reactors. More extreme performance applications enabled by TRISO-matrix type concepts include nuclear thermal propulsion (NTP) for space, VHTR, Subterrene tunneling, and nuclear ramjets.

How it's Made

The kernel of the TRISO particle is the nuclear fuel, which can be made of a carbide or oxide of a sufficiently enriched fissile element, either U, Pu, or Th. Due to material availability, the most prevalent kernel type is UO2 formed via the sol-gel process in two reactions. First, U3O8 powder is dissolved in nitric acid to make a broth solution of uranyl nitrate. Spheres are formed by vibrational dripping of droplets through an ammonia atmosphere into an ammonium hydroxide solution. Sphericity is maintained by ensuring the droplets solidify before colliding with the solution to form ammonium diurinate kernels. The kernels are washed to remove water and any reaction products, calcined to form UO3, and finally reduced and sintered to achieve high density UO2.

Multiple coatings, each with a specific purpose, are added to the fuel kernel via chemical vapor deposition (CVD) in a fluidized bed, a system designed to batch coat particles through CVD in a continuous process. Layer order and thickness is optimized to achieve necessary chemical and structural properties and can vary with the type of fissile fuel being used and the required burnup. Pyrolytic Carbon (PyC) layers of varying porosity are formed with acetylene or propylene in Ar atmosphere. The SiC layer is formed with methyl trichlorosilane with H2.

Layer Roles

Challenges

The layers were designed to deal with the challenges of nuclear fuel:

1. Swelling and cracking of the UO2 or other nuclear fuel. This is called radiation induced swelling and fragmentation and can result in significant volume changes of around 30%.
2. Release of fission gases which can increase the pressure substantially in the range of 40-100 MPa. Besides high pressure, many of the fission gases are also mobile, able to diffuse through most materials.
3. High temperatures in the range of 500-2000 °C. The fuel is generating a lot of heat and the temperatures will be high. The materials must maintain their properties across these high temperatures.
4. Radiation damage on the materials significantly degrades their properties. This can include reduce thermal conductivity, swelling, reduced strength, embrittlement, reduced fission product retention etc.
5. High thermal throughput from the fuel to the coolant. The layers must allow the heat to pass through to the fuel without getting too hot.

1. Porous Pyrolytic Carbon

The first coating deals with Challenge 1. It is a porous pyrolytic carbon (PyC) buffer layer (Buffer) that acts as a gas chamber for fission product gases and provides sacrificial volume to absorb fission product recoils and any swelling from the fuel kernel. PyC is used for its high conductivity at high temperatures, very low thermal expansion, and chemical stability. Higher porosity is achieved by increasing deposition rate which also increases isotropy. Excessive anisotropies can lead to stress concentrations and eventually particle failure.

2. Dense Inner Pyrolytic Carbon

A second but denser inner PyC (IPyC) layer is added to protect the fuel kernel from the subsequent processing steps. In particular, the SiC deposition involves hydrochloric acid products which can damage the layers underneath. The PyC has maximum density to prevent diffusion of heavy metal fission products like Palladium that can the attack SiC.

3. Silicon Carbide

SiC solves Challenge 2. The Silicon Carbide layer acts as the pressure vessel and diffusion barrier for the fission product gases. Microstructurally, smaller equiaxed grains are desired as it lowers diffusion of fission products. These are achieved by using lower temperatures during deposition.⁵

4. Dense Outer Pyrolytic Carbon

A final outer PyC (OPyC) layer protects the SiC from further processing steps and provides a compliant layer between the SiC and the matrix that binds the particles into a pellet. The interaction between the outer and inner PyC layers, which shrink during irradiation, provides compressive stress to the Silicon Carbide layer. This means that the SiC starts in compressive stress and transitions to tensile stress as the fission gas pressure increases.⁶

5. Matrix or Filler

The pellet matrix, or inter-particle filler material, is ordinarily sintered graphite but can also be sintered SiC as in the FCM fuel.⁷ This enhances fission product retention and structural stability of the fuel form.

Challenge 3 and 4 are met because the SiC and graphite layers offer excellent radiation and temperature tolerance, maintaining their performance characteristics across the intended temperature and irradiation conditions. The melting temperatures of SiC and graphite are about 2800 °C and 3800 °C, respectively. Both ceramics have good and resilient thermal conductivities, allowing the fuel form to deal with Challenge 5.

Physical Properties of SiC

The key to TRISO’s performance is the β Silicon Carbide layer which possesses the right mix of properties and resilience over the encountered conditions. As shown in Figure 1, materials down selection for nuclear fuels involves minimizing cross sections and various trades on thermal, structural, and chemical properties.⁸ Depending on the particular reactor type, the materials selection for bulk materials (i.e. significant volumes) is roughly limited to Zirconium alloys and various ceramics including SiC, ZrC, and graphite. There are actually a few ceramics with comparable performance including β-Si3N4, SiAlON, AlN, Al2O3, ZrC, and TiC. However, SiC leverages its good enough properties with the most data, qualification, and established recipes for its manufacture at moderately low temperatures.¹⁰

Neutronically benign

From a neutronics perspective, silicon and carbon atoms have small enough cross section for thermal neutrons to not interfere with fission reactions.

High temperature capability

β SiC is the allotrope with a cubic crystal and sp3 hybridization. Strong covalent and somewhat ionic bonding give β SiC nearly best in class thermal, structural, and chemical properties in the context of Figure 1. It has a melting temperature on order of 2800°C, low thermal expansion, high thermal conductivity, high strength, and is largely chemical inert.

Fission product barrier

It has negligible diffusion for fission gases in the temperature ranges of interest, even along grain boundaries, for most fission products including Helium gas.⁹ When incorporated into the TRISO particle, the fission product diffusion through SiC has been measured to be excellent up to 1400 K with Ag, Eu, Kr posing limited risks at higher temperatures.

Extreme irradiation tolerance

But SiC’s most significant advantage is its behavior under extreme irradiation and temperature. While most materials swell, transmute, and deteriorate to the point of crumbling, irradiated SiC converges to a quite acceptable equilibrium, at least when it is highly crystalline, pure, and cubic. The swelling is indicative of the overall degradation and it should be noted that elastic modulus and thermal conductivity trend with the swelling and also equilibrate at acceptable values.

Swelling occurs when neutron irradiation creates interstitials and vacancies, with the interstitials usually migrating faster towards dislocations, which leaves excess vacancies to form voids and cause the material to swell. The time rate of swelling is determined by the flux of vacancies compared to the flux of interstitials, simply as the difference between the fluxes ɸ.¹¹ In SiC at temperatures between 423 K and 1273 K, defect formation is low due to thermal recombination, and the interstitial flux is high enough to match the vacancy flux so that the defects annihilate and the swelling stops.^{12 13}Swelling behavior has been measured up to 1600 °C and reveals 3 regimes. The temperatures of interest for nuclear fuel are between 400 to 1000 °C for which SiC shows saturated swelling behavior.¹⁴

Suppliers

CNNC

Current world leader in at-scale production of TRISO particles at their Baotou facilities, reportedly with the capacity to supply more than ten HTR-PMs. They currently supply 2 HTR-PM reactors operating at full power. The Chinese licensed technology reactor and fuel technology from the dying German HTGR efforts in the 80s and 90s.

JAEA

Small batch capabilities to supply the HTTR reactor while it was operating.

Ultra Safe Nuclear Corporation

Factory plans in Washington state partnering with Framatome. Pilot facility in Knoxville, Tennessee. Ultra Safe Nuclear is the only supplier to have improved on the original TRISO particle fuel form by encapsulating TRISO particles in Silicon Carbide rather than graphite, which has several advantages. Ultra Safe also aims to supply TRISO particle or TRISO-like fuels for Nuclear Thermal Propulsion.

BWXT

Small batch capabilities to supply the DOD Pele Program transportable reactor. Also a contender for Nuclear Thermal Propulsion fuels.

TRISO-X

Planned fuel fabrication facility funded by the US DOE in Tennessee. TRISO-X has been pursuing TRISO fuels since 2015 with a DOE grant of $40M and delivering lab-scale TRISO samples.

The Future of TRISO is Bright

There are dozens of advanced nuclear reactors at various stages of development, and about a dozen aim to use TRISO-based nuclear fuels. For regulators and the public, the demonstration of these reactors and fuels will provide a useful comparison relative to the molten fuel and ordinary fuel reactors that also in development. We will finally be able to understand whether or not fuel performance is an important factor in nuclear reactor safety and commercial viability.

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1. K. Verfondern, H. Nabielek & J. M. Kendall. Coated particle fuel for HTGCR.pdf. Nuclear engineering and technology 39, 603 (2007).↩
2. PELE PROGRAM PHASE I.↩
3. Petti et al., Triso-Coated Particle Fuel Performance.↩
4. Hales et al., “Multidimensional Multiphysics Simulation of TRISO Particle Fuel.”↩
5. Petti et al., “Key Differences in the Fabrication, Irradiation and High Temperature Accident Testing of US and German TRISO-Coated Particle Fuel, and Their Implications on Fuel Performance.”↩
6. Konings and Verfondern, TRISO Fuel Performance Modeling and Simulation.↩
7. Mehner et al., “Spherical Fuel Elements for Advanced HTR Manufacture and Qualification by Irradiation Testing.”; Heit et al., “Status of Qualification of High-Temperature Reactor Fuel Element Spheres.”; Sawa, TRISO Fuel Production; Date and Phase, “VDR Focus Area 04 MMR-REM Fuel Design and Qualification.”↩
8. Hosemann et al., “Materials Selection for Nuclear Applications: Challenges and Opportunities”; Azevedo, “Selection of Fuel Cladding Material for Nuclear Fission Reactors.”↩
9. Petti et al., Triso-Coated Particle Fuel Performance.↩
10. Snead et al., “Handbook of SiC Properties for Fuel Performance Modeling.”↩
11. Was, Fundamentals of Radiation Materials Science.↩
12. Katoh et al., “Microstructural Development in Cubic Silicon Carbide during Irradiation at Elevated Temperatures.”↩
13. Katoh et al., “Stability of SiC and Its Composites at High Neutron Fluence.”↩
14. Snead et al., “Handbook of SiC Properties for Fuel Performance Modeling.”↩