The upward displacement of molten fuel material (or actinides) from the core region due to a reactor-scale fuel bubble expansion–compression, during energetic core disruptive accident (CDA), is essential in evaluating the accident source term for a pool-type sodium fast reactor (SFR). A numerical model is developed to estimate the molten fuel mass distribution in the reactor vessel at the end of the fuel bubble’s expansion–compression phase. The model evaluates the Lagrangian trajectories of representative fuel droplets during the upward displacement from the damaged core. Subsequently, the fuel mass removed from the core region, the fuel mass absorbed by the sodium pool, and the fuel mass that remains in the bubble region after the cessation of the bubble’s expansion–compression cycles are estimated. The fuel droplet diameters are evaluated by considering both mechanical and thermal modes of fragmentation.

The initial fuel bubble temperature ranges from 4200 to 4700 K, and the fuel bubble mass is between 1000 and 3000 kg. These values are typical for a medium-sized pool-type SFR under energetic CDA conditions. The fuel droplet trajectories are evaluated by considering drag and gravity as dominant forces under a dilute flow regime. The local fluid (fuel vapor) velocity required to evaluate droplet trajectories is supplied by a fuel bubble model, which accounts for sodium entrainment and the fuel–sodium heat transfer.

Analysis results show that the fuel droplet diameter strongly depends on the initial fuel bubble temperature, and it is insensitive to the fuel bubble mass. Also, the sodium entrainment in the bubble region retards the fuel droplet absorption by the sodium pool. Fuel droplets generated by mechanical fragmentation are effectively removed from the bubble region by gravitational settlement. Similarly, inertial impaction effectively removes the thermal fragmentation–generated fuel droplets from the bubble. Results show that the fuel droplets with diameters less than 3 µm (up to 183 kg) remain suspended in the fuel bubble region at the end of the bubble expansion. These suspended fuel droplets can reach the cover gas space during the subsequent buoyancy rise of the fuel/sodium vapor bubble swarm. Results form the basis for the estimation of the accident source term in a reactor containment building due to the fuel bubble phenomenon under energetic CDA in a pool-type SFR.