TRISO Particle Behavior Analyses of an HTR-10 Type Reactor Employing the OTTO Operation Scheme

Abstract High-temperature gas-cooled reactors (HTGRs), renowned for their safety features and capacity to provide high-temperature heat, are anticipated to find widespread applications. The HTR-10 serves to demonstrate the intrinsic safety characteristics of modular HTGRs. Different refueling schemes result in varied pebble bed flows, with the Once-Through Then Out (OTTO) scheme causing fuel pebbles to remain in the core for longer periods. This increased residency in the core exposes the pebbles to more intense operating conditions, especially those pebbles that traverse the core’s center. In this work, the VSOP code coupled with THERMIX is used to calculate the pebble bed power distribution and radius-wised graphite temperature while tracing the flow of representative pebble and TRISO. Obtained data are fed to a 1-D OFFBEAT TRISO model to simulate the temperature distribution and variation, as well as the deformation of each layer of the particle. By this method, the fuel temperature, mechanistic properties, and fission gas release during the OTTO cycle can be estimated. The results show that in the core center, the fuel temperature reaches over 1100K while at the edge of the core is about 1050K, well below the criteria of 1473K. Mechanical analyses show that the maximum equivalent stress of TRISO particles in the core is about 350 MPa with an FGR of about 17.5%. This work offers a foundation for further analysis of fuel failure probability and fission product diffusion and release under normal operating conditions. This work established a method to analyze the fuel behavior of HTR TRISO fuel using legacy code and high-fidelity solver and play as a foundation for further fuel failure and fission products diffusion analyses.