Fragmentation of a single molten metal droplet penetrating sodium pool I copper droplet and the relationship with copper jet

The progression of hypothetical core disruptive accidents in metallic fuel fast breeder reactors is strongly affected by the fragmentation of molten metallic fuels due to the molten fuel-coolant interaction (FCI). As a basic study of FCI, the present paper focuses on the fragmentation of a single molten copper droplet with mass from 1 to 5 g, which penetrated a sodium pool at instantaneous constant interface temperatures (Ti) from 995 to 1,342degC. Intensive fragmentation of a single molten copper droplet was clearly observed even if Ti values are below the melting point (1,083degC) of copper besides the higher Ti range. The intensive fragmentation shows that the mass median diameters (Dm) of copper droplets with a fivefold difference in mass or the same mass have little difference, i.e., they are nearly the same. Under the lower Ti condition, the Dm data of droplet fragments of both the same and different masses scatter widely. It is found that the present Dm/D0 data of mass median diameter normalized by the diameter before touching sodium (D0) give a distribution with larger values than those of molten copper jets with large mass from 20 to 300 g under the lower Ti condition, which were previously reported by the authors, because of the limited amount of heat of droplets. The present Dm/D0 data under the higher Ti condition are found to show an effective fragmentation compared with those of molten copper jets with a large mass of 4 kg. (author)