Reduced-order modal characteristics and dynamic evolution of nuclear–thermal–hydraulics coupled systems under multi-parameter driving

The operational stability of advanced solid-fuel reactors is governed by nonlinear bidirectional coupling between neutron transport and thermal-hydraulics. Conventional approaches often decouple these physics or rely on time-averaged turbulence models, thereby obscuring transient flow–neutron interactions. In this study, a high-fidelity finite-volume framework is developed to tightly couple neutron transport with direct numerical simulation (DNS). A composite state vector integrating velocity, temperature, and neutron flux is constructed, and synchronous Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are applied for full-field reduced-order diagnosis. The results show that solid-fuel Doppler feedback is the primary stabilizing mechanism: through conjugate heat transfer at the solid–liquid interface, it suppresses power surges and drives divergent eigenvalues toward quasi-equilibrium or limit-cycle states. The neutron cross-section acts as a critical bifurcation parameter, inducing symmetry breaking and transforming ordered periodic structures into multi-scale fragmented vortices. Intensified convection broadens the dynamic energy spectrum and accelerates the transition to spatiotemporal chaos. These modal indicators and stability boundaries provide guidance for heat-removal design, operating-parameter selection, and early warning of thermo-neutronic instabilities in advanced reactor cooling systems.