Mapping Distribution of Molten Boron Debris Could Help Determine Re-Criticality Risk within Nuclear Reactors

Japanese researchers are a step closer to decommissioning the Fukushima Daiichi Nuclear Plant. They have been able to successfully map the distribution of boron compounds in a model control rod, which will help to determine the re-criticality risk within the reactor.

Compilation of control rod cross-sectional images, showing results of high-temperature steam oxidation. Japanese researchers have mapped the distribution of boron compounds in a model control rod, paving the way for determining re-criticality risk within the reactor. (Credit: Kyoto University)

To this day, the exact condition within the Fukushima Daiichi Nuclear Plant is not clear.

Removing fuel debris from the reactor contaminant vessel is one of the top priorities for decommissioning.

Ryuta Kasada, Kyoto University

Boron has the ability to absorb neutrons that are released by splitting atoms, so boiling water reactors, including the Fukushima Daiichi Nuclear Plant, are equipped with boron carbide-filled stainless steel tubes to control the output energy. Nuclear fission reactions occur at steady rates when these control rods function properly. However, in extreme conditions like the Fukushima accident, the rods are exposed to overheated vapors, and consequently a reaction between boron and surrounding material - such as stainless steel - occurs to form 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. 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. It's crucial to get a picture of how boron atoms are distributed inside the reactor, so that we know 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.

Ryuta Kasada, Kyoto University

In order to replicate the conditions of an extreme nuclear accident, Kasada and collaborators filled a model control rod with steam at 1250°C and mapped the molten boron debris distribution. Also, the chemical state of the molten debris was determined using a soft X-ray emission spectrometer integrated to a type of scanning electron microscope.

A new diffraction grating was coupled to a highly-sensitive X-ray CCD camera in the soft X-ray emission spectrometer. Different peak structures were observed on the X-ray spectrum for various boron compounds, such as iron boride, boron carbide, and boron oxide.

Previously this was only possible to visualize in large synchrotron radiation facilities. We've shown that the same is possible with laboratory-sized equipment. This finding demonstrated on a micro-scale what needs to be done in Fukushima. 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.

Ryuta Kasada, Kyoto University

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