Conditions for bubble recirculation in a molten salt natural circulation loop and heater failure effect on loop conditions and argon bubble dynamics

Off-gas systems employing bubble injection have been proposed as an effective method for removing gaseous fission products in molten salt reactors (MSRs). In a semi-closed loop, bubbles may remain entrained in the fluid even after the gas has passed the point where the bubble should escape the fluid, and then recirculates through the loop. Bubble recirculation poses risks to reactivity control and flow stability in the operation of the off-gas systems of MSRs. The longer the bubble is in the reactor the higher the concentration of fission products in the bubble. This can cause changes in local reactivity and power profiles. This study experimentally investigates the mechanisms behind bubble recirculation in a natural circulation molten salt loop under degraded thermal conditions. Advanced flow diagnostics such as Particle image velocimetry (PIV), particle tracking velocimetry (PTV), and proper orthogonal decomposition (POD) were employed to characterize bubble trajectories, slip velocities, and associated flow structures. A downcomer heater failure created asymmetric heating, leading to partial salt freezing and flow stagnation, while the blocked expansion tank prevented bubble venting. Over time, recirculating bubbles coalesced, increasing in size and altering local flow behavior. Measured slip velocities diverge significantly from drift-flux model predictions, underscoring the limitations under off-normal conditions. POD analysis revealed dominant wake structures and vortex interactions, and temporal cross-correlations revealed a sequence of flow phenomena induced by bubble motion. These findings underscore the need to maintain thermal uniformity and suggest refinements to computational models of off-gas systems in MSRs. • Bubble recirculation observed in molten-salt natural circulation loop. • Slip velocities diverged from drift-flux predictions under off-normal conditions. • Downcomer heater failure caused asymmetric heating and salt freezing. • POD revealed coherent wake structures and Kármán-like vortex interactions.