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TRISO fuel uses coated nuclear fuel particles, each with a fuel kernel surrounded by four layers: a porous carbon buffer, an inner dense pyrolytic carbon layer, a silicon carbide layer and an outer pyrolytic carbon layer. The name abbreviates tristructural isotropic: the four layers use three coating material forms.^[ref]

The layers perform complementary functions. The porous carbon buffer accommodates released fission gas; the pyrolytic carbon supports the particle mechanically; and the silicon carbide supplies a strong barrier to fission products. Coated particles can be embedded in a graphite matrix. Their retention performance depends on coating integrity, material behaviour under irradiation, and the conditions experienced in service. The architecture makes a small particle itself part of the containment system, linking nuclear engineering closely to materials science.^[ref]