Fragmentation of a Single Molten Metal Droplet Penetrating Sodium Pool I Copper Droplet and the Relationship with Copper Jet

Abstract

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 contact interface temperatures (T i ) from 995 to 1,342 C. Intensive fragmentation of a single molten copper droplet was clearly observed even if T i values are below the melting point (1,083 C) of copper besides the higher T i range. The intensive fragmentation shows that the mass median diameters (D m ) 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 T i condition, the D m data of droplet fragments of both the same and different masses scatter widely. It is found that the present D m =D 0 data of mass median diameter normalized by the diameter before touching sodium (D 0 ) give a distribution with larger values than those of molten copper jets with large mass from 20 to 300 g under the lower T i condition, which were previously reported by the authors, because of the limited amount of heat of droplets. The present D m =D 0 data under the higher T i condition are found to show an effective fragmentation compared with those of molten copper jets with a large mass of 4 kg.

Fragmentation of a Single Molten Metal Droplet Penetrating Sodium Pool II Stainless Steel and the Relationship with Copper Droplet

“…Therefore, the fragment size characteristics of molten metal jets can be evaluated reasonably by the fragmentation of the single metal droplet, which has been verified in coppersodium experiments. 5) The present data also link well to the stainless steel data of Armstrong et al 6) despite the mode of streamlets and droplets with large masses from 9.1 to 29.1 g. It is also worth noting that even at T i ¼ 994 C < T tp , one datum of the streamlets and droplets with a D m value of 4.5 mm obtained by Armstrong et al also fairly agrees with the present data. Based on these comparisons, it is suggested that the fragment sizes to be applied for the safety evaluation can be selected without considering the dropping modes and dropping mass of the molten core structural material, provided that the mode of the molten core material and coolant mixing is not significantly different from the presently studied conditions.…”

“…5) The relationship between the thermal conditions and the shapes of fragments observed in the present experiment is qualitatively consistent with those observed in the copper-sodium experiments 5) as explained below despite the different thermophysical properties between them.…”

“…The present results of a single molten droplet are found to be in fair agreement with those of streamlets and droplets (9.1-29.1 g) reported by Armstrong et al 6) and those of jets (2.5 and 4 kg) conducted by Schins' group 7,8) in the molten stainless steel experiments. Moreover, the authors also report that at T i > 1;000 C, the fragmentations of the single molten stainless steel and copper droplets 5) possessing the different thermophysical properties have almost the same D m =D 0 values. Figure 1 shows a schematic diagram of the present experimental apparatus, which consists of an induction heater, a sodium pot, and an alumina crucible for holding molten stainless steel.…”

“…They proposed a thermal fragmentation mechanism of sodium-entrapment type combined with the rapid release of latent heat within the droplet (hereafter referred to as fragmentation of entrapment type). The present authors 5) conducted an experiment on a single copper droplet (1-5 g) penetrating a sodium pool in a large range of T i from below to above the melting point of copper, and confirmed the fragmentation of entrapment type. In addition to this, the authors reported that the fragmentation of a molten metal jet with a large mass can be evaluated by the fragmentation of a single molten metal droplet with a small mass.…”

“…5) Many small droplets of sodium, 2-8 mm in diameter, were splashed from the molten sodium pool and adhered to the bottom surface made of stainless steel of the containment box. This kind of violent splash was not observed in the copper experiment.…”