Abstract
To contain the impact of core disruptive accidents (CDAs) of sodium-cooled fast reactors in the reactor vessels, it is important to retain molten core material within the vessel during CDAs. This concept is called in-vessel retention (IVR). Molten core material discharged into the lower sodium plenum has the potential to impose a significant thermal load on the lower structures of the reactor vessels and thus may compromises IVR. However, if the molten core material is fragmented into smaller particles well before it reaches the lower structures, such the thermal load should be significantly reduced by enhanced quenching of this core material. Hence, the fragmentation of molten core material is crucial for achieving IVR. In this paper, based on the experimental results of a series of fragmentation tests (FR tests), in which around 10 kg of molten alumina (melting point: approximately 2300 K) was discharged into a sodium pool (depth: 1 m, diameter: 0.4 m, sodium temperature: 673 K) through a duct (inner diameter: from 40mm to 63 mm) by using an experimental facility in National Nuclear Center of the Republic of Kazakhstan, dominant fragmentation mechanisms of molten core material discharged into sodium are discussed. In the FR tests, molten alumina was finely fragmented in the sodium pool, thereby forming a debris bed. The mass median diameters of solidified alumina particles were around 0.3 mm, which are comparable to particle sizes predicted by hydrodynamic instability theories such as Kelvin-Helmholtz instability. However, even though hydrodynamic instability theories predict that particle size decreases with the increase of Weber number (We), such the dependence of particle size on We was not observed in the FR tests. Considering that in the tests, the distances for fragmentation of the molten alumina were approximately from 60 % to 70 % (i.e., around 65 %) below the values predicted using an existing representative correlation that regards hydrodynamic instabilities as a dominant fragmentation mechanism, it can be interpreted that the tendency of measured mass median diameters (i.e., non-dependence on We) suggests that before hydrodynamic instabilities sufficiently grow to induce fragmentation, thermal phenomena such as local coolant vaporization and resultant vapor expansion accelerate fragmentation.
