Bubble-Scale Multi-Physics Analysis of Local Power Distribution Perturbations Induced by Helium Bubble Morphology in a Localized Molten-Salt Domain

In molten salt reactors (MSRs), helium bubbling systems can be employed for the continuous removal of gaseous fission products. However, helium injection generates local gas–liquid two-phase flow in the fuel salt and may induce corresponding local perturbations in the calculated neutronic field. To investigate these bubble-scale interactions, this study developed a coupled multi-physics framework integrating the volume of fluid (VOF) method with a multigroup neutron diffusion model. The framework was applied to a localized 20 mm × 40 mm fuel-salt domain containing a single 1 mm helium injection nozzle. Planar 2D and axisymmetric calculations were performed to examine the influence of geometrical representation on bubble growth, detachment, transport, and the corresponding local power response. The axisymmetric formulation was further used to evaluate the sensitivity of the calculated response to the helium mass flow rate. Within this restricted numerical test problem, the coupled framework resolved the evolution of helium bubbles and the associated local changes in the power field under the prescribed boundary conditions. For the centered circular nozzle and symmetry-preserving near-inlet conditions considered, the axisymmetric formulation provided a more geometrically consistent representation of rotational volume weighting and interfacial curvature than the planar 2D formulation. Variations in helium mass flow rate also modified the calculated local bubble behavior and power-response metrics. These results constitute a numerical demonstration of local bubble-resolved multi-physics coupling and should not be interpreted as reactor-scale power predictions, core-wide safety metrics, or design criteria for an MSR helium bubbling system.