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2025.03.25
Fig. 1 Overview of the developed visualization method
The Nuclear Science and Engineering Center (NSEC) has developed a method to visualize the phenomenon of liquid breaking up into a large number of small droplets in three dimensions (3D) and has deepened the understanding of the debris formation process.
In a severe accident at a nuclear reactor, the fuel in the reactor melts and falls into a lower coolant pool, where it breaks up into a large number of small droplets and spreads out. When the molten fuel and broken-up droplets cool and harden, they form fuel debris. Especially in the case of a shallow pool, the molten fuel impinging on the floor breaks up into droplets, so the fuel debris is formed in such a very complex situation. If we can clarify the fuel debris formation process, we will be able to contribute to the decommissioning of the Tokyo Electric Power Company Holdings, Inc, Fukushima Daiichi Nuclear Power Station (1F) by interpreting the actual debris formation process. In addition, we will be able to further improve the safety of nuclear reactors by managing severe accidents in advance. However, because it is extremely difficult to experimentally visualize and measure the phenomenon of liquid breaking up into a large of small droplets, we have not been able to obtain a detailed understanding of the debris formation process.
In this study, we have developed a method to visualize the phenomenon of liquid breaking up into droplets in 3D. In addition, by processing 3D visualization data with a computer, it became possible to measure the size and velocity of each droplet with high precision (see Fig. 1). Using these methods, we conducted experiments simulating the situation where a molten fuel falls into a shallow pool in a severe accident. As a result, it was found that droplets form with the “surfing pattern” caused by the velocity difference between the two simulated liquids and a centrifugal force, or the “liquid film rupture pattern” caused by gravity. Thus, for the first time in the world, we achieved to observe in detail the phenomenon of liquid breaking up into a large number of small droplets. In addition, through more detailed observation and high-precision measurement, we have deepened the understanding of the debris formation process. This achievement will contribute to the decommissioning of 1F and to the improvement of reactor safety.
This achievement was published in the professional journal, Physics of Fluids, on March 10, 2025.
【Paper information】
Journal: Physics of Fluids
Title: Atomization Mechanisms in the Vortex-like Flow of a Wall-impinging Jet in a Shallow Pool
Authors: Naoki Horiguchi1, Hiroyuki Yoshida1, Akiko Kaneko2, Yutaka Abe3
Affiliation: 1Nuclear Science and Engineering Center, Japan Atomic Energy Agency, 2Institute of System and Information Engineering, University of Tsukuba, 3Professor Emeritus, University of Tsukuba
DOI：10.1063/5.0253743
JAEA HP Press release (Japanese only)
2025.01.23
Global-warming-induced alterations in precipitation patterns can influence carbon dioxide (CO2) emission from soils. A new study, conducted by the Nuclear Science and Engineering Center, in collaboration with Niigata University and Kyushu University, reveals that soil drying and rewetting cycles largely increase CO2 emission from soils.
Increasing the atmospheric CO2 concentration causes global warming and alters the global water cycling and precipitation patterns. Extreme heavy rainfall and extended drought are often observed around the world, which intensify soil drying and rewetting cycles and influence the decomposition of soil organic carbon and consequently the CO2 emission from soils. Since the CO2 emission from the world’s soils is estimated to be about 5 times the amount of CO2 emission from human activities, even a small change in CO2 emission from soils could significantly impact the Earth’s climate system. The research team studied overall trends in the effects of soil drying and rewetting cycles on the CO2 emission by conducting an 84-day laboratory incubation experiment using 10 Japanese forest and pastureland soils. They found a 1.3 to 3.7-fold increase in CO2 emission due to drying and rewetting cycles across all soils. Furthermore, they analyzed relations between the increasing magnitude of CO2 emission and soil properties, and suggested that the increase in CO2 emission occurred mainly through destructions of microbial cells and organo-mineral complexes during drying and rewetting cycles. The findings of this study have important implications for understanding the climate–carbon cycle feedback and for improving the ability to predict the future of Earth’s climate.
The results of this study have been published in “SOIL,” an international scientific journal issued by the European Geoscience Union, on January 16, 2025.
Article information: https://soil.copernicus.org/articles/11/35/2025/
JAEA HP Press release (Japanese only)
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