Multiphysics simulations of the Advanced Test Reactor using Pronghorn and Griffin

The Advanced Test Reactor (ATR) at Idaho National Laboratory is used for the development of accident-tolerant fuels, isotope production, and other advanced nuclear applications. To support these missions, the reactor must accommodate frequent insertion of experimental materials, which introduces potential safety challenges. This paper evaluates the role of multiphysics simulations in predicting the core behavior of the ATR under various accident scenarios. The proposed multiphysics workflow integrates Pronghorn, a coarse mesh computational fluid dynamics code; Griffin, a deterministic reactor physics code; and OpenMC, a Monte Carlo neutronics code. OpenMC is used to generate homogenized cross sections at various temperatures, which are loaded in Griffin. Griffin computes the power distribution, which is then transferred to Pronghorn. Pronghorn then updates the temperature field, which is transferred back into Griffin until convergence is achieved. The Pronghorn model shows good agreement with the expected pressure drop and temperature values. Initial neutronics simulations using Griffin reveal reactivity differences of 284 pcm and 904 pcm for the 8-group and 18-group structures, compared to OpenMC. The discrepancies are primarily attributed to geometry homogenization and differences in energy group resolution. Moderator feedback effects show strong agreement with OpenMC results, supporting realistic safety transient simulations. The multiphysics coupling effects were found to be weak-to-moderate at the nominal steady-state operating point, showcasing 1%–3% shift in the power profile and a 1-2 K shift in the fuel temperature in a representative assembly. The transient scenarios presented in this paper, however, highlight the importance of capturing the coupling between thermal-hydraulics and neutronics.