Dynamic Modeling and Transient Analysis of Micro Reactor with Direct Helium Brayton Cycle Based on Modelica

Micro Gas-Cooled Reactor (MGCR) has garnered attention in relevant domains, owing to its advantages of miniaturization and transportability, which is capable of providing stable electrical power to off-grid and special regions. As a typical multi-physics coupled system, a dynamic model for the MGCR integrating nuclear, thermal-hydraulic, mechanical, and electrical subsystems was developed in this study using the multi-physics modeling language Modelica. Steady-state validation results indicate that the maximum deviation between the simulated values and the design parameters is merely 1.05%. Meanwhile, transient validation demonstrates a high degree of consistency with the outcomes generated by the MGT-3D code, with the relative error of the maximum temperature remaining below 3%. Based on the established dynamic model, transient analyses were carried out for three typical accident scenarios: reactivity insertion accidents, loss-of-flow accidents, and load rejection events. The results demonstrate that in the case of reactivity insertion and loss-of-flow accidents, the reactor can achieve a new steady state or autonomously reduce power through its inherent feedback mechanisms, thereby ensuring operational safety; specifically, the maximum temperature rise at the reactor outlet reaches 33.89°C during a reactivity insertion accident and 23°C in the event of a loss-of-flow accident. For load rejection scenarios, the opening of the isolation valve can effectively protect the helium turbine: upon a complete loss of external load, the rotor speed increases by up to 5.6%, which does not exceed the safety limits. This study verifies the inherent safety of MGCRs under accident conditions and demonstrates the application potential of Modelica in multi-physics dynamic analyses of such advanced nuclear systems.