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The present study is aimed at investigating the ability of a CFD modeling of liquid–liquid jet breakup to resolve the principal mechanisms relevant to jet breakup as well as submillimeter-scale drop size. It is generally known that jet leading edge breaks up by boundary layer stripping (BLS), and jet lateral surface breaks up by Kelvin–Helmholtz instability (KHI). The jet breakup rate as well as the resulting particle size are important parameters that would largely govern the intensity of a steam explosion in severe reactor accidents. First, a two-dimensional simulation of KHI along the melt-liquid coolant interface was performed using the VOF model in ANSYS Fluent with fine meshes as small as 0.02 mm. The dominant wavelength obtained by FFT analysis of calculated melt volume fractions showed that the fastest growing wavelength from the linear analysis of KHI is seen only at the very early development of the instability, and it increases gradually. Second, a three-dimensional simulation of BLS was performed, and the shapes and sizes of the melt particles were obtained. The particle size distributions from KHI and BLS simulations were compared with COLDJET experimental data of Woods metal and water, and it showed that the finer drops of one millimeter or smaller are produced by Kelvin–Helmholtz instability, and the drops of a few millimeters in diameter are mainly produced by boundary layer stripping. [ABSTRACT FROM AUTHOR]