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| Fig. 1: STM photo of a cracked TRISO fuel pellet showing its layered nature. [4] (Courtesy of the DOE. Source: Wikimedia Commons.) |
Tristructural isotropic fuel particles, or TRISO particles, are a robust nuclear fission fuel form primarily designed for use in high-temperature gas reactors and small modular fission reactors. Serving as a replacement to traditional fuel rods, TRISO fuel particles exhibit higher burnup and resistance to higher reactor temperatures and neutron flux, which has resulted in their widespread adoption and research pursuit since the 1960s by startups and government agencies alike.
Unlike fuel in traditional fuel rods, TRISO fuel particles have three thin outer layers that contribute to the fuels safety, mechanical stability, and streamlined waste management. Since the outer layers act as a pressure vessel, the release of fission products is limited in the event of a reactor meltdown or other reactor disaster. This natural self-containment mechanism results in increased safety over traditional methods, as well as easier waste-management of spent fuel. [1] Additionally, the structure of this fuel type effectively bypasses the issue of grid- to-rod fretting, a significant challenge for traditional fuel rods. [2] These tiny fuel particles are encapsulated together in a graphite matrix in a spherical or cylindrical geometry about the size of a hand, depending on the intended use of the fuel. [3]
A TRISO fuel particle, as pictured in Fig. 1, is around 800 μm and is composed of an internal fuel particle, a porous carbon buffer, and three outer layers that act as diffusion barriers and maintain the particles structural integrity under internal pressure, neutron bombardment, and high thermal loads. [1,4]
The fuel kernel, which is at the center of the TRISO pebble, is generally composed of ceramic uranium dioxide (UO2). After neutrons pass through the three outer layers and reach the fuel kernel, they are absorbed by Uranium atoms, causing them to undergo fission, which results in increases in fuel kernel volume.
This carbon buffer serves several important purposes in mitigating the internal stresses that build up within the fuel particle. Aside from accommodating the fuel kernels increased volume, the buffer absorbs and reduces internal pressure buildup from gas production within the kernel.
Encapsulating the carbon buffer are three thin layers which act as diffusion barriers. The first of these layers is the inner pyrolytic carbon layer, which protects the fuel kernel from corrosive gases used for the deposition of the next layer which is made of silicon carbide (SiC). The SiC layer is the main pressure vessel for the fuel pellet, maintaining structural integrity against the high pressure buildup within the fuel pellet that exerts an outward force on this layer. The final layer is the outer pyrolytic carbon layer protects the TRISO particle from chemical attack during operation.
The carbon buffer and outer layer materials (pyrolytic carbon and silicon carbide) were chosen in part for their low neutron absorption cross-sections, meaning that their relative transparency to neutrons allows the neutrons to reach the fuel kernel. Despite their transparency to neutrons, these layers act as a diffusion barrier that limits the release of fission products and radioactive gases from the fuel pellet, ensuring the safe containment of these dangerous fission byproducts.
Additionally, both the inner and outer pyrolytic carbon layers shrink in response to initial neutron bombardment, aiding the SiC layer in counteracting the outward force from internal gas pressures.
All four coating layers on the uranium dioxide fuel kernel are fabricated with the use of fluidized bed chemical vapor deposition (FBCVD). In this fabrication method, the fuel kernels are suspended in a high temperature gas stream and introduced to varying precursor gases. When these precursor gases break apart at high temperatures, their carbon parts deposit onto the particles surface, causing the growth of even, thin films to coat the kernel. Varying the precursor composition and temperature of the reactor bed allows the tuning of density and material composition, which result in differences between the four coating layers. Each layer is formed through the thermal decomposition of hydrocarbons in the precursor gases, but only the PyC and porous carbon layers are considered pyrolytic upon deposition. This distinction is due to their viscoelastic behavior and anisotropic grain orientation, which are characteristic of pyrolytic carbons, unlike the deposited SiC layer, which distinctly forms a ceramic crystalline structure. [5]
Throughout the intended lifespan of the fuel kernels, production of CO, CO2, and gaseous fission products creates an internal pressure buildup that exerts a significant outward force on the encapsulating carbon and SiC layers. [1]
As uranium atoms in the fuel kernel undergo fission, the oxygen atoms they are attached to get left behind to be absorbed into the remaining uranium dioxide lattice. Although the lattice has some capacity to accommodate additional oxygen atoms beyond its ideal stoichiometric ratio, high temperatures cause the lattice to vibrate and release oxygen atoms into the carbon buffer. Here, the oxygen atoms react with carbon to create CO and CO2 gas, which exert outward pressure on the structural layers of the fuel pebble.
Additionally, as fission occurs, gaseous fission products like Xenon and Krypton are produced. Since they are chemically inert and do not react with the uranium dioxide or carbon, they occupy voids in the porous carbon buffer layer and contribute to the fuel pellets internal pressure exerted on the structural layers. These gases can also get trapped in the fuel kernel, forming voids within the lattice and increasing its overall volume. The fuel kernel also expands since uraniums fission products take up more space than uranium atoms. This increase in volume exerts an outward force, compressing the porous carbon layer and decreasing the available volume within the particle for gas products.
This internal gas pressure buildup is the dominant cause of TRISO fuel particle failure, so multiple design changes are being explored to reduce internal pressure buildup. Among these are decreasing the stoichiometric ratio of oxygen in the uranium dioxide kernel (ie. UO1.7) so the fuel kernel begins with additional vacancies to accommodate oxygen product within the lattice itself.
Coating layers undergo stresses and strains caused by internal gas pressure buildup as well as from mismatched thermal expansion of each coating layer and irradiation creep from neutron bombardment. [1] The first failure mechanism is dominated by internal pressure buildup, and is considered a pressure vessel failure. Internal pressures from gas production within the fuel pebble exert a tensile stress that exceeds the structural layers fracture strength (~350-400 MPa), causing a through-thickness crack in the encapsulating layers of the fuel pebble.
Additionally, since each coating layer is made from different materials, they respond to both thermal and irradiation changes in different ways, resulting in strains and stresses between encapsulation layers. In particular, desintification of pyrolytic carbon during initial neutron bombardment can cause debonding from SiC before ultimately exerting outward force on the layer due to swelling and void formation. Other failure mechanisms include particle asymmetry, kernel migration, SiC coat thinning, and more.
© Daniella Fenster. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. J. Powers and B. D. Wirth, "A Review of TRISO Fuel Performance Models," J. Nucl. Mater. 405, 74 (2010).
[2] C. D. Rusch, "Nuclear Fuel Performance: Trends, Remedies and Challenges," J. Nucl. Mater. 383, 41 (2008).
[3] M. Moorehead et al., "Accelerated Thermal Property Mapping of TRISO Advanced Nuclear Cuel," Mater. Today Adv. 21, 100455 (2024).
[4] R. L. Siebert et al., "Production and Characterization of TRISO Fuel Particles With Multilayered SiC," J. Nucl. Mater. 515, 215 (2019).
[5] C. Tang et al., "Design and Manufacture of the Fuel Element for the 10 MW High Temperature Gas-Cooled Reactor," Nucl. Eng. Des. 218, 91 (2002).
