Method for 3D visualization of molten fuel droplet formation developed by JAEA

During nuclear power plant accidents, when fuel inside the reactor melts and falls into the coolant pool, it breaks into numerous fine droplets that then spread. Fuel debris forms as the molten fuel and fragmented droplets cool and solidify. When the pool is shallow, the molten fuel forms droplets while colliding, resulting in a very complex fuel debris formation process. A research group led by Researcher Naoki Horiguchi and Principal Researcher Hiroyuki Yoshida from the Nuclear Science and Engineering Center at the Japan Atomic Energy Agency (JAEA), along with Professor Akiko Kaneko and Professor Emeritus Yutaka Abe from the University of Tsukuba, has observed the phenomenon of molten fuel fragmenting into droplets while colliding with the floor in a shallow pool. With the aim of understanding this, the research group developed a method to visualize in three dimensions the phenomenon of liquid fragmenting into numerous droplets. By processing the 3D visualization data, they can now accurately measure the size and velocity of individual droplets. Their research was published in Physics of Fluids.

Laser-Induced Fluorescence (LIF) obtains cross-sectional shapes by causing a cross-section through which laser light passes to fluoresce. However, to observe droplets extending before and after the cross-section and to measure the quantity and individual sizes of droplets, it was necessary to change the position of the fluorescing cross-section rapidly and arbitrarily.

The group succeeded in rapidly and arbitrarily changing the position of this fluorescing cross-section by incorporating a galvano scanner (reflective mirror) into the experimental apparatus. This led to the development of the 3D-LIF method, which can visualize in three dimensions the phenomenon of liquid fragmenting into numerous droplets. By processing the three-dimensional shape data of the liquid simulating molten fuel with a computer, they can now accurately measure the size and velocity of individual droplets.

Horiguchi commented: "We were able to visualize the phenomenon of liquid fragmenting into nearly 1,000 droplets within 0.5 seconds. As a result, we found that droplets are generated through surfing patterns due to velocity differences between the two liquids and centrifugal forces, as well as liquid-film breaking patterns due to gravity. By collecting various data while changing liquid types and combining the data with numerical simulations, we can contribute to the decommissioning of the Fukushima Daiichi Nuclear Power Plant and improve the reliability of nuclear reactor core design."

Journal Information
Publication: Physics of Fluids
Title: Atomization mechanisms in the vortex-like flow of a wall-impinging jet in a shallow pool
DOI: 10.1063/5.0253743