Atomization plays a crucial role in various nuclear field scenarios, such as cooling a molten material during severe accidents or boiling transitions in annular dispersed flow under the steady operation of a boiling water reactor. This study explores the atomization of a wall-impinging jet in a shallow pool using numerical simulation^*1.
 In severe accidents, molten fuel material jets can enter shallow water pools, impinge on the floor, and spread in three dimensions, leading to jet atomization. However, a direct and comprehensive investigation of jet atomization is difficult. To address this, we simplified the molten fuel material jet as a simulated fluid and investigated its atomization through (1) detailed numerical simulation using the detailed two-phase flow analysis code, TPFIT^*2, (2) validation of numerical results against experimental results^*3, (3) investigation of droplet formation as a fundamental atomization behavior, (4) development of a theoretical formulation for droplet formation, and (5) comparison of theoretical estimates with numerical results.
 Figure 1 shows the numerical simulation results related to (1). We confirmed that numerical simulations effectively reproduced jet atomization and spreading observed in experiments. Then, we focused on droplet formation, for which we developed a theoretical formulation based on interfacial instabilities and force balances. The numerical data closely aligned with estimates from the formulation, clarifying jet atomization mechanisms. This research enhances the fundamental understanding of jet atomization mechanisms and is anticipated to be a valuable reference for other nuclear field applications.

^＊1 N. Horiguchi, et al., Phys. Fluids. 35, 073309 (2023).
^＊2 H. Yoshida, et al., Trans. At. Energy Soc. Japan. 3 (3) (2004).
^＊3 S. Yamamura, et al., Phys. Fluids. 34, 082110 (2022).

This research was conducted with the supercomputer HPE SGI8600 in the Japan Atomic Energy Agency.