Natural circulation characteristics of compact petal-shaped fuel rod bundle channel under rolling conditions

This study develops a multiscale computational model to examine the thermal–hydraulic response of a natural circulation loop equipped with a compact arranged petal-shaped fuel rod bundle under rolling motion. Rolling motion changes the gravity projection of loop sections and introduces inertial forces, thereby modifying the driving pressure. The results indicate that oscillatory flow is governed by the dynamic balance between time-varying driving pressure and flow resistance. Under static conditions, the rod bundle exhibits geometric periodicity and centrosymmetric temperature characteristics. Local temperature distributions along helical auxiliary lines show distinct thermal responses in petal-root and petal-tip regions. Petal-root regions generally have higher wall temperatures and weaker axial fluctuations, whereas petal-tip regions show lower average temperatures but stronger periodic fluctuations. Under rolling motion, the transverse coolant redistribution breaks temperature centrosymmetry, especially in the upper rod-bundle region and outer subchannels. These findings support thermal safety assessment and reliability evaluation of marine natural circulation systems.