Integration of TRISO Fuel Particles with Open-Cell Foam for Increased Safety, Heat Transfer, and Fuel Performance

Tristructural isotropic (TRISO) fuel is a key component of advanced small modular nuclear fission reactors due to its inherent safety at high temperatures and irradiation levels and decreased proliferation risk relative to current reactor fuels. A more efficient cooling method for TRISO fuel compacts would increase heat transfer, allow for an increase in fuel density, and further improve accident tolerance. In previous work for NASA and DoD, Ultramet developed open-cell ceramic foams with an interconnected hollow- ligament structure. In recent work for DOE, Ultramet and Oak Ridge National Laboratory (ORNL) demonstrated the initial feasibility of infiltrating a graphite matrix containing surrogate TRISO fuel particles into the large void space surrounding silicon carbide (SiC) foam ligaments. The result was a structure comprising surrogate TRISO particles embedded in graphite reinforced with SiC foam containing an interconnected network of cooling channels. The potential exists to substantially increase heat transfer, relative to conventional TRISO particle-in-graphite compacts, by flowing helium gas inside the hollow-ligament foam cooling channels. This will eliminate the current need in prismatic assemblies for machining coolant channels in the graphite surrounding TRISO-graphite compacts, which are relatively large and not in close proximity to much of the fuel. This design is anticipated to permit a significant increase in fuel density, enabling either lower enrichment scenarios for the fuel or smaller, higher power density cores, with no change in fission product release. The TRISO particle-in-foam concept will be further developed and demonstrated by Ultramet, in collaboration with ORNL and X-energy, as an alternative to poorly cooled gun-drilled prismatic assemblies. The design offers the inherent safety features of large thermal mass graphite blocks, but with substantially greater thermal efficiency and control. In addition, the SiC foam-reinforced compacts will not sinter or crack under thermal cycling. ORNL will perform computational fluid dynamics (CFD) analysis of the cooling flow and thermomechanical analysis, including characterization of volumetric nuclear heating, of this new compact geometry. ORNL will also perform fuel assembly and plenum design, as well as compact optimization including the size and spacing of the coolant passages and response to loss of forced flow. Ultramet will fabricate compacts composed of SiC foam infiltrated with a fuel particle- graphite mixture, using surrogate TRISO particles provided by X-energy. Characterization will be performed by Ultramet, in conjunction with X-energy, to ascertain surrogate fuel density and uniformity of particle distribution throughout the compacts. Pressure and flow testing of prototypes will be performed by Ultramet for validation with the CFD models.The proposed technology is anticipated to be ideal for small modular reactors currently under development by X-energy and others. These advanced reactors have dramatically improved performance and operational safety margins and decreased proliferation risk compared with existing nuclear power generation technologies.