Effect of Operating Void Fraction on Wall-Deposited Decay Heat in the Main Loop of a Molten Salt Reactor

In molten salt reactors, multiphase transport of fission products produces nonuniform nuclide inventories throughout the primary loop, thereby affecting subsequent decay heat evolution. Although fission product transport has been widely studied, its influence on post-shutdown wall-deposited decay heat remains insufficiently characterized. Here, the bubble transport capability of the Thorium Fission Products Migration Code (ThorFPMC) was extended using a drift-flux model to investigate the effects of operating void fraction on the magnitude, spatial distribution, nuclide composition, and subsequent evolution of wall-deposited decay heat under an idealized post-drainage condition. Results showed that, for the baseline bubble diameter of 0.508 mm, the initial wall-deposited decay heat decreased from 41.33 to 6.63 kW as the operating void fraction increased, corresponding to an 84.0% reduction; bubble diameter sensitivity calculations preserved the same monotonic trend. The source remained strongly nonuniform, with the heat exchanger consistently dominant, while increasing the void fraction reduced the core fraction and increased the relative external-loop contribution. Nuclide contributions evolved from a broad early distribution to long-term dominance by 95Nb and 103Ru, with precursor feeding shaping several important daughter nuclides. These results link operating period multiphase transport to component-resolved residual decay heat sources and provide a basis for operating-condition-dependent source terms in shutdown thermal and system analyses.