The technique is expected to improve understanding of how fuel debris forms during nuclear reactor accidents, which would contribute both to the decommissioning of the Fukushima Daiichi and to the improvement of reactor safety in general.
In their research, the two teams focused their attention on the phenomenon in which molten reactor fuel falls into a coolant pool during severe accidents and shatters into many fine droplets, which in turn become fuel debris once cooled and solidified. Also, they realized that the droplet formation process is especially difficult to understand when the coolant pool is shallow, as the molten fuel may hit the bottom as it breaks apart, complicating the behavior of fuel debris formation.
The joint researchers from JAEA and the University of Tsukuba have been advancing their study of droplet formation by developing experiments as well as detailed numerical simulation techniques.
Using two types of liquids to simulate molten fuel and coolant, they successfully reproduced the phenomenon of massive microdroplet generation at the laboratory scale, but had been unable—until recently—to measure the quantity and individual size of each droplet.
In their latest effort, the teams developed a technique they termed the “3D-LIF method,” enabling three-dimensional visualization through a reflective mirror known as a galvo (galvanometer) scanner, which can control laser light. They then acquired three-dimensional data of the liquid simulating the molten fuel, and processed the data using computers to accurately measure the size of each individual droplet along with the speed in which it spreads.
The researchers had first tried using the 3D-LIF method in a pool experiment, but found that the droplets spread in an extremely complex manner, beyond the capability of the human eye to comprehend. However, by combining the method with other ones, they clarified two primary mechanisms, as follows:
- A “surfing” pattern that emerges due to velocity differences and centrifugal force between the two liquids.
- Another pattern that appears when the liquid film ruptures, and which is driven by gravity.
The researchers are actively aiming for broader applications that the new 3D-LIF method—which enables the detailed tracking of particle motion—can eventually be applied in a wide variety of fields, including research into internal combustion engines and pharmaceutical research.
