Fragmentation Mechanisms of a Single Molten Copper Jet Penetrating a Sodium Pool—Transition from Thermal to Hydrodynamic Fragmentation in Instantaneous Contact Interface Temperatures below Its Freezing Point—

Abstract

To clarify the fragmentation mechanism of a molten metallic fuel jet in a sodium pool under highejection-velocity conditions that correspond to the medium-and high-burnup conditions in the metallic fuel core of liquid-metal-cooled fast breeder reactors, a series of experiments with molten copper as a metallic fuel simulant and a sodium pool was carried out. Under low-ejection-velocity conditions in the range of an ambient Weber number ðWe a Þ < 200, the fragmentation of the molten copper jet depends on the initial superheating of the jet. The size of copper fragments decreases with increasing initial superheating. Under high-ejection-velocity conditions in the range of We a ! 200, the size of the fragments is confirmed to be almost independent of the initial superheating of the jet. Furthermore, the size of the fragments agrees well with that evaluated using the Rayleigh-Taylor instability model, in which the fragment size is assumed to be equal to half the fastest growing wavelength. This result is qualitatively consistent with the characteristics that the molten jet column with large inertia force owing to the high ejection velocity, which transports enthalpy downwards, can penetrate the decelerated leading edge and can directly come into contact with sodium successively.

Flow transition criteria of a liquid jet into a liquid pool

“…As mentioned above, the hydrodynamic interaction is considered to be an important factor in meltjet breakup. Nishimura et al (2010) suggested a hydrodynamic fragmentation model that agreed well with their copper/sodium experiments at high ambient Weber numbers (> 200). The ambient Weber…”

“…Fig. 12 shows the droplet MMD obtained from the present experiments and the fragment MMD obtained from the previous works: U-alloy78/water system (Matsuo et al, 2008), copper/sodium system (Nishimura et al, 2010), and aluminum-oxide/sodium system (Matsuba et al, 2016b). The vertical axes in Fig.…”

“…Here we use the mass-median diameter (MMD) as the index of droplet size. The MMD has been employed by several authors to evaluate the fragment size during melt-coolant interactions (e.g., Matsuba et al, 2016b;Matsuo et al, 2008;Nishimura et al, 2010). Therefore, we can compare our results with the available data on the same basis.…”

“…Instead, a scoping study of the fundamental process is an effective approach to understanding the actual situation. To date, there have been many experimental efforts under various conditions that have been aimed at clarifying the melt-coolant interaction, including UO2 into sodium (Magallon et al, 1992;Schins and Gunnerson, 1986), molten oxide into sodium (Kaiser et al, 1998;Matsuba et al, 2015;Schins and Gunnerson, 1986), molten metal into sodium (Kaiser et al, 1998;Matsuba et al, 2016a;Nishimura et al, 2010;Schins and Gunnerson, 1986), corium into water (Huhtiniemi and Magallon, 2001;Magallon, 2006;Magallon and Huhtiniemi, 2001;Spencer et al, 1994), molten oxide into water (Kaiser et al, 2001;Karbojian et al, 2009;Kudinov et al, 2013;Manickam et al, 2014Manickam et al, , 2016Moriyama et al, 2005) and molten metal into water (Abe et al, 2006;Bang et al, 2003;Bang and Kim, 2014;Bürger et al, 1995;Cho et al, 1991;Dinh et al, 1999;Iwasawa et al, 2015bIwasawa et al, , 2015cKondo et al, 1995;Mathai et al, 2015;Matsuo et al, 2008;Pillai et al, 2016;Spencer et al, 1986;Wei et al, 2016). The aim of all these experiments and tests was not limited to understanding a CDA in an SFR; a severe accident in a light-water reactor was also included as a possible scenario.…”

“…is often used to characterize the jet breakup length (Bürger et al, 1995;Ginsberg, 1985;Thakre et al, 2015) and the diameters of the resulting fragments (Matsuba et al, 2016b;Nishimura et al, 2010). Matsuo et al (2008) concluded from their melt-water experimental data that the dominant effect on jet breakup was the shear force that acted on the interface.…”