Thermal-Hydraulic-Mechanical Modeling and Simulation of Sodium-Potassium–Cooled MARVEL Microreactor Core

—The U.S. Department of Energy’s Microreactor Program, along with Idaho National Laboratory’s development of a nuclear microreactor applications platform named MARVEL, support research and development for deploying small, transportable reactors across civilian, industrial, and defense sectors. The MARVEL microreactor, an 85-kW(thermal) fission reactor, incorporates TRIGA nuclear fuel and a sodium-potassium eutectic as its primary coolant. It is designed for safe and efficient operation using natural circulation and a low power density core. The reliance on natural circulation for primary cooling means the reactor avoids using fuel spacers to minimize core pressure drop, which could disrupt the primary coolant’s natural flow. However, the core’s tight pitch-to-rod diameter ratio of 1.056 raises the risk of fuel rod contact and elevated peak cladding temperatures. To ensure reactor safety, this study conducted computational modeling and simulations to examine the reactor’s thermal-hydraulic-mechanical characteristics, including the reactor core heat transfer coefficients, potential for rod-to-rod contact, and its impact on peak cladding temperature. The analyses indicated that the thermal deformation of the fuel rods under a worst-case scenario may lead to fuel rod contact, but peak cladding temperatures remained well below safety criteria, ensuring safe reactor operation without fuel spacers under steady-state normal operating conditions.