![]() The net effect is that HTGRs using conventional TRISO fuel require a large core volume to minimize neutron leakage and to achieve criticality with a small uranium loading. Moreover, the mechanical pressing process also limits the geometric complexity of the fuel to relatively simple shapes. This limitation is compounded by the use of a graphite matrix, which is typically fabricated using mechanical pressing and which limits the maximum fuel volume fraction to less than ∼40% due to TRISO failure caused by particle-to-particle interactions. In addition, the relatively small size of the kernel (∼425 µm in recent works ) and the relative thickness of the TRISO coatings (∼215 µm in total) limit the fuel volume fraction. First, the fissile kernel is composed of a mixture of uranium oxide and uranium carbide (commonly termed UCO), which has a lower uranium density compared to uranium carbonitrides. Despite the obvious advantages of TRISO fuels, there are challenges in deploying this fuel form for some applications beyond HTGRs. The operating temperatures of HTGRs generally preclude the use of most metal alloys to retain fission products, which necessitates the use of ceramic materials in the fuel, such as TRISO particles. Tristructural-isotropic (TRISO) fuel particles have a long history of research, development, and deployment, particularly when integrated within a graphite matrix for high-temperature gas-cooled reactor (HTGR) applications, ,,. The discussion focuses on the importance of understanding matrix density distributions and the particle-matrix interface properties to prevent matrix cracks from causing TRISO particle failures. In any case, the matrix cracks propagated through the coatings of TRISO particles located in the high-density matrix regions on the peripheries of the compacts, resulting in measurable fission gas release. ![]() Therefore, fuel failure was likely not caused by thermal stresses and may have been related to leakage currents from the electrical heaters and erratic fuel surface temperatures that were only observed in the test for which failure was observed. Thermal stress-induced matrix cracks also would not cause complete fracture because the tensile stresses transition to compression in the higher temperature regions. Calculated thermal stresses in the failed compacts were far less than the measured strength of the SiC matrix and the stresses in some failed compacts were less than those in compacts that did not show FGR. ![]() Initial testing at higher (∼700–750 ☌) fuel surface temperatures showed fission gas release (FGR) and complete fracture of three compacts, but no FGR was observed in later high temperature tests (∼300–750 ☌) of both fueled compacts and loose TRISO particles. Fission gasses were fully retained in all loose particle tests and in integral compacts irradiated at low (<250 ☌) surface temperatures. This work summarizes initial low-burnup, high-power irradiation testing of TCR fuel materials, including loose UCN TRISO particles and integral fuel compacts with ∼55% TRISO particles by volume, to evaluate fission gas retention. The Transformational Challenge Reactor (TCR) fuel form was designed to contain large, densely packed uranium carbonitride (UCN) tristructural-isotropic (TRISO) fuel particles within a 3D printed SiC matrix, increasing the uranium density compared to conventional TRISO fuel forms and offering full geometric freedom for core design.
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