Analyzing molten core-concrete interactions in the Fukushima disaster cleanup efforts
When an earthquake and subsequent tsunami heavily damaged the Fukushima Daiichi nuclear power plant in 2011, the heat from the cores melted a variety of materials in and around the reactors: uranium oxide pellets, zirconium cladding, steel, concrete, and more. The resulting glass-ceramic mixture, known as molten core-concrete interactions, now surrounds what remains of the cores.
Over a decade after the incident, the reactor is still emitting high levels of heat. It’s cooled with 400 cubic meters of water every day, which must then be decontaminated. The Fukushima cleanup process is expensive and hazardous, and on a few occasions, it has resulted in leakage of caesium-137.
The goal is to fully decommission the plant by containing and stocking the radioactive compounds from the core. The challenge, however, is breaking through the molten core-concrete interactions, which are dangerously radioactive — so much so that humans cannot work in the area. The alternative is to use robots designed to pierce the material. But how strong do they need to be?
Determining the strength of molten core-concrete interactions for the Fukushima cleanup
To find out, a team led by Charilaos Paraskevoulakos of the Technical Institute of Denmark used a combination of synchrotron radiation imaging, digital volume correlation, and advanced data analysis to see how samples similar to molten core-concrete interactions respond to stress tests.
The material from Fukushima is too dangerous to handle, so the team tested a safer replica created by researchers at the University of Sheffield. The test itself was cyclical: First, the team used Hertzian indentation to apply an increasing amount of force to the material, starting at 20 Newtons. They then used synchrotron radiation imaging and digital volume correlation to monitor the formation of pores and cracks, global volume variation, and local deformation. After the fourth load — 160 Newtons — the sample finally failed.
Graph of the stress tests showing how the material responded to each load and ultimately failed. Source: https://doi.org/10.1038/s41529-022-00264-y
The team used Thermo Scientific Avizo Software to visualize and analyze the data captured from each stage of the stress test. The software helped identify pores and ratio of materials in the mixture and understand the material deformation process by computing 3D full-field displacement and strain map.
A. Identified pores, uranium, and iron phases. B. Segmentation of the different phases shown in a 2D slice. C. 3D volumes of the segmented uranium, iron, and pore phases. Source: https://doi.org/10.1038/s41529-022-00264-y
Digital volume correlation with Avizo Software
The initial imaging data did not reveal any microcracks or measurable variation on the porous phase. To get a better idea of what was happening within the material, the team turned to Avizo Software’s digital volume correlation module, which they used to compare data collected before and after each physical stress was applied. The technique is particularly suitable for capturing complex phenomena, such as localization induced by heterogeneities, thermal mismatch between constituents, micro-cracking, fatigue behavior, and phase transitions when the force was applied.
The analysis revealed that deformation was not homogeneous within the sample and detected discontinuities within the deformation pattern. This information suggests that microcracks did form within the material but that the imaging resolution was not sufficiently high to resolve them.
Axial displacements in the simulated MCCI material measured by DVC as weight was added during the stress test. Source: https://doi.org/10.1038/s41529-022-00264-y
The researchers noted that, while their sample is rather representative of the material at the incident site, they expect the mixing ratios there to vary widely due to higher concentrations of some materials in certain areas and the lack of mixing. However, their results still provide valuable insights that will aid the Fukushima cleanup efforts and help equip the robots to successfully extract the radioactive material.
Ultimately, the work is an important step in protecting the recovery workers, the people of Fukushima, and the environment from the dangers of the site.
To learn more about Avizo Software, watch our introductory webinar on demand.
Luigi Raspolini is a Product Marketing Manager at Thermo Fisher Scientific
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