Japanese researchers have mapped the distribution of boron compounds in a model control rod, which they say paves the way for determining re-criticality risk within the damaged reactors at the Fukushima Daiichi Nuclear Plant.

More than five years after the core meltdowns at three reactors onsite, the precise situation inside the plant is still unclear. Nonetheless, removing fuel debris from the reactor contaminant vessel remains a top priority for decommissioning.

Stainless steel tubes filled with boron carbide are used to control energy output in boiling water reactors, including at Fukushima Daiichi, as boron absorbs neutrons resulting from splitting atoms. With these control rods functioning properly, nuclear fission occurs at a steady rate. In an extreme situation, such as during the Fukushima accident, in which overheated vapors come into contact with the rods, boron reacts with surrounding materials like stainless steel to create molten debris.

"When melting happens, phenomena like relocation occur such that the boron atoms—trapped in the debris—accumulate towards the bottom of the reactor," explains Ryuta Kasada, of Kyoto University. "This can lead to a lack of control agents in the upper core structure and thus a higher risk of re-criticality in those areas."

According to Kasada, it is critical to get a picture of how boron atoms are distributed inside the reactor to understand which areas have higher risk of re-criticality. "It's also important to know the chemical state of boron, as some boron compounds can affect the formation of radioactive materials released to the environment," he adds.

Kasada and colleagues filled a model control rod with steam at 1,250 degrees Celsius to imitate the conditions of a severe nuclear accident. The team then mapped the distribution of molten boron debris and simultaneously determined its chemical state with a soft X-ray emission spectrometer, in which they combined a new diffraction grating with a highly-sensitive X-ray CCD camera equipped to a type of scanning electron microscope. The boron compounds—including boron oxide, boron carbide and iron boride—each showed different peak structures on the X-ray spectrum.

Control rod cross-sectional images showing results of high-temperature steam oxidation. Image credit: Kyoto University.Control rod cross-sectional images showing results of high-temperature steam oxidation. Image credit: Kyoto University. "Previously this was only possible to visualize in large synchrotron radiation facilities. We've shown that the same is possible with laboratory-sized equipment," Kasada says.

"This finding demonstrated on a micro scale what needs to be done in Fukushima," he adds. "This can't yet be applied in the field, but in the meantime we plan to visualize the chemical state of other elements so as to create a sound materials base for decommissioning Fukushima."

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